Displays

A significant number of studies in recent years have demonstrated that earthquakes in the lower crust are more abundant than previously thought. Specifically in continental collision zones, earthquakes are suggested to play a crucial role in permitting fluid infiltration and driving metamorphic transformation processes in crustal portions that are typically considered dry and metastable. However, the mechanisms that trigger brittle failure in the lower crust remain debated and the sequence of events that ultimately lead to seismic slip is unclear. To further understand this process we performed field and microstructural observations on an amphibolite facies fault (0.9-1 GPa) in granulite facies anorthosite from the Bergen Arcs, Western Norway. The fault preserves an exceptional record of brittle deformation and frictional melting that allows us to constrain the temporal sequence of deformation events. Most notably, the fault is flanked on one side by a damage zone where wall rock minerals are fragmented with little to no shear strain (pulverization). The fault core consists of a zoned pseudotachylyte bound on both sides by fine-grained cataclasites. Spatial relationships between these structures reveal that asymmetric pulverization of the wall rock and comminution preceded the seismic slip required to produce melting. These observations are consistent with the propagation of a dynamic shear rupture. Our study implies that high differential stress levels may exist within the dry lower crust of orogens, causing brittle faulting and earthquakes in a portion of the crust that has long been assumed to be characterized by ductile deformation.

How to cite: Petley-Ragan, A., Ben-Zion, Y., Austrheim, H., Ildefonse, B., and Renard, F.: Direct observations of a dynamic earthquake rupture in the lower crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8965, https://doi.org/10.5194/egusphere-egu2020-8965, 2020.

Natural and induced seismicity is widely investigated, and extensive knowledge has been acquired in the last years, but exact earthquake mechanisms remain elusive and poorly understood. The high impact of earthquakes on human society emphasizes that a deeper understanding of earthquake processes must be a priority in order to improve seismic hazard assessment and mitigate associated risks. Pervasive fluid flow is a key process significantly influencing rock physics and mechanics, and thus has a crucial impact on natural and induced earthquakes. Seismo-hydro-thermo-mechanical (SHTM) modelling is an important nascent branch of geodynamic modelling, which investigates evolution of coupled fluid-solid systems under conditions of both slow tectonic and fast seismic deformation rates. Here, we present a new fully coupled two-dimensional seismo-hydro-mechanical numerical code, with a poro-visco-elasto-plastic rheology, based on fully staggered finite differences with marker-in-cell technique, adaptive time stepping and global Picard iterations. The presented numerical code combines inertial mechanical deformation with pervasive fluid flow. The adaptive time stepping allows the resolution of co-seismic and interseismic phases with time steps ranging from milliseconds to years.

First, we demonstrate how fluid-bearing subducting rocks are intrinsically seismic and how seismic events in the form of highly localized ruptures spontaneously nucleate along the subduction interface. Nucleation and propagation of such events are driven by rapid fluid pressurization caused by visco-plastic compaction, counterbalanced by a nearly simultaneous and equivalent poroelastic decompaction inside the propagating and rupturing fault. Successive post and interseismic fluid pressure release, generated by poroelastic compaction along the fault, allows strength recovery of the megathrust. The model reproduces the broad range of seismic events present at the subduction interface, including slower events, regular earthquakes, and earthquakes that rupture the entire megathrust and reach velocities on the order of m/s, without employing slip rate dependent frictional properties.

Next, we show how our approach can be successfully adapted for fluid injection setups to model induced seismicity phenomena. The numerical code successfully modelled fluid induced seismic events and the resulting seismic wave propagation.  Preliminary results show that faults can form spontaneously and grow aseismically at the injection site thereby creating favorable conditions for the development of broad induced seismicity region where different faults can be activated seismically at different time.

Finally, we outline in short future SHTM modeling directions that should account for fracture-induced dilatation and dynamic permeability variations, thermal effects and three-dimensionality.

How to cite: Gerya, T., Petrini, C., and Yarushina, V.: Poro-visco-elasto-plastic seismo-hydro-thermomechanical geodynamic models for subduction zones and induced seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19456, https://doi.org/10.5194/egusphere-egu2020-19456, 2020.

In this study, we use a state-of-the-art 2D numerical algorithm solving the standard thermo-mechanically coupled equations of continuum mechanics for slow flowing viscoelastoplastic material to model the evolution of rifting, thermal relaxation and convergence-to-collision of Alpine-type orogens in three stages. (1) A ca. 360 km wide basin that is floored by exhumed mantle and bounded by two conjugate magma-poor hyper-extended passive margins is generated during a 50 Myrs rifting period. An absolute extension velocity of 1 cm/yr is applied. (2) The passive margin system is thermally equilibrated during a subsequent cooling period of 60 Myrs without significant deformation in the lithosphere (no extension velocity). At this stage, we parameterise a serpentinization front on top of the exhumed mantle by replacing the dry peridotitic mantle by serpentinized mantle in one series of simulations. The thermally equilibrated system is used as a self-consistently generated initial configuration for the subsequent period of convergence lasting for 70 Myrs applying an absolute convergence velocity of 1.5 cm/yr. Values for the duration of deformation periods and for deformation velocities are chosen to allow for comparison between simulation results and petrological data from the Central and Western Alps. Density of all materials is either precomputed for characteristic bulk rock compositions and read in from precomputed thermodynamic look-up tables (Perple_X), or calculated during run time via a linearized equation of state (EOS).

We quantify (1) the impact of a serpentinization front of the exhumed mantle on the subduction dynamics by increasing systematically the strength of the serpentinites, (2) the peak pressure and temperature conditions of subducted crustal material from the passive margins of the overriding and subducting plate by tracking pressure (P)-temperature (T)-time (t)-depth (z) paths of selected particles and (3) the driving forces of the system. Last, (4) the impact of metamorphic phase transitions is investigated by parameterising densification of crustal material. We compare the results of simulations in which density is computed as a simple linearized EOS to results of simulations in which density is a more realistic function of P and T using precomputed thermodynamic look-up tables.

We discuss geometric similarities between the simulation results and 2D geodynamic reconstructions from field data, quantify the P-T-t-z-history of selected particles and compare it to P-T-t data obtained from natural rocks. First results indicate that the strength of the serpentinites controls whether the deformation within the orogenic core is driven by buoyancy forces (subduction channel model) or by far-field tectonic forces (orogenic wedge model). There is a transition from subduction channel to orogenic wedge model from low to intermediate strength of the serpentinites.

How to cite: Candioti, L. G., Schmalholz, S. M., and Duretz, T.: Subduction channel vs. orogenic wedge model: numerical simulations, impact of serpentinites and application to the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10209, https://doi.org/10.5194/egusphere-egu2020-10209, 2020.

The core complexes of western North America are generally thought to exhume deeply buried rocks (as much as 30 km) from the Cordilleran infrastructure, from beneath an inferred orogenic plateau to the surface today. However, how deep these rocks were buried has been intensely debated over the past three decades, especially for the Ruby Mountain-East Humboldt Range (RER) and northern Snake Range core complexes, eastern Nevada: published thermobarometry calculations, including robust modern techniques, suggest deep burial to 2-3x stratigraphic depths (as much as 30 km), whereas generations of field studies support burial only to roughly stratigraphic depths (~12-15 km). This has led to fierce debate that either field geologists are missing major structures or geobarometric estimates may neglect important considerations, such as reaction overstepping. Here we propose that a model of non-lithostatic conditions can resolve both field and petrologic datasets, and therefore the North American core complexes represent an example of tectonic overpressure. Western North America is covered by a remarkably well-characterized ~12-15-km-thick passive margin sequence that allows for careful structural reconstructions. Our observations focus on the RER geology, including new detailed geologic mapping (1:24,000 scale), structural traverses, thermochronology, and peak temperature (Tp) estimates. In particular, peak P-T conditions that suggest deep burial require (1) relatively low geothermal gradients of ≤20°C/km and (2) enigmatic structures that are not observed and would be atypical of other Cordilleran fold-thrust belts or even other analogous intra-plateau thrust systems. Instead, our Tp compilation (e.g., Raman spectroscopy of carbonaceous material, Conodont color alteration index, thermochronology) across continuous stratigraphy suggests high geothermal gradients (≥40°C/km) that are consistent with the region being extensively intruded and mineralized—i.e., the region underwent major Jurassic, Cretaceous, and Eocene intrusive episodes and hosts an Eocene(?) world-class Carlin-type gold deposit—and matches thermal gradients observed in other eastern Nevada studies and analogous orogens. Systematic mapping does not reveal any structural break across a section of Neoproterozoic to undeformed Permian passive margin strata that was supposedly deeply buried beneath an additional entire stratigraphic section. The approach of using a Tp traverse to test deep burial models allows for self-consistent evaluation of the data. That is, interpretations are based on a trend of temperature variations deduced from numerous measurements rather than relying on a single (or few) pressure data point(s). Our observations suggest that non-lithostatic pressure may have affected Cordilleran core complexes. We explore how the local rheologically heterogeneous rock types and specific tectonic setting may have created conditions favorable for tectonic overpressure in North American core complexes. For example, paleo-stress estimates from across several shear zones demonstrate significant strength variations that may have facilitated mean stress (pressure) perturbations.

How to cite: Zuza, A., Levy, D., Henry, C., Long, S., and Dee, S.: Non-lithostatic pressure in North American core complexes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11435, https://doi.org/10.5194/egusphere-egu2020-11435, 2020.

Pressure recorded in metamorphic rocks is typically assumed to represent a hydrostatic stress and thus depends linearly on depth. Recently, work in the Monte Rosa nappe in the western Alps has challenged this lithostatic assumption. Observable pressure differences of 0.8 ± 0.3 GPa between chloritoid, talc, and phengite-bearing lithologies (locally known as ‘whiteschists’) at ca. 2.2 - 2.5 GPa, and metagranite lithologies at 1.4 – 1.6 GPa have been recorded. These pressure variations, rather than being attributed to variable rock kinetics, partial retrogression, or tectonic mixing, have been interpreted to be mechanically induced. As part of this ongoing investigation, we will present work undertaken on newly discovered staurolite-chloritoid bearing metapelites belonging to the Monte Rosa basement, in order to constrain the peak pressure and temperature conditions during burial within the Alpine orogeny and the resulting tectono-metamorphic and geodynamic implications.

Metapelitic samples from the Monte Rosa basement show a rich polymetamorphic history from high-T Variscan garnet growth through to high-P Alpine equilibration and decompression. Extensive phase petrology calculations have been undertaken on staurolite + chloritoid + phengite + paragonite assemblages, as well as garnet + chlorite + phengite + paragonite assemblages, representing equilibration at peak Alpine conditions. Various mixing models were employed due to non-negligible amounts of ZnO recorded in staurolite (~5% wt% and ~1 a.p.f.u) and the lack of available solution models. These result in peak Alpine conditions of 1.6 ± 0.2 GPa and 580 ± 15 ºC. These findings confirm the presence of significant disparities in pressure of 0.6 ± 0.2 GPa within the coherent Monte Rosa nappe.

Vital for the reconstruction and tectonic history for the western Alps is the maximum burial depth of units involved. We argue that the maximum burial depth of the Monte Rosa unit was significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at > 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal or less than ca. 60 km, estimated from pressures of 1.4 - 1.6 GPs recorded frequently in metagranite and metapelitic lithologies. This depth is compatible with burial and exhumation within an orogenic wedge, rather than a complex exhumation mechanism such as within a weak and long subduction channel. Equally, the relatively slower exhumation rates from shallower crustal depths fit more reasonable tectonic velocities.

How to cite: Vaughan-Hammon, J. D., Luisier, C., Schmalholz, S., and Baumgartner, L.: Pressure variations in the Monte Rosa nappe: new results from staurolite bearing metapelites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8284, https://doi.org/10.5194/egusphere-egu2020-8284, 2020.

Intermediate-depth earthquakes are registered in convergence zones where crustal rocks are expected to deform by ductile flow. This paradox is also evidenced in exhumed crustal rocks where brittle structures (e.g., pseudotachylytes and breccias) associated to high-pressure metamorphism have been documented. If the link between brittle deformation and metamorphic reactions appears obvious today, the mechanism involved is still a burning issue. We propose that the initial heterogeneity of rocks, by itself, is sufficient to trigger both metamorphic reaction and brittle deformation. Based on a mechanically consistent dynamic model, we show that local pressure variations due to pre-existing heterogeneities can be high enough to reach the thermodynamic conditions required for reaction initiation. Brittle behaviour is then controlled by the strength difference between the untransformed host rock and its reaction product. This continuous process also explains the higher pressures recorded in eclogite facies rocks of ductile shear zones compared to their brittle host rock. Our results, constraint by natural data, have therefore significant implications for intermediate-depth seismicity.

How to cite: Yamato, P. and Baïsset, M.: A new model for brittle failure at depth involving high-pressure metamorphism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13673, https://doi.org/10.5194/egusphere-egu2020-13673, 2020.

Metamorphic reactions involving hydration and dehydration frequently occur during orogenic cycles, for example, when ambient pressure and temperature conditions change due to subduction and subsequent exhumation, or when fluids infiltrate metastable mineral assemblages at constant ambient conditions. Such (de)hydration reactions can be associated with significant volume changes, which may cause significant differential stresses in the rock, potentially leading to fracturing. The impact of (de)hydration reactions on the rock’s stress state and on the magnitudes of associated differential stresses is still controversially debated. One reason for the debate is due to the different theoretical models used to quantify and simulate (de)hydration reactions coupled with rock deformation. In many models, the rock deformation is frequently simplified, by either completely ignoring rock deformation or by considering volume deformation only. Additionally, the fluid flow is often simplified, by for example considering constant porosity. Here, we present a method to derive a system of governing equations to describe coupled Hydro-Mechanical-Chemical processes, which is suitable to quantify rock deformation coupled to (de)hydration reactions. Reactions are mainly treated as density changes whereby the density changes are determined by tabulated densities from thermodynamic Gibbs free energy minimizations in pressure, temperature and composition space. The rock deformation is quantified by the continuum mechanics force balance equations, here the Stokes equations. Considered flow laws describe either linear viscous deformation or dislocation and diffusion creep. Equations for reactions and rock deformation are coupled by several equations for the conservation of mass, such as total mass or mass of solid components stored in the solid. The governing system of equations is solved with a pseudo-transient finite difference method. For simplicity, we apply the numerical model here to several Brucite – Periclase (de)hydration reactions and show results of models with different levels of coupling, for example, constant or variable porosity. We also quantify the differential stresses associated with the (de)hydration reactions. Furthermore, we compare the modelled stresses with microstructural observations and stress estimates from high-resolution EBSD measurements in natural rock.

How to cite: Schmalholz, S. M., Plümper, O., Moulas, E., and Podladchikov, Y.: Hydro-Mechanical-Chemical modelling of Brucite – Periclase (de)hydration reactions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6649, https://doi.org/10.5194/egusphere-egu2020-6649, 2020.

We present the hypothesis that material instabilities based on multiscale and multiphysics dissipative waves hold the key for understanding the universality of physical phenomena that can be observed over many orders of scale. The approach is based on an extended version of the thermodynamic theory with internal variables (see related abstract by Antoine Jacquey et al. for session EMRP1.4 entitled: “Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials”). The internal variables can, in many cases, shown to be related to order parameters in Lev Landau’s phase-transition theory. The extension presented in this contribution consists of replacing the jump condition for the symmetry-breaking order parameter at the critical point (e.g., density difference at the liquid-gas transition) through considering a second-order phase transition, where the internal variables change continuously from the critical point due to the propagation of material-damaging dissipative waves. This extension to the first-order theory allows assessing the dynamics of coupling the rates of chemical reactions, failure and fluid-flow as well as thermo-mechanical instabilities of materials. The approach gives physics-based insights into the processes that are commonly described by empirical relationships. Here, we present a first analytical model extended by numerical analyses and laboratory and field observations that show the existence of these precursor phenomena to large-scale instabilities. In the event that the propagating waves lead to a large-scale instability, the dissipation processes are predicted to leave tell-tale multi-scale structures in their wake, which can be used to decipher the dynamic processes underpinning the event.

First analyses from a laboratory analogue experiment are presented, illustrating the slow speed of the waves and their peculiar dispersion relationships and reflection from boundaries. An idealized 1-D (oedometric) compaction experiment of a highly porous (45% porosity) carbonate rock investigates the emergence of localized compaction bands proposed to be formed by long-term resonant collision of the transient dissipation waves. Complementary numerical models of the phenomenon allow in-depth analysis of the dynamics and illustrate the physics of the formation of dissipative waves.

For field application, we propose that a multiscale analysis - from the grain- over the outcrop- up to the lithospheric scale - can be used to extract quantitative information directly from natural deformation bands, fractures, and fault zones on, for example, the state of stress, the size of the underlying earthquakes, the flow and mechanical properties of the host rock, and the spatiotemporal evolution of fluid and mechanical pressure associated with faulting. The experimental investigation of the fundamental instability has broader applications in the fields of industrial processing of multiphase materials, civil, mechanical, and reservoir engineering and solid mechanics.

How to cite: Regenauer-Lieb, K., Schrank, C., Gaede, O., Marks, B., Hu, M., Clavijo, S. P., Jacquey, A., Blacy, T., Chen, X., and Roshan, H.: Multiphysics dissipative waves as a multiscale precursor phenomenon to geodynamic instabilities , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20764, https://doi.org/10.5194/egusphere-egu2020-20764, 2020.

A wide variety of fluid-rich natural systems exhibit a distinct pulsating signature on geophysical measurements. Identifying the processes leading to these observed pulses are key to further understand important multi-scale and multi-physics valve-like dynamics in natural environments such as gas flow in volcanic systems, magma transport in the crust, tremors and slip or subsurface flow migration. These natural two-phase systems share common features as they can be described as viscously deforming saturated porous media. They exhibit a time-dependant deformation of their porous matrix, buoyant pore-fluid, an effective pressure dependant bulk viscosity and a nonlinear porosity-permeability relation.

We here investigate the role of coupled hydro-mechanical processes to trigger pulsating localised fluid expulsions. We show that the pulsating regime may be a natural outcome of the interactions between a viscously deforming porous matrix and a nonlinear pore-fluid flow. We rely on high-resolution direct numerical two-phase flow calculations in three dimensions to explore what parameters control the main characteristics of the pulsating signal. We are particularly interested in how amplitudes, wave lengths and frequencies of the signal relate to the input model parameters.

We show that repeated fluid pulses are a natural outcome of the coupled Stokes and Darcy equations within the nonlinear viscous two-phase flow regime. We discuss the relevance of our findings in light of the valve-like behaviour in a variety of natural fluid-rich environments. We propose to use the characteristic of the pulsating signal to gain further insight in the dynamics of complex natural systems.

How to cite: Räss, L., Simon, N. S. C., and Podladchikov, Y. Y.: Pulsating localised fluid expulsions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22105, https://doi.org/10.5194/egusphere-egu2020-22105, 2020.

The improved resolution of recent seismic surveys has made seismic chimney structures a common observation in sedimentary basins worldwide and on the Norwegian Continental Shelf. Focused fluid flow in vertical chimneys is an important and poorly understood feature in a petroleum system. Oil and gas migrate through preferential pathways from source rocks to structural traps where they form reservoirs. Further migration or leakage from reservoirs leads to formation of shallow hydrocarbon accumulations and gas pockets. In some cases, leakage through preferential pathways can be traced up to the surface or to the sea floor, where it leads to formation of mud volcanoes, mounds and pockmarks. Here, we present results of an integrated case study, which is performed on a 3D seismic data set that covers an area of approximately 3000km2. The seismic sequence stratigraphic interpretation is complemented with a study of seismic fluid migration paths. Detection of seismic chimneys is a challenging task. State-of-the-art chimney cube technology based on self-educating neural networks was used to automatically identify possible structures. The results of seismic inversion in combination with available well data provided a set of surfaces distinguishing various stratigraphic layers and their properties. Obtained geological model was used as a basis for coupled geo-mechanical / fluid flow modelling that reconstructed the fluid flow processes in the geological past that lead to formation of chimney structures. Our numerical model of chimney formation is based on the two-phase theory of fluid flow through (de)compacting porous rocks. Viscous bulk rheology and strong nonlinear coupling of deforming porous rocks to fluid flow are key ingredients of the model. Chimney formation is linked to pressure build-up in the underlying reservoir. We reconstruct the fluid flow processes in the geological past that lead to formation of chimney structures and provide expectations for their present-day morphology, porosity and fluid pressure. Conditions of chimney formation, their sizes, spatial distribution and times of formation are investigated. The fate of the chimney after it has been created and its role as a fluid pathway in the present-day state is studied.

How to cite: Yarushina, V., Lakhlifi, A., Wang, H., Connolly, D., Wangen, M., Kocsis, G., and Fæstø, I.: Integrated seismic and geomechanical/flow modelling study of focused fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7724, https://doi.org/10.5194/egusphere-egu2020-7724, 2020.

The relative motion of tectonic plates is accommodated at boundary faults through slow and fast ruptures that encompass a wide range of source properties. Near the Parkfield segment of the San Andreas fault, deep tremors and slow slip take place deeper than most seismicity, at temperature conditions typically associated with stable sliding, which should inhibit stick slip. However, laboratory experiments indicate that the strength of granitic gouge decreases with increasing temperature above 350$^\circ$C, providing a possible mechanism for weakening if temperature is to vary dynamically. Here, we argue that recurring tremor and slip at these depths may arise due to shear heating and the temperature dependence of frictional resistance and contact healing. Assuming a lower thermal diffusivity in the fault zone than in the surrounding rocks, numerical simulations can explain the recurrence pattern of the mid-crustal tremors and their correlative slip distribution, predicting peak temperatures exceeding the solidus of wet granite during sliding. We conclude that shear heating associated with slow slip can be sufficient to generate pseudotachylyte injection veins in host rocks even when fault slip is domin.

How to cite: Wang, L. and Barbot, S.: Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20139, https://doi.org/10.5194/egusphere-egu2020-20139, 2020.

Episodic brittle-ductile behaviour reflects the complex interplay of micromechanical hardening and softening, often with some type of fluid pressure associated with introduction of new material that acts as the switch from coseismic to interseismic response. Brittle features observed in nature can in general be characterized as discrete surfaces or narrow zones across which fast particle displacements have occurred, with or without dilatant behaviour; this descriptively meets the criteria for generation of earthquakes. Likewise, non-brittle flow is a priori associated with slower particle velocities. This reduces the problem to one of how and why rocks cycle between slow and fast displacements. Particle displacement in the solid-state is limited to three processes: individual atoms, glide of packets of atoms and frictional displacement across an essentially free surface. Each of these processes, however large the feature being studied or rapid the displacements, necessitates the sequential overcoming of extant atomic bonding energies. Within the rock record, evidence of seismic events are embedded as new or reconstituted material introduced to the deforming host as a consequence of brittle deformation; for example, veins and pseudotachylyte. This new material acts as an important sink for strain energy whereby brittle responses are suppressed until such time as a new critical state is reached. In turn, the strain rate softening abetted by the new material provides a ductile overprint of their syn-fracture origin. Consequently, rheological transitions within Earth’s crust are spatially and temporally transient, evidence for which may be routinely lost. As part of this cyclic behaviour, localization of deformation can be viewed as the default state, with macroscopic deformation a result of organization into required dissipative structures.

How to cite: White, J. C.: Cyclic micromechanical controls of transient crustal deformation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11353, https://doi.org/10.5194/egusphere-egu2020-11353, 2020.

Aftershock sequences follow three empirical laws; Gutenberg Richter, Omori, and Bath. Unless they don't. This raises the question as to why most earthquakes follow empirical laws, while other earthquakes generate few, if any, aftershocks. For example, a magnitude 7.1 earthquake in Mexico in 2017 and a magnitude 8 earthquake in Peru in 2019 generated no aftershocks, while a magnitude 7.1 earthquake in 2019 in California and a magnitude 6.4 earthquake in 2020 in Puerto Rico generated thousands of aftershocks. In this work, I show from numerical modelling and comparisons with data that the differing behaviours rests with the presence of high-pressure fluids at depth. Using a simple model of non-linear diffusion, I compare model results with well-located aftershocks from four Southern California earthquakes and show strong spatial correlation between measured hypocenters and calculated fluid pressure emanating from a high-pressure source. I also show that Omori's Law arises from permeability dynamics. That is, permeability: 1) is effective-stress dependent, 2) undergoes a co-seismic step-like increase, and 3) exponentially heals through either precipitation processes or tectonic re-compaction. I find excellent temporal correlation (Omori's Law) between the number of measured and modelled earthquakes from the strike-slip earthquakes of Joshua Tree and Landers (1992), the strike-slip Hector Mine earthquake (1999), and the thrust Northridge earthquake (1994). Finally, I demonstrate that the fit to the Omori-Utsu Law depends only on the rate of permeability recovery, and argue that all rich aftershock sequences are fluid-driven, while fluid-absent geodynamic settings produce few, if any, aftershocks. 

How to cite: Miller, S. A.: No aftershocks, fluid-driven aftershocks, and Omori’s Law, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13994, https://doi.org/10.5194/egusphere-egu2020-13994, 2020.

In this presentation I will review geodetic constraints on the rheology of the mid- to lower continental crust from observations and models of all phases of the earthquake deformation cycle. I will focus on observations of slow interseismic strain accumulation and rapid postseismic strain transients, both of which result primarily from deformation in the mid- to lower crust. I will argue that, with a few exceptions, interseismic strain is focused in zones around faults with widths that are compatible with strain at depth being focused on a fault or distributed in a shear zone up to ~3 x the seismogenic layer thickness. I will show that for the North Anatolian Fault, the strain accumulation rate appears to be approximately constant for the entire earthquake cycle, once the postseismic transient has decayed. This is consistent with observations at other fault where geodetic measurements were made prior to major earthquakes; the broad agreement between geological and geodetic estimates of slip rate is also consistent with interseismic strain accumulation rates being relatively time invariant. Time-invariant interseismic strain accumulation rates require a relatively strong mid- to lower crust, where relaxation times are equal to or greater than the average earthquake revisit time. Postseismic deformation transients are commonly observed following most earthquakes, but they are interpreted using a variety of very different deformation mechanisms. By compiling all observations of postseismic deformation we show that the largest transient postseismic velocities decay following a simple t^-1 power-law, analogous to Omori’s law for aftershock decay. This is consistent with frictional afterslip and/or power-law creep in a narrow shear zone. This model of a weak shear zone embedded within a stronger substrate can explain most observations of the earthquake deformation cycle. Exceptions to this simple model might occur in locations where the lower crust is weaker, perhaps due to the presence of partial melt. Geological constraints on rheology are critical for making further progress in understanding the earthquake deformation cycle – geological models for the mid- to lower crust can be tested by comparing geodetic observations with geologically-realistic earthquake cycle models.

How to cite: Wright, T., Ingleby, T., and Hussain, E.: Constraints on the rheology of the mid- to lower continental crust from geodetic studies of the earthquake deformation cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11951, https://doi.org/10.5194/egusphere-egu2020-11951, 2020.

The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of an anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone rock, defined by the interlayering of amphibolite with leucocratic domains. Within the outcrop, quartz-filled fractures and their associated amphibolite alteration haloes crosscut the granulite. These fractures are relicts of the initial fluid infiltration event. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is equivalent to that occurring locally within leucocratic domains of the shear zone. Due to the textural and compositional similarities between quartz-filled fractures and leucocratic domains, the compositional layering of the shear zone rock may be directly linked to fracturing during initial fluid infiltration. Mass-balance calculations indicate quartz-filled fractures and compositional differentiation of the shear zone form by internal fractionation rather than partial melting or precipitation of minerals from an eternally derived fluid. This inferred fluid connectivity combined with the enhanced local dissolution indicates the presence of a continuously replenished fluid along fracture pathways. The overall conclusion is that the mass transfer processes that result in metamorphic differentiation of the shear zone lithologies are dependent on both continuous fluid flux and heterogeneous strain distribution.

How to cite: Putnis, A., Moore, J., Beinlich, A., Piazolo, S., and Austrheim, H.: From granulite hydration to metamorphic differentiation: Evolution of a shear zone., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12862, https://doi.org/10.5194/egusphere-egu2020-12862, 2020.

The classical eclogite assemblage consists of the non-hydrous minerals garnet and omphacite. Nevertheless, it is widely accepted that the transformation of mafic magmatic rocks into eclogite requires fluid infiltration. The most common fluid pathway referred to are cracks acting as brittle precursor for fluid-supplied eclogitization, followed by subsequent strain localization, possibly enhancing further eclogitization. While this seems to be a common observation, it is still not fully understood by which processes fluids enhance the metamorphic processes. Herein a set of eclogites from the type-locality (Hohl, Koralpe, Austria, Eastern Alps) representing three different strain stages has been analyzed by means of their microstructure and petrology. Additionally, thermodynamic forward modelling has been performed to constrain pressure, temperature and water activity during eclogitization. All samples are composed of garnet (grt), sodic-clinopyroxene (cpx), quartz (qtz) and a fine grained polycrystalline aggregate (fgpa) of kyanite (ky), clinozoisite (czo) and retrograde plagioclase (pl). While the mineral assemblage is identical in all investigated samples, we do observe minor variation in the volume fraction of each mineral, the specific mineral chemistry and the microstructure with respect to the different eclogite types. 
Almost unstrained eclogites are characterized by grt coronas surrounding cpx in a fgpa matrix. Locally the replacement of coarse crystals of sodium-poor pyroxene by a polycrystalline mixture of qtz and cpx can be observed. In intermediate strained eclogites grt occurs in elongated clusters surrounded by cpx and fgpa matrix. Clinopyroxene grains start to develop a shape preferred orientation (SPO) together with a weak crystallographic preferred orientation (CPO). Highly strained eclogites are characterized by a pronounced foliation defined by a SPO of cpx and elongated layers of fgpa. Garnet again occurs as elongated clusters locally starting to disaggregate perpendicular to the foliation. Though cpx matrix grains develop a more pronounced CPO with increasing strain hardly any intracrystalline deformation can be observed. In all samples we observe symplectites composed of diopside and pl surrounding elongated cpx grains indicating that deformation occurred at eclogite-facies conditions. 
Thermodynamic modelling yield formation conditions of approximately 2.4 GPa, 670 °C and a H2O activity slightly lower than 1 suggesting that fluid supply did play an important role during eclogitization and deformation. Nevertheless, different to above mentioned studies, we do not observe any positive correlation between fractures and reaction front. Our microstructural and petrological investigations instead reveal the formation of a micro-porosity along new developed grain boundaries allowing fluids to migrate to the reaction front, slowly consuming the original gabbroic protolith and replacing it with the stable eclogitic mineral paragenesis. This rather static-type of eclogitization seems to be dominated by dissolution-reprecipitation processes and is resulting in a volume reduction of about 12 %. Subsequent volumetric and tectonic strain is further accommodated by dissolution-reprecipitation resulting in the development of foliated eclogites. Finally, lack of chemical zoning in minerals suggests that formation and deformation of the investigated eclogites occurred under stable P-T-fluid conditions. This study emphasizes that the planar and linear fabric of eclogites might not always be directly related to eclogite facies shear zones.

How to cite: Rogowitz, A. and Huet, B.: Fluid assisted formation and deformation of eclogites - dislocation vs. dissolution-reprecipitation creep, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13093, https://doi.org/10.5194/egusphere-egu2020-13093, 2020.

Antigorite is a key constituent of subducted slabs, and its dehydration is thought to be responsible for the generation of intermediate-depth earthquakes. The mechanical behaviour of antigorite at elevated pressure and temperature remains difficult to constrain experimentally: intracrystalline slip systems are hard to activate under typical laboratory timescales and microstructures do not always provide unambiguous evidence for dislocation creep. Here, we present recent laboratory data showing that antigorite might deform due to intracrystalline frictional slip and delamination, at least in the low temperature regime (<400°C). This behaviour is typical of the semi-brittle regime. Based on a time-independent rheology including friction and potential compaction at elevated pressure, we formulate a model for coupled deformation and dehydration of antigorite. We show that a pore pressure and compaction localisation instability can develop when the net volume change associated with the reaction is negative, i.e., at intermediate depth in subduction zones. Unstable compaction and fluid pressure build-up may provide a mechanism for the nucleation of intermediate-depth earthquakes.

How to cite: Brantut, N., David, E., Hansen, L., Hirth, G., Sulem, J., and Stefanou, I.: Antigorite deformation and dehydration-induced compaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16979, https://doi.org/10.5194/egusphere-egu2020-16979, 2020.

Mylonites are ubiquitous structural features of dynamic plate boundaries, and are widely assumed to represent the product of localized deformation at high pressure and temperature. There are two features of mylonites that distinguish them from typical host rocks: grain-sizes that may be reduced by orders of magnitude and mineral phases that generally well-mixed.  Together, these microstructural characteristics are thought to promote rheological weakening over long geologic intervals, an essential feature of Earth-like plate tectonics.  In this contribution we describe experiments that seek to reproduce deformation processes and resulting microstructures that occur during mylonitization. Experiments were conducted at high pressure (1-2 GPa) and temperature (500-750 C) on dense synthetic composites of calcite (Ca) and quartz (Qz), anhydrite (An), or fluorite (Fl).  These composites were selected to investigate the influence of viscosity contrast on the phase mixing process. Shear strains of γ > 50 were produced using the Large Volume Torsion Apparatus (LVT) at Washington University in St. Louis.  Ex situ microstructural analysis was performed with optical microscopy, SEM, EBSD, and TEM. Experiments are interpreted to have deformed by either viscoplastic (Ca+Fl and Ca+An) or semi-brittle mechanisms (Ca+Qz). We show that the evolution of the protolith towards recrystallized and well-mixed microstructures occurs over a large range of shear strains. The critical strain depends on the mechanism of mixing, the viscosity contrast between the two phases, and the microstructure of the starting material. Phase mixing is determined to be the product of several independent mechanisms, the relative importance of which depends on pressure, stress, strain, composition, viscosity contrast, and the ratio of the initial grain-size to the recrystallized grain size.

How to cite: Skemer, P., Bollinger, C., Cross, A., and Couvy, H.: Experimental constraints on mylonite formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2672, https://doi.org/10.5194/egusphere-egu2020-2672, 2020.

Based on experimental observations, there have been claims that deviatoric stresses may trigger high pressure phase transitions below their equilibrium transition pressures. This implies that the phase assemblages observed in exhumed rocks may reflect stresses induced by tectonic overpressure rather than mere lithostatic pressure, thus resulting in overestimated maximum depths of burial. Despite the numerous studies that have addressed whether mean or principal stress may trigger polymorphic phase changes, the case is still not completely clear. The aim of this study is therefore to investigate the role of deviatoric stress on phase transitions at high PT conditions. In this study, we investigated the α-β transition of quartz, which is one of the most common mineral of the Earth’s crust. This transition has a particular importance for the lower continental crust because of the significantly different elastic properties of the two polymorphs. The α-β quartz transition is also a good experimental candidate because of its displacive and quasi-instantaneous nature.

A series of experiments was performed with a new high pressure Griggs-type apparatus equipped with ultrasonic monitoring, at the ENS Paris. Cored rock samples of Arkansas Novaculite (mean grain size of 5.6 mm) were subjected to pressure and temperature conditions of 0.5-1.5 GPa and ~ 850 °C. The deviatoric stress was increased to cross the transition while keeping the temperature constant. Two p-wave transducers were used on top and bottom of the assembly as transmitter and receiver to measure travel times across the assembly. The quartz a-b transition was directly observed by a time-shift of the p-wave arrival in the order of 10 ns. The mechanical data clearly show that the phase transformation is controlled by mean stress. The quartz α-β transition induces a softening behavior on our sample because of the volume change induced by the reaction. According to the elastic properties of α and β quartz, the variation of p wave velocity for the quartz α-β transition is in the order of 10 %. The present active monitoring method allowed us to detect variations smaller than 5 %, which can be explained by a partial transformation due to local stress heterogeneities in the sample, since microscopic stress at the grain scale can be different than the macroscopic stress that we measure.

How to cite: Moarefvand, A., Gasc, J., Fauconnier, J., Deldicque, D., Labrousse, L., and Schubnel, A.: Experimental study of the effect of stress on α → β quartz transformation at lower continental crust pressure and temperature conditions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15125, https://doi.org/10.5194/egusphere-egu2020-15125, 2020.

It is generally known that creep deformation of the rocks, occurring in the Earth under high stress level, influences the fluid flow, as well as other processes related to the strain accumulations. Strain localization across multiple scales is a complex process in any tectonic environment, and is still poorly understood. Because of some technical complications, the majority of laboratory researchers prefer to make a rock testing under deformation control mode, rather than under stress control mode. Three-day multistage loading testing of the mudstone/shale sample collected from the Barents See was conducted in Skoltech in the frame of international project. The loading of the sample was done under 20 MPa confining pressure as a series of consecutive 20 MPa axial stress-steps. After each step, the axial load was kept constant for at least 3 hours’ time interval to study the creeping of the sample, while the monitoring of axial and radial strain allowed to calculate the rock viscosity.
In addition, sixteen Acoustic Emission (AE) sensors were glued to the cylindrical surface of the rock. They were used as well for localization of microcracking within the rock, as for periodical measurement of P-wave velocities along different directions. During the early stage of the rock loading, all velocities demonstrated initial increase related to the compaction of the rock. However, after application of approximately 50% of the maximal axial stress, a strong heterogeneity of P-wave velocity within the rock was recorded, and the decrease of the velocities along some traces indicated occurrence of local dilatancy of the sample. The results of these observations are well correlated with the beginning of AE clustering in the fracture nucleation zone, and both processes were detected during the secondary, steady-state stage of the creep. It was found that the location of AE nucleation zone correlates well with the position of natural crack detected in the sample before the testing by 3D CT X-Ray scanner. 
Macroscopic failure of the sample occurred approximately two minutes after the application of the final stress-step equal to 280 MPa. Analysis of AE signals shows close correlation between the onset of macroscopic fault acceleration, accompanied by significant increase of AE signal amplitudes, and the beginning of tertiary creep stage, detected approximately 25 seconds before the final failure of the sample. Preexisted in the sample natural crack could be considered as healed natural fault, and during our test, we studied activation and creeping of this fault during the stressing of the sample up to the failure, causing observed changes of P-wave velocities, clustering of AE events and variations of rock viscosity.

How to cite: Stanchits, S., Yarushina, V., Sabitova, A., Stukachev, V., and Bobrova, M.: Creep of rock from Barents See, monitored by Acoustic Emissions, Ultrasonic Transmissions and deformation measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10578, https://doi.org/10.5194/egusphere-egu2020-10578, 2020.

Strain localization is a natural deformation process that has been variously attributed to brittle, chemical or geometrical precursors. Despite some theoretical consideration, experimental evidence for temperature softening was lacking. We report thermally-activated strain localization in prismatic samples of homogenous and isotropic glassy polymer. Uniaxial compression was performed at room temperature and at different but constant displacement rates while the temperature was captured with an infrared camera. Results show temperature increase due to viscous heating along planar zones before any rupture along these zones. We validated the experimental results with a thermo-mechanical numerical model. This experimental investigation extrapolated to geological conditions shows that viscous heating can induce strain localization in all levels of deforming lithosphere.

How to cite: Madonna, C., Podladchikov, Y., and Burg, J.-P.: Viscous heating triggers strain localization: Experimental evidence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21705, https://doi.org/10.5194/egusphere-egu2020-21705, 2020.

Key Words: numerical modelling, elasto-plastic analytical solutions, shear bands, geomechanics.

The correct analysis of wellbore stability in unconventional reservoirs receives much interest from the industry as shale rock and tar sands demonstrate perceptible plastic behavior which influences the estimation of rock failure. To tackle this problem the 3D finite element code has been developed for computing the stress-strain state in the elastoplastic medium near a borehole. The accuracy of the results, obtained due to the application of the finite element technique, can be affected by various numerical effects. Since the theory of plasticity assumes infinitesimal load increments, errors associated with finite increments are almost inevitable. The accuracy of the numerical solution can be verified by comparing the numerical results with the analytical solutions. Elasto-plastic analytical solutions [1], [2] stand out among others because they are the only ones among many others, mentioned in the cited monographs, that consider analytical solutions under conditions of non-hydrostatic loading.

In this study, the numerical and analytical solutions were verified and relative errors were calculated for different loading paths. It turned out, for example, that Galin’s analytical solution works well not only in the field of its applicability, but also outside of it, despite different errors. This work discusses questions related to the influence of the increment of the applied load on the structure of a stationary elasto-plastic solution, including in the case of the formation of zones of localized plastic deformation. The issue of the appearance of shear bands zones is also considered: these bands develop directly around the hole under certain boundary conditions or gradually grow out of the zones of elliptical plastic deformation.

The first, third and fifth authors acknowledge support of research by Geosteering technologies company within the scope of Geonaft project sponsored by Skolkovo foundation, Russia.

The second and fourth authors acknowledge support of research by Government of Russian Federation under grant 2019-220-07-9139.

REFERENCES

[1] Detournay, E. (1986). An approximate statical solution of the elastoplastic interface for the problem of Galin with a cohesive-frictional material. International Journal of Solids and Structures, 22(12), 1435–1454.

[2] Galin, L.A. (1946). Plane elastoplastic problem. Applied Mathematics and Mechanics, 10 (3), 365–386.

How to cite: Grishko, E., Myasnikov, A., Sabitov, D., Podladchikov, Y., and Garavand, A.: The applicability of analytical elasto-plastic solutions and issues of the formation of shear bands zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14810, https://doi.org/10.5194/egusphere-egu2020-14810, 2020.

KeyWords: FEM, Legandre element, plasticity, localization, shear band

The phenomenon of strain localization is widespread and can reveal both during the geodynamic sliding

of plates at macro scale length and at scales, character to a wells and mining. Herein we propose

accurate way to solve problems based on the spectral Legendre element with incremental formulation,

elastoplastic deformations, a consistent linearized matrix for governing relations. Two models of materials

are taken into account: the Drucker-Prager (pressure dependent) model and the Mises (pressure

insensitive) model. This report presents a qualitative and quantitative analysis of the kinematic pattern of

the lines of plastic deformations at different characteristic scales and types of stress states. It is shown for

general case pressure dependent Drucker-Prager model, in contrast to Mises model, solution can not possess

symmetric and continuous values: both radial and hoop stresses in the case of thick-walled cylinder

under compression can have periodic symmetry, but are discontinuous along the thickness.

How to cite: Krapivin, K.: Fully Lagrangian Method For Shear Band Capturing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20886, https://doi.org/10.5194/egusphere-egu2020-20886, 2020.

Strain localization problems, i.e., shearbandings, have received a lot of interest, especially when strain softening is disregarded from the elasto-plastic consistution relationship. Indeed, reproducing correctly oriented shear bands without softening allows to overcome the mesh-depenency problem. Our work focuses on a Material Point Method (MPM) implementation of strain localization to i) study the behavior of shear bands in order to ii) assess the capabilities of this quite recent numerical method.

To study strain localization and shear banding, we developped an efficient numercial Material Point Method (MPM) solver in Matlab, based on the Update Stress Last (USL) scheme enriched with the Generalized Interpolation Material Point (GIMP) variant, which fixes a major flaw of any MPM solver: the cell-crossing error due to discontinuous gradient of the basis functions. This home-made solver allows us to study strain localizations in either a fixed or continuously deforming continuum. The algorithm solves explicitly momentum equations in an updated lagrangian manner similarly to an explicit FEM solver. We therefore investigate the compression of an elasto-plastic domain under pure shear condition, thus reproducing the geometrical settings and pure shear conditions used in Duretz et al. (2018). Strain softening is disregarded since we do not want any mesh dependence within the solver. A Mohr-Coulomb yield criterion was selected and plasticity was computed by a return mapping algorithm, i.e., we did not use consistent tangent operator. Localization is triggered by a weaker circular inculsion in the center of the domain..

Preliminary results demonstrates the suitability of the MPM solver to reproduce the correct shearbanding behavior under compression, for both static and dynamic meshes. The higher the resolution, the more accurate are the shear bands. Naturally, this implies future implementations of the solver in a GPU-accelerated environment to increase the numerical resolution.

How to cite: Guerin, A., Wyser, E., Podladchikov, Y., and Jaboyedoff, M.: Fast and efficient MPM solver for strain localization problems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18464, https://doi.org/10.5194/egusphere-egu2020-18464, 2020.

The classification of the strain localization modes is attempted around brittle-ductile transition. The stresses are high. The are a number of suspects: earthquake-like thermal runaway (Braeck et al. 2009), stable sliding as shear heating zones oriented 45 degrees to the principal stresses (Kiss et al. 2019), brittle faults/shear bands oriented ca. 30 degrees to the maximum compressive principal stress and mode 1 fracture. The coupling to the porous fluid hydrology is accounted for.  High resolution numerical simulations are compared to classical and newly derived composite asymptotic solutions.

References

Braeck, S., Podladchikov, Y., & Medvedev, S., 2009. Spontaneous dissipation of elastic energy by self-localizing thermal runaway, Phys. Rev. E , 80, 046105, doi:10.1103/PhysRevE.80.046105.

Kiss, D., Podladchikov, Y., Duretz, T., & Schmalholz, S., 2019. Spontaneous generation of ductile

shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the

lithosphere, Earth Planet. Sci. Lett., 519, 284–296, doi:10.1016/j.epsl.2019.05.026.

How to cite: Podladchikov, Y.: On brittle-ductile strain localization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11843, https://doi.org/10.5194/egusphere-egu2020-11843, 2020.

The rheology/mechanical behavior of rock is controlled by several processes including thermal, hydraulic, mechanical, and chemical conditions. (Braeck et al., 2009; Kiss et al., 2019)

We conduct a systematic parametric study within a fully coupled Thermo-Hydro-Mechanical-Chemical (THMC) numerical rheological model to identify regions of stable and unstable (brittle?) deformation. The rheological model assumes incompressible viscous deformation and is governed by the equations of conservation of mass, linear momentum, and energy; a constitutive equation, and a creep flow law. Three parameters control the deformation: background strain rate, shear heating, and a Brinkman number that captures the interplay between viscosity and temperature.

We setup a grid of points using these parameters, use each grid point as a starting instance of the
rheological model, and let each instance evolve with time. We are able to perform a fine-grained study of the parameter space by using a high-performance GPU cluster. Our initial results show that the background strain rate requires relatively low values (near 1) for the computation to remain stable. While keeping a constant (low) strain rate, we next observe how each model instance evolves with respect to shear heating and Brinkman values. This approach allow us to map stable/unstable regions in the 3-parameter space. 

Next we analyze the rheological conditions of each model instance (in the stable and unstable regions) and its potential as a rock-weakening mechanism.

References
Braeck, S., Podladchikov, Y., & Medvedev, S., 2009. Spontaneous dissipation of elastic energy by self-localizing thermal runaway, Phys. Rev. E , 80, 046105, doi:10.1103/PhysRevE.80.046105.

Kiss, D., Podladchikov, Y., Duretz, T., & Schmalholz, S., 2019. Spontaneous generation of ductile shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the lithosphere, Earth Planet. Sci. Lett., 519, 284–296, doi:10.1016/j.epsl.2019.05.026.

How to cite: Alvizuri, C. and Podladchikov, Y.: Results from a systematic analysis of fully coupled Thermo-Hydro-Mechanical-Chemical rock models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12896, https://doi.org/10.5194/egusphere-egu2020-12896, 2020.

We present a 3D numerical model for hydraulic fracturing and damage of low permeable rock in an anisotropic stress field. The 3D numerical model computes the intermittent damage propagation, microseismic event-locations, microseismic event-distribution, damaged rock volume, and injection pressure. The model builds on concepts from invasion percolation theory, where cells in a regular grid are connected by transmissibilities, also called bonds. A numerical pressure solution provides the pressure in each cell at each time step during the hydraulic fracturing operation. The numerical solution is based on a cell-centered finite volume scheme. A fast version of the numerical scheme is suggested by restricting fluid flow to the damaged rock volume. The hydraulic fracture and the damaged rock volume propagate by one cell when a bond breaks. An intact bond breaks when the fluid pressure exceeds the least compressive stress and a random uniformly distributed bond strength. The model is different from a pure invasion percolation model by using the fluid pressure in combination with a random bond strength to decide which bond to break, instead of only the random strength. The volume of damaged rock is estimated with a simple expression for cases with high permeability of the damaged rock volume. The model is tested with a published case from the Barnett Shale. It reproduces the observed main features of the Barnett case, such as the spatial and temporal distribution of the events, the magnitude – frequency distribution and the injection pressure. It is found that the microseismic event-distribution and the b-value depend on the permeability of the damaged rock volume. The b-value increases with decreasing permeability from 0.6 to a value above 2 for the maximum possible permeabilities. The damaged rock volume is non-compact and similar to a percolation cluster for ‘‘high’’ damaged rock permeabilities, and it becomes increasingly compact with decreasing permeabilities. The resulting loop-less fracture network is found to have similar characteristics for different damaged rock permeabilities.

How to cite: Wangen, M.: A 3D numerical model of hydraulic fracturing, injection pressure and microseismicity in anisotropic stress fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5006, https://doi.org/10.5194/egusphere-egu2020-5006, 2020.

Elastodynamic hydro-mechanical coupling based on Biot’s theory describes an upscaling of the fluid-solid deformation at a porous scale. Examples of applications of this theory are near surface geophysics, CO2 monitoring, induced seismicity, etc. The dynamic response of a coupled hydro-mechanical system can produce fast and slow compressional waves and shear waves. In many earth materials, a propagating slow wave degenerates into a slow diffusion mode on orders of magnitude larger time scales compared to wave propagation. In the present work, we propose a new approach to accelerate the numerical simulation of slow diffusion processes. We solve the coupled Biot elastodynamic hydro-mechanical equations for particle velocity and stress in the time domain using the finite volume method on a rectangular grid in three dimensions. The MPI-based multi-GPU code is implemented using CUDA-C programming language. We prescribe a fluid injection at the center of the model that generates a fast propagating wave and a significantly slower fluid-diffusion event. The fast wave is attenuated due to absorbing boundary conditions after what the slow fluid-diffusion process remains active. A Courant stability condition for the fast wave controls the time-step in the entire simulation, resulting in a suboptimal short time step for the diffusion process. Once fast waves are no longer present in the model domain, the hydro-mechanical coupling vanishes in the inertial terms allowing for an order of magnitude larger time steps. We accelerate the numerical simulation of slow diffusion processes using a pseudo-transient method that permits to capture the transition in time step restrictions. This latest development enables us to simulate quasi-static and dynamic responses of two-phase media. We present benchmarks confirming the numerical efficiency and accuracy of the novel approach. The further development of the code will capture inelastic physics starting from the dynamic events (earthquake modeling) to quasi-static faulting.

How to cite: Alkhimenkov, Y., Khakimova, L., Raess, L., Quintal, B., and Podladchikov, Y.: GPU-based solution of Biot’s elastodynamic equations to account for fluid pressure diffusion , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10287, https://doi.org/10.5194/egusphere-egu2020-10287, 2020.

Fluid flow instability in deforming porous rock, commonly known as porosity waves, has been used to explain formation of seismic chimneys, one of the most important expressions for the localized fluid flow in the subsurface. Experiments show that volumetric deformation of rocks is strongly coupled with shear deformation, leading to shear-induced decompaction at low confining pressure and shear-enhanced compaction at higher confining pressure. Previous studies introduce a weakening factor of R for bulk viscosity in the viscous deforming regime. While it has successfully reproduced the channelized fluid flow in numerical models, it cannot investigate the effect of shear deformation. More controversially, negative effective pressure (P_t-P_f) is required for the channel formation. Here, we develop a viscoplastic rheology that takes into account effects of shear stress and plastic failure on the volumetric deformation, consistent with experimental data. A dilation pressure is naturally introduced through viscoplastic strain-rate when plastic failure occurs under high fluid pressure and shear stress condition. Our model results show that this new rheology can produce channelized fluid flow without negative effective pressure in the model.

In order to apply our models into real geological setting, we test the effects of reservoir properties, geological layering, transport properties of the layers and faults.  Our results show that fluid channel initiates at local topography highs in the reservoir and a high-permeability fault can also trigger the initiation of fluid channels. Fluid channels can have different length and time scales in different layers, depending on bulk viscosity and permeability of the layers.

How to cite: Wang, H., Yarushina, V., and Podladchikov, Y.: Modelling channelized fluid flow: failure physics and geological setting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13829, https://doi.org/10.5194/egusphere-egu2020-13829, 2020.

Developing new numerical reactive transport models is essential for predicting and describing natural and technogenic petroleum and geological processes at different scales. Examples of such processes are pore fluid migration in subduction zones, causing seismic and volcanic activity, chemical and thermal enhanced oil recovery activities, etc. New numerical reactive transport models must be validated against analytical or semi-analytical solutions to ensure its correct numerical implementation. In this study, we construct thermo-hydro-chemo-mechanical model which takes into account multi-phase fluid flow in porous matrix associated with inter- and intra-phase chemical reactions with significant temperature and volume effect and treats porosity and permeability evolution. All equations are derived from basic principles of conservation of mass, energy, and momentum and the thermodynamic admissibility of all equations is verified. We solve the proposed system of equations both with a finite difference approach on a staggered grid and characteristic-based Lax-Friedrichs different order schemes to treat the disintegration of discontinuities.  Resolving the problem of large discrepancies during the time evolution of coupled physical processes is challenging. For that, we use pseudo-iterations which force slow modes to attenuate quickly. Furthermore, we perform dimensionless analysis of the proposed model which allows us to detect proper dimensionally independent, dimensionally dependent and non-dimensional parameters. A new semi-analytical is derived which is based on a relaxation method of defining the stationary solution of system of partial differential equations, so detection specific regimes for reaction front propagation are possible. As a result, reaction front velocity dependence on Peclet, Damkohler and Lewis nondimensional parameters is obtained.

How to cite: Khakimova, L., Alkhimenkov, Y., Cheremisin, A., and Podladchikov, Y.: Modelling of nonlinear processes in deforming and reacting porous saturated rocks: different regimes for reaction front propagation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19684, https://doi.org/10.5194/egusphere-egu2020-19684, 2020.

Short timescale processes such as earthquakes, tremors and slow slip events may be influenced by reactions, which are known to proceed rapidly in the presence of water (typically several days). Here, we developed a theoretical framework to introduce the influence of mineralogical reactions on fluid flow and deformation. The classical formalism for dissolution/precipitation reactions is used to consider the influence of the distance from equilibrium and of temperature on the reaction rate and a dependence on porosity is introduced to model the evolution of the reacting surface area during reaction. The thermodynamic admissibility of the derived equations is checked and an analytical solution is derived to test the model. The fitting of experimental data for three reactions typically occurring in metamorphic systems (serpentine dehydration, muscovite dehydration and calcite decarbonation) indicates a systematic faster kinetics on the dehydration side than on the hydration side close from equilibrium. This effect is amplified through the porosity term in the reaction rate. Numerical modelling indicates that this difference in reaction rate close from equilibrium plays a key role in microtextures formation during dehydration in metamorphic systems. The developed model can be used in a wide variety of geological systems where couplings between reaction, deformation and fluid flow have to be considered.

How to cite: Malvoisin, B. and Podladchikov, Y. Y.: Role of kinetics on the couplings between fluid flow, deformation and reaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20626, https://doi.org/10.5194/egusphere-egu2020-20626, 2020.

The Chilean margin is amongst the most active seismic and volcanic areas on Earth. It hosts active and fossil geothermal and mineralized systems of economic interest documenting significant geofluid migration through the crust. By comparing numerical models with field and geophysical data, we aim at pinning when and where fluid migration occurs through porous domains, fault zone conduits, or remains stored at depth awaiting a more appropriate stress field. Dyking and volcanic activity occur within fault zones along the SAVZ, linked with stress field variations in spatial and temporal association with –short therm- seismicity and -long term- oblique plate convergence. Volcanoes and geothermal domains are mostly located along or at the intersection of margin-oblique fault zones (Andean Transverse Faults), and along margin-parallel strike slip zones, some which may cut the entire lithosphere (Liquiñe-Ofqui fault system). Whereas the big picture displays fluid flow straight to the surface, at close look significant offsets between crustal structures occur. 3D numerical models using conventional elasto-plastic rheology provide insights on the interaction of (i) an inflating magmatic cavity, (ii) a slipping fault zone, and (iii) regional tectonic stresses. Applying either (i) a magmatic overpressure or (ii) a given fault slip can trigger failure of the intervening rock, and generate either i) fault motion or ii) magmatic reservoir failure, respectively, but only for distances less than the structures' breadth even at low rock strength. However, at greater inter-distances the bedrock domain in between the fault zone and the magmatic cavity undergoes dilatational strain of the order of 1-5x10-5. This dilation opens the bedrock’s pore space and forms «pocket domains» that may store up-flowing over-pressurized fluids, which may then further chemically interact with the bedrock, for the length of time that these pockets remain open. These porous pockets can reach kilometric size, questioning their parental link with outcropping plutons along the margin. Moreover, bedrock permeability may also increase as fluid flow diminishes effective bedrock friction and cohesion. Comparison with rock experiments indicates that such stress and fluid pressure changes may eventually trigger failure at the intermediate timescale (repeated slip or repeated inflation). Finally, incorporating far field compression (iii) loads the bedrock to a state of stress at the verge of failure. Then, failure around the magmatic reservoir or at the fault zone occurs for lower loading. Permanent tectonic loading on the one hand, far field episodic seismic inversion of the stress field on the other, and localized failure all together promote a transient stress field, thus explaining the occurrence of transient fluid pathways on seemingly independent timescales. These synthetic models are then discussed with regards to specific cases along the SVZ, particularly the Tatara-San Pedro area (~36°S), where magnetotelluric profiles document conductive volumes at different depths underneath active faults, volcanic edifices and geothermal vents. We discuss the mechanical link between these deep sources and surface structures.

How to cite: Ruz, J., Gerbault, M., Cembrano, J., Iturrieta, P., Novoa Lizama, C., Hassani, R., and Saez Leiva, F.: Faults and magma reservoirs along the Southern Andes Volcanic zone (SAVZ): linking oservations and numerical models of stress change controlling magmatic and hydrothermal fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11473, https://doi.org/10.5194/egusphere-egu2020-11473, 2020.

The main mechanism of transport of magma in the Earth’s crust is the formation of cracks, or dikes, through which the melt moves towards the surface under the action of buoyancy forces and tectonic stresses. Due to the structural features of the crust or external stress fields, dikes often do not reach the surface, but penetrate the localized region in which the rocks melt, leading to the formation of magmatic chambers, whose volume can exceed thousands of cubic kilometers. We present a model of the formation of a magma chamber during the intrusion of dikes at a given flow rate. The model is based on the solution of heat equation and considers the actual melting diagrams of magma and rocks. It Is shown that, in case of magmatic fluxes typical of island arc volcanoes, magma chambers are formed over hundreds of years from the beginning of magma intrusion. The influence of the magma flow rate, the size of the dikes and their orientation on the volume of the formed magma chamber and its shape was investigated. The size of the chamber significantly exceeds the area of dike intrusion due to the displacement of magma and rocks of the crust, their heating up and melting. To calculate displacement of rock and magma in a numerical simulation, a hybrid method based on PIC/FLIP interpolation is developed, making it possible to avoid unphysical mixing due to numerical dissipation, thus preserving the fine details of the formed magma chamber.

This work was supported by RFBR, project number 18-01-00352

How to cite: Utkin, I. and Melnik, O.: Magma chamber formation by magma intrusion into the Earth's crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9566, https://doi.org/10.5194/egusphere-egu2020-9566, 2020.

A recent focus of studies in geodynamic modeling and magmatic petrology is to understand the coupled behavior between deformation and magmatic processes. Here, we present a 2D numerical model of an upper crustal magma (or mush) chamber in a visco-elastic host rock, with coupled thermal, mechanical and chemical (TMC) processes. The magma chamber is isolated from deeper sources of magma and it is cooling, and thus shrinking. We quantify the mechanical interaction between the shrinking magma chamber and the surrounding host rock, using a compressible visco-elastic formulation, considering several geometries of the magma chamber.

We present a self-consistent system of the conservation equations for coupled TMC processes, under the assumptions of slow (negligible inertial forces), visco-elastic deformation and constant chemical bulk composition. The thermodynamic melting/crystallization model is based on a pelitic melting model calculated with Perple_X, assuming a granitic composition and is incorporated as a look-up table. We will discuss the numerical implementation, show the results of systematic numerical simulations, and illustrate the effect of volume changes due to crystallization on stresses in the host rocks.

How to cite: Kiss, D., Moulas, E., Rummel, L., and Kaus, B.: 2D thermo-mechanical-chemical coupled numerical models of interactions between a cooling magma chamber and a visco-elastic host rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9853, https://doi.org/10.5194/egusphere-egu2020-9853, 2020.

The formation of alkaline magmas observed worldwide requires that low degree-melts, potentially formed in the asthenosphere, were able to cross the overlying lithosphere. Fracturing in the upper, brittle part of the lithosphere may help to extract this melt to the surface. However, the mechanism of extraction in the lower, ductile part of the lithosphere is still contentious. Metasomatic enrichment of the lithospheric mantle demonstrates that such low-degree melts interact with the lithosphere, but the physical aspect of this process remains unclear. The aim of this study is to better understand the percolation of magma in a porous viscous medium at pressure (P) and temperature (T) conditions relevant for the base of the lithosphere. We study such melt percolation numerically with a Thermo-Hydro-Chemical model of reactive transport coupled with thermodynamic data obtained via Gibbs energy minimisation. We perform Gibbs energy minimisation with Matlab using the linprog algorithm. We start with a simple ternary system of Forsterite/Fayalite/Enstatite solids and melts. All variables are a function of T, P and composition of the system (C), and are computed in both the Gibbs energy minimisation and in the reactive transport code, and can therefore vary freely.

How to cite: Bessat, A., Pilet, S., Schmalholz, S. M., and Podladchikov, Y.: Coupling between Thermo-Hydro-Chemical reactive transport and Gibbs minimisation: magma evolution in evolving multiphase porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13322, https://doi.org/10.5194/egusphere-egu2020-13322, 2020.

Pressure-temperature paths are a major tool for tectonic reconstruction as proxies of the burial and exhumation history of the rocks during subduction-exhumation phases. The mineral assemblages are commonly considered to reflect lithostatic pressure and near-equilibrium regional geothermal gradients. These axioms ground on the assumptions that the rock cannot support high differential stress in one place, and that heat diffusion in rocks is fast enough to defocus localized thermal anomalies, respectively.

The rare but systematic occurrence, in actual mountain ranges, of ultrahigh-pressure and/or high-temperature rocks within lower grade metamorphic rocks rise a major challenge for developing a consistent geodynamic model for exhumation of such deep seated rocks. Subduction zones are, in fact, efficient player driving material from the surface down into the Earth's mantle. However, the mechanisms to exhume part of this material (and particularly the denser oceanic rocks) back to the shallow crust are still highly debated.

In this contribution, we present new structural, petrological and thermochronometric data from an exhumed subduction zone - the Cima di Gagnone in the Central Alps– where small ultramafic inclusions (peridotite) preserving high temperature and high pressure record are enveloped within amphibolite-facies gneisses, defining a classical inclusion-in-matrix system. We found evidence of heterogeneous metamorphic and temperature records in both peridotite and felsic rocks, being the gneisses generally characterized by much lower pressure. However, we detect also in the matrix gneiss close to peridotite inclusions high-pressure and high-temperature remnants, which are structurally and temporally associated with those of ultramafic bodies.

The coexistence, at the outcrop scale, of such different conditions implies either extreme mechanical decoupling or extremely variable metamorphic equilibrium during Alpine subduction and exhumation. A possible alternative explanation is to consider part of the metamorphic record as due to mechanical deviations from lithostatic pressure and equilibrium temperature. We compare the observed metamorphic pattern with the outcome of numerical simulations obtained from elasto-visco-plastic 2D Finite Difference models. The evolution of rocks strength and viscosity is furthermore monitored to control the effectiveness of physical conditions simulated with the analytical dataset. Finally, we discuss a possible positive feedback of tectonic stress on the development of apparently incompatible metamorphic patterns.

How to cite: Maino, M., Casini, L., Corvò, S., Langone, A., Schenker, F., and Seno, S.: Evidence of deformation control on the P-T record in compositionally heterogeneous shear zone during subduction-exhumation cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6637, https://doi.org/10.5194/egusphere-egu2020-6637, 2020.

Metamorphic differentiation, resulting in the segregation of minerals into compositional bands, is a common feature of metamorphic rocks. Considering the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation have received very little attention. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of an anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone rock, defined by the interlayering of amphibolite with leucocratic domains. Detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variation with assemblage microstructure. Within the outcrop, quartz-filled fractures and their associated amphibolite alteration haloes, are observed crosscutting the granulite. These fractures are demonstrated to be relict of the initial fluid infiltration event. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is equivalent to that occurring locally within leucocratic domains of the shear zone. Due to the textural and compositional similarities between quartz-filled fractures and leucocratic domains, the compositional layering of the shear zone rock may be directly linked to fracturing during initial fluid infiltration.

            Mass-balance and thermodynamic calculations indicate quartz-filled fractures and compositional differentiation of the shear zone form by internal fractionation rather than partial melting or precipitation of minerals from an eternally derived fluid. The process of internal fractionation within the shear zone is attributed to enhanced dissolution along fracture pathways, resulting in the loss of MgO, Fe₂O₃ and K₂O within leucocratic domains. These elements, being more mobile in the fluid, are then transported and ultimately either precipitated in amphibolite lithologies or escape with the fluid, resulting in an overall volume loss in the shear zone. This inferred fluid connectivity combined with the enhanced local dissolution indicates the presence of a continuously replenished fluid along fracture pathways, leading to the overall conclusion that the mass transfer processes that result in metamorphic differentiation of the shear zone lithologies are dependent on both continuous fluid flux and heterogeneous strain distribution.

How to cite: Moore, J., Beinlich, A., Piazolo, S., Austrheim, H., and Putnis, A.: Metamorphic differentiation via enhanced dissolution along high strain pathways, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12939, https://doi.org/10.5194/egusphere-egu2020-12939, 2020.

Tectonic stresses at the base of decollement thrusts are generally expected to be low due to the presence of mechanically weak evaporites. Yet, the presence of abundant micro-seismicity in the region expected to correspond to the evaporitic layer remains paradoxical. We study here a fossil thrust zone from the base of the Digne nappe (SE France) where exotic thrust slices formed by brecciated Paleozoic basement micaschists are observed within the Mio-Pliocene decollement. Petrographic investigations reveal the presence of highly-substituted phengitic rims (up to Si=3.43 apfu) around pre-alpine muscovitic cores. Similar micaschists sampled in a basement high further North do not exhibit these phengitic rims around muscovite, thus suggesting that white mica zoning relates to a younger overprint. Such high-Silica phengites are commonly found in high-pressure terranes (i.e. 7-15 kbars depending on the buffering assemblage) but are not expected in foreland regions, such as in the Digne area where the overburden has never been thicker than c.5km (i.e. approximately 1.3 kbar). We propose that the mica zoning observed reflects the former presence of non-lithostatic stresses (possibly on the order of several kilobars) related to the elastic charging of a thrust slice “squeezed” at the base of the moving nappe. This finding sheds light on stress distribution as well as on the origin of micro-seismicity along active decollement thrusts in orogenic belts.

How to cite: Häkkinen, M., Angiboust, S., Dubacq, B., and Simoes, M.: Squeezed Under the Sheet: White Mica Records High Tectonic Stresses Within a Decollement Thrust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9363, https://doi.org/10.5194/egusphere-egu2020-9363, 2020.

During collisional orogeny, the lower continental plate is typically subjected to pressures no greater than 3 GPa (~100 km). Locally, however, ultrahigh-pressures (UHP) in excess of 5 GPa have been recorded, most commonly in included metamorphosed mafic-ultramafic rocks. Such pressures would suggest burial of continental crust to mantle depths; however, continental subduction to such depths is not observed in active orogens as it is hindered by the positive buoyancy of sialic crust relative to the mantle. An alternative explanation for extreme pressures recorded in continental crust is that they reflect non-lithostatic conditions, an idea that has been limited to modelling experiments and thus its applicability to natural systems is highly debated. Specifically, it was proposed that mechanical heterogeneities could explain extreme non-lithostatic pressures of c. 5.5 GPa obtained in enstatite eclogite veins cross-cutting a peridotite hosted in the archetypal subducted continental terrane, the Western Gneiss Complex (WGC) in Norway. Here, we use thermobarometry and Lu-Hf garnet geochronology to determine at what conditions and at what point in the burial cycle the enstatite eclogite assemblages actually equilibrated. The results show that the enstatite eclogites equilibrated at pressures of 4-5.5 GPa and at c. 393 Ma; these conditions are greater than those typical of ‘normal’ eclogites in the WGC and the age represents a time when the terrane had already exhumed to crustal depths (<2.5 GPa). Finite element modeling of mechanical pressure distribution can explain the seemingly spurious conditions recorded in these unusual rocks and demonstrates that these late extreme pressure excursions are feasible for the given rock system. Although the occurrence of non-lithostatic UHP conditions in deeply buried continental crust may, indeed, be unusual, it allows crucial simplification of models for continental subduction and validates the importance of integrating rock thermo-mechanics with geochronology and thermobarometry in interpreting observations from collision zones.

How to cite: Cutts, J., Smit, M., and Vrijmoed, J.: Non-lithostatic eclogitization in exhuming continental crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12588, https://doi.org/10.5194/egusphere-egu2020-12588, 2020.

Rheological properties of continental lithosphere are key controls on the behavior of continental deformation. Using thermal structure, constrained by surface heat flow data and measured thermal properties of rocks, the present study calculates different thermo-rheological structure scenarios for the ocean–continent transition (OCT) at the northern margin of the South China Sea, using two different models: a conventional model, taking into account frictional sliding and power-law creep, and a model that additionally includes a high-pressure brittle-fracture mechanism. Two compositions of the lower part of the lithosphere are considered: a soft case with felsic granulite lower crust and wet peridotite lithospheric mantle, and a hard case with mafic granulite lower crust and dry peridotite lithospheric mantle. The former scenario shows a major rheological change from a “jelly sandwich” to a “Christmas tree” type of rheology from north to south along the margin. This complex rheological structure explains lateral changes in earthquake distribution and geometries of extensional faults of the OCT at the northern margin of the South China Sea. Further, our analyses indicate that the initial lithospheric rheology profile probably has only one ductile layer in the lower part of upper crust. Such an initial lithospheric rheology model predicts focused extension to form asymmetric margins, which is the case for the SCS.

Keywords: Ocean-continent transition; Crustal strength; Thermo-rheology; South China Sea; Pearl River Mouth basin

How to cite: Hu, J., Tian, Y., Long, Z., Hu, D., Huang, Y., Wang, Y., and Hu, S.: Thermo-rheological structure of the northern margin of the South China Sea: structural and geodynamic implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22508, https://doi.org/10.5194/egusphere-egu2020-22508, 2020.

A physical property that is important for understanding the geodynamics of Earth’s lithosphere and asthenosphere is the effective viscosity η_eff (the ratio of stress and strain rate). This is particularly important for accurate Glacial Isostatic Adjustment (GIA) calculations, which are increasingly crucial for estimating ice loss and sea level rise from the Greenland and Antarctic ice sheets. Mantle viscosity cannot be measured directly, but can be inferred from strain rate, for example as observed by ground uplift following deglaciation or a seismic event. Empirically, mantle strain rate is mainly controlled by stress, temperature, grain size, and composition (water content and partial melt). The influence of these controlling parameters can be inferred from geophysical observations such as seismic and magnetotelluric (MT) measurements, which are useful for imaging the subsurface of the Earth but do not directly constrain viscosity. These observations can be used to improve constraints on viscosity using a three-step conversion process: (1) constrain temperature from MT, seismic, and other data; (2) constrain compositional structure from MT and seismic data (water content of nominally anhydrous minerals from MT, partial melt content from MT and seismics); and finally, (3) convert the calculated thermal and compositional structures into a constrained viscosity structure. In each conversion process, we can assess and quantify the involved uncertainties. Furthermore, we determine the dominant deformation regime in order to accurately interpret the sensitivity of viscosity to its controlling parameters. For instance, water content strongly affects viscosity for the dislocation-accommodated grain-boundary sliding (dis-GBS) and dislocation creep regimes, while diffusion creep and dis-GBS are highly sensitive to grain size. Stress and grain size are important parameters for determining where these critical transitions may occur. Although neither MT nor seismic velocity observations place strong constraints on grain size, information about seismic attenuation or tectonic history can potentially provide information about grain–size. Overall, we find that seismic and MT observations together can significantly improve estimates of mantle viscosity, and in particular can place useful constraints on the amplitude of regional variations in mantle viscosity. Such constraints will be particularly useful for studies to estimate the impact of such variations on GIA processes.

How to cite: Ramirez, F., Selway, K., and Conrad, C.: Using magnetotelluric and seismic geophysical observations to infer viscosity for Glacial Isostatic Adjustment calculations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9826, https://doi.org/10.5194/egusphere-egu2020-9826, 2020.

Seismic chimneys have been observed in sediments overlying reservoirs containing different fluids, such as water, hydrocarbons, or CO2. Furthermore, such chimneys have been linked to pockmarks and gas seepages on the seafloor. Visco-plastic models show how these chimneys can form by focused fluid flow through viscous, porous materials. However, the mechanisms that cause fluid flow to focus along such relatively narrow pathways with transiently elevated permeability have not been investigated thoroughly in experiments.

We present analogue experiments carried out in a transparent Hele-Shaw cell, in which a fluid is injected into an aggregate of viscous grains, leading to transient focused fluid flow. Fluid flow is imaged using a digital camera, and our observations are compared to models describing chimney formation.

How to cite: van Noort, R. and Yarushina, V.: Experimental imaging of focused fluid flow through a viscous porous rock-analogue., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8821, https://doi.org/10.5194/egusphere-egu2020-8821, 2020.

Strength profiles through the crust and upper mantle typically show the brittle to viscous transition as a change in deformation mechanism from frictional sliding to crystal plastic (dislocation creep) mechanisms. Even though such a change may conceivably take place, experimental evidence and natural observations indicate that a transition from semi-brittle to diffusion creep mechanisms rather than dislocation creep is more common.

In experiments we have carried out on granitoid and mafic rock material we can distinguish 3 main processes for the brittle to viscous transition: (1) Grain size comminution by cracking produces a sufficiently small grain size (sub-micron) to cause a switch to diffusion creep. (2) Amorphous material forms (aseismically) from mechanical wear at high stresses (high dislocation densities or high work rate) without melting. The amorphous material is observed to be weak and deforms viscously. (3) Nucleation of new minerals as a consequence of metastability of existing minerals at given P,T, fluid-conditions produces fine-grained and well-mixed aggregates causing a switch to diffusion creep as in (1).

The viscously deforming part of the crust or upper mantle is not the region where most earthquakes occur, because low stresses commonly are associated with viscous deformation. However, the transitions observed in experiments described above are transformational processes the material progressively evolves over a period of time in terms of microstructure, grain size, and/or composition, i.e., they are deformation-history-dependent transitions. In other words, during the transformation, only parts of the material deform by viscous processes while others have not evolved and are still brittle (and stronger). The bulk material strength of partially transformed rock depends on the connectivity of the weaker transformed material. The weaker material causes stress concentrations at the tips of transformed zones. The coalescence of transformed zones and/or a sufficiently large amount of transformed material is expected to cause catastrophic failure and thus seismic rupture. In such a way, transformation to viscously deforming weaker material may cause seismic behavior rather than according to the conventional view, where material properties change as a result of seismic deformation first, leading to creep.

How to cite: Stunitz, H., Marti, S., Mansard, N., Pec, M., Raimbourg, H., Précigout, J., and Heilbronner, R.: The brittle-to-viscous transition and its potential relationship to seismic deformation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5522, https://doi.org/10.5194/egusphere-egu2020-5522, 2020.

Abundant observations of field- and micro-structures in marble rocks in both natural and laboratory settings indicate that these rocks have deformed by various combinations of mechanical twinning, dislocation motion, and dilatant fracturing. To better constrain the systematics of this semi-brittle flow, we performed a set of about 80 experiments at eight different temperatures (20°C<T<800°C). At each T, deformation conditions included different confining pressures (50 < P_C <300 MPa) and strain rates (10^-6 < ε’ <10^-4 s^-1). Under almost all these conditions, both the strength (σ) and the hardening coefficient (Θ=∂σ/∂ε) are affected by changes in P_C and ε’, but the functional relationships of σ(P_C, ε’) and Θ(P_C, ε’) are unique. For example, at 20°C, σ is a non-linear function of both P_C and ε’, while Θ depends on P_C alone. In contrast, at 600°C, the dependence of σ on P_C is very weak, and Θ depends on ε’ alone.

At T<650°C (less than half the absolute melting point of calcite), and P_C greater than 50 MPa, the hardening coefficients are substantial (1% or more of the shear modulus), similar to steels and hexagonal metals that deform in a regime called twinning induced plasticity (TWIP). During TWIP, deformation proceeds with “easy” mechanical twinning, combined with dislocation glide on several slip systems whose glide planes are at high angles to the twin plane. In the calcite rocks, depending on conditions, the hardening resulting from twinning may be reduced by dilation and failure owing to brittle processes (at low pressures and temperatures), or by recovery and recrystallization (at higher temperatures or slower strain rates). Thus, both microstructural observations and mechanical deformation data are consistent with the interpretation that the hardening coefficient and strength are determined by the relative partitioning of inelastic strain amongst mechanical twinning, dislocation mechanisms, and dilatant fracturing. One important aspect is the nature of the mechanism that accommodates of discontinuous inelastic strain at the termination of twins at grain boundaries.

How to cite: Rybacki, E., Niu, L., and Evans, B.: Semi-brittle transient creep in Carrara marble: Hardening and Twinning-induced Plasticity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17998, https://doi.org/10.5194/egusphere-egu2020-17998, 2020.

Seismic rupture of the lower continental crust requires a high failure stress, given large lithostatic stresses and potentially strong rheologies. Several mechanisms have been proposed to generate high stresses at depth, including local amplification of stress heterogeneities driven by the geometry and rheological contrast within a shear zone network. High dynamic stresses are additionally associated with the subsequent slip event, driven by propagation of the rupture tips. In the brittle upper crust, fracturing of the damage zone is the typical response to high stress, but in the lower crust, the evolution of combined crystal plastic and brittle deformation may be used to constrain in more detail the stress history of rupture, as well as  additonal parameters of the deformation environment. It is crucial to understand these deep crustal seismic deformation mechanisms both along the fault and in the wall rock, as coseismic damage is an important (and sometimes the only) method of significantly weakening anhydrous and metastable lower crust, whether by grain size reduction or by fluid redistribution.

A detailed study of pyroxene microstructures are used here to characterise the short-term evolution of high stress deformation experienced on the initiation of lower crustal earthquake rupture. These pyroxenes are sampled from the pseudotachylyte-bearing fault planes and damage zones of lower crustal earthquakes linked to local stress amplifications within a viscous shear zone network, recorded in an exhumed granulite-facies section in Lofoten, northern Norway. In orthopyroxene, initial low-temperature plasticity is overtaken by pulverisation-style fragmentation, generating potential pathways for hydration and reaction. In clinopyroxene, low-temperature plasticity remains dominant throughout but the microstructural style changes rapidly through the pre- and co-seismic periods from twinning to undulose extinction and finally the formation of low angle boundaries. We present here an important record of lower crustal short-term stress evolution along seismogenic faults.

How to cite: Campbell, L. and Menegon, L.: Pyroxene low-temperature plasticity and fragmentation as a record of seismic stress evolution in the lower crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7380, https://doi.org/10.5194/egusphere-egu2020-7380, 2020.

Impact rocks often reveal particular structures, e.g. shock-induced amorphization and melting of crystals, that formed due to high stresses during shock metamorphism. This experimental study presents four granulite samples that were deformed in a D-DIA apparatus at 2.5 GPa and 3 GPa and at either 1023 K, 1173 K, or at 995 to 1225 K. During deformation of two samples (2.5 GPa and either 995-1125 K or 1173 K) 82 and 794 acoustic emissions (AEs) were recorded, respectively, whereas less than 10 AEs were recorded while deforming the other two granulite sample (3 GPa and 995-1225 K; 2.5 and 1073 K). Microstructures of the samples that emitted 82 and 794 AEs reveal amorphous patches that are absent in the samples corresponding to the runs in which <10 AEs were recorded, indicating a link between AE-activity and amorphization of plagioclase. The contacts between amorphous patches and host-plagioclase crystals are very sharp and amorphization predominantly occurred along two distinct planes inclined at approx. 45° towards the direction of maximum compression. Surrounding the patches, the hosts show extensive fragmentation. Chemical analyses of the amorphous patches demonstrate an enrichment in potassium and silicon relative to the initial plagioclase chemistry and the growth of euhedral quartz crystals within the patches. Such microstructures were previously found in naturally or experimentally shocked rocks and interpreted as shock melts. The occurrence of structures, revealing striking similarities to shock melts, in experimental samples that underwent embrittlement at high-pressure, high-temperature conditions below the sample’s solidus (~1377 K) suggests melting due to elevated transient stresses, e.g. during rupture processes.

How to cite: Incel, S.: Stress-induced melting of plagioclase during laboratory earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8294, https://doi.org/10.5194/egusphere-egu2020-8294, 2020.

Deformed eclogites often reveal interconnected layers of omphacite and intercalated elongated garnet clusters. The evolution of such fabrics is usually associated with strain localization and rheological weakening. To better understand the onset of strain localization in eclogite, we experimentally investigate the strain-dependence of microfabrics in omphacite-garnet aggregates. Eclogites were synthesized by hot-pressing omphacite-garnet powders (with a volume fraction of 25% garnet) in a piston-cylinder press at 3 GPa and 1100 °C for 24 h. These synthetic eclogites were then axially shortened by ~3.5%, ~4.7%, ~17% and ~40% in a Griggs-type deformation apparatus at 2.5 GPa, 900°C, and a strain rate of 6.4·10^-6 s^-1. The low-strain experiments document microstructures developing near the material’s yield point at ~4% axial strain, whereas the highly strained samples reached nearly mechanical steady state with minor strain weakening. The recovered samples were analyzed using an X-ray microtomography instrument (μCT) which provides quantitative volume, shape and spatial arrangement data in three dimensions. By utilizing optical light microscope, scanning electron microscope and electron backscatter diffraction analyses in combination with the μCT data we identified the dominant deformation mechanisms operating at different strains and linked them to the microfabric. At low strain, omphacite exhibits a weak shape preferred orientation (SPO) and garnet tends to form clusters. The highly strained samples show a strong foliation with elongated omphacite crystals exhibiting a pronounced SPO and garnet clusters being arranged into elongated layers perpendicular to the maximum compressive stress. A reduction in grain size and an increase in density of low-angle grain boundaries with increasing strain indicate deformation of omphacite by dislocation creep. Elongated garnet clusters show brittle deformation in the form of micro-cracking. Evidence for minor crystal-plastic deformation in garnet occurs locally at the proximity of the grain boundaries where high differential stresses tend to localize resulting in increased misorientation. Similar to naturally deformed eclogites, we observe a layering of omphacite and garnet in our experimental samples, in which omphacite generally accommodated most of the strain while garnet grains behaved essentially like rigid bodies.

How to cite: Thospann, P., Incel, S., Fusseis, F., Butler, I. B., Renner, J., and Rogowitz, A.: Strain dependent microfabric evolution of experimentally deformed synthetic eclogites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9462, https://doi.org/10.5194/egusphere-egu2020-9462, 2020.

When implicated in convergence zones, granulites of the lower continental crust are expected to eclogitize at depth.When exposed in the field such units show a bimodal rheological behavior between fracturing of the protolith rock (granulites) and ductile flow of the transformed parts (eclogites). It seems therefore that a competition exists between the rate at which the rocks are loaded in stress and the rate at which they transform, i.e. the overall eclogitization kinetics. The aim of the work presented here is to quantify the kinetics of the metamorphic reactions involved in eclogitization by estimating the reaction rates in plagioclase-bearing assemblages  submitted to different P-T conditions over different time spans. For this, experiments have been performed in piston-cylinder apparatus on aggregates derived from natural granulites. Special attention is paid to the location where nucleation starts and how it propagates in and between the grains. In this prospect, the presence of garnet and cpx in the plagioclase matrix is a first order control on the reaction process. This work follows previous experimental studies (e.g. Shi et al., 2017, Incel et al., 2018) which show that reaction-enhanced embrittlement may be key for fracturing at high pressure. It has been proposed that transient properties of the rocks induced by the very beginning of the reaction (e.g. volume change, small grain size nucleation products) can lead to brittle instabilities. As we assume that the rheological behavior of the crust is controlled by a competition between reaction rate and strain rate, experiments involving deformation of granulites while undergoing eclogitization are required. Preliminary results performed on Griggs-type apparatus, which constitutes the best tool for that, will also be presented.

How to cite: Baisset, M., Labrousse, L., and Schubnel, A.: Eclogitization kinetics of continental granulites : quantification and implications , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8351, https://doi.org/10.5194/egusphere-egu2020-8351, 2020.

It is known that understanding of long-term hydrocarbon recovery or CO2 storage problems depend on proper addressing the physical coupling between the fluid flow and mechanical deformation. The success of geo-energy applications such as hydraulic fracturing, wellbore stability, and geological storage of CO2 is directly connected to the comprehensive formulation of appropriate rock rheology. Effective viscosity is an important parameter that allows to couple fluid flow and deformation processes occurring in the Earth. However, this parameter is rarely measured in the laboratories as it is a challenging task. Moreover, few existing measurements were made in the compaction regime and make no reckoning of decompaction. However, decompaction may affect fluid flow distribution in a porous medium and create highly porous channels such as chimneys observed in the subsea reservoirs and caprocks. In this study, we present results of multistage laboratory creep and relaxation experiments that were conducted on different materials including artificial specimens, limestones, heterogeneous shales with sandstone inclusions, and pure sandstones and shales. Both compaction and (de)compaction regimes were considered. We studied the influence of historical changes in the thermal regime during the glaciation and deglaciation cycles, water saturation, preliminary damage of the samples on their viscous behavior. The first stage of the experiment is the initial fast loading to dilation point, where the transition from compaction to dilatancy occurs. The second stage is a purely viscous creep. The third stage is the stress relaxation phase. During the fourth stage, repeated cycles of fast visco-elasto-plastic loading/ unloading were conducted. Effective viscosity was calculated for all samples. Experimental curves are explained using the theoretical model for visco-elasto-plastic (de)compaction of porous rocks.

How to cite: Sabitova, A., Yarushina, V., Stanchits, S., Stukachev, V., Myasnikov, A., and Cheremisin, A.: Experimental study of visco-elasto-plastic deformation of sedimentary rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9419, https://doi.org/10.5194/egusphere-egu2020-9419, 2020.

Preservation of mechanically-controlled microstructures can help us to unravel the long-term stress state in geological materials. To better understand the stress state in such a microstructure, we need to quantify the processes in a coupled, chemo-mechanical, point of view. One of such a mechanically-controlled microstructure is oscillatory zoning in high-temperature metamorphic rocks. The presented example is a sharp zoned plagioclase of 150 x 200 µm size with thin compositional lamellae of 1-10 µm alternating from the core towards the rim. This microstructure is interpreted to be mechanically-controlled, since conventional diffusion failed to preserve the observed microstructure within timescales that would be reasonable from a regional geology point of view. In contrast, considering that chemical diffusion is coupled to mechanical deformation the observed zoning can be maintained over the geologically-relevant timescales.

Despite of the recent valuable progress in our understanding of these microstructures, the mechanisms controlling its evolution from slowly cooled rocks are still not complete. Here, we numerically investigate the coupled, chemo-mechanical, effect that generates oscillatory zones mechanically maintained over geologically relevant timescales. We test the possibility of modelling oscillatory zoning in minerals that is similar to the exsolution process. We apply a classical Cahn-Hilliard-type equation where we add more complexity considering the impact of deformation during the process.

How to cite: Schmidt, K., Tajmanova, L., and Khakimova, L.: Application of the Cahn-Hilliard-type approach to the development of oscillatory zoning in minerals , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10384, https://doi.org/10.5194/egusphere-egu2020-10384, 2020.

Phase transitions affect the physical properties of rocks (e.g. rheology) and control geodynamic processes at different spatial and time scales. However, the influence of deformation on phase transitions and their coupling is not well understood. 
Previous experiments, with both assembly-induced and additionally placed mechanical heterogeneities, have shown patterns in the phase transition distribution. Numerical modelling (2D, viscous finite difference models) have been used to correlate the experimental observations with the mechanic stress state. The locally increased mean stress in the models shows the best correlation with the formation of high-pressure polymorphs in experiments (Cionoiu et al. 2019).
Besides the distribution of polymorphs, grain-size and deformation patterns also vary across the samples due to stress, strain and pressure variations. To better understand the mechanisms contributing to these variations, we used advanced numerical models (3D, viscoelastic) to calculate the local distribution of first order parameters as pressure, stress and strain. The modelled stress and strain patterns are compared to the experimentally produced phase transformation distribution and previous (2D) modelling results. The 2D and 3D models differ partially regarding the quantification of local stresses – an effect that mainly depends on sample geometry (coaxial vs. general-shear). However, the qualitative fit between experiments, 2D and 3D models persists (i.e. the localisation of increased stresses or strain).
This contribution shows how numerical models, that closely represent the sample, can further improve the understanding of processes occurring in deformation experiments. Our new results emphasize that mechanically-induced stress-variations influence the grain-size and mineralogy of rocks which feeds back on their rheology.

References:
Cionoiu, S., Moulas, E. & Tajčmanová, L. Impact of interseismic deformation on phase transformations and rock properties in subduction zones. Sci Rep 9, 19561 (2019)

How to cite: Cionoiu, S., Tajčmanová, L., and Khakimova, L.: Advances in understanding localised variations in deformation experiments using numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10427, https://doi.org/10.5194/egusphere-egu2020-10427, 2020.

The recent careful theoretical, numerical and experimental investigations of exsolution miscrostructures focus only on chemical aspect of the exsolution process. Interestingly, mechanics, i.e. stress and pressure redistribution around the exsolved lamellae, may play an important role on its evolution. In this contribution, we investigate the coupled, chemo-mechanical, effect around the exsolved lamellae. We apply a classical Cahn-Hilliard-type equation and we add more complexity considering deformation during the exsolution process. We also discuss the general importance of the exsolution process in geomaterials and its effect on rheology. At the time of the exsolution lamellae formation (coherent at initial stage), large stresses are built-up inside the host grain. The reason that we still partially see the microstructure preserved is that the stress variations were maintained during the further evolution. In other words, a strong rheology is needed to preserve such large stresses on geological timescales so that we can now detect by the analytical techniques.

How to cite: Tajcmanova, L., Khakimova, L., and Podladchikov, Y.: The application of chemo-mechanical coupling in the modeling of exsolution lamellae in minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9533, https://doi.org/10.5194/egusphere-egu2020-9533, 2020.

Dehydration of serpentinites in subduction zones plays a major role in Earth's deep water cycle. The fact that there is still water present at the Earth's surface indicates an efficient fluid release mechanism that is able to keep up with transport of water into the mantle by subduction. Rock dehydration itself is a multi-scale process that spans several orders of magnitudes in both time and spatial scales. 
Plümper et al. (2016) showed that on small scales (µm-mm) dynamic porosity generation and fluid flow is mainly controlled by intrinsic chemical heterogeneities in the rock. However, field observations indicate that on larger scales the process might be mechanically dominated by the formation of a channelized system of hydraulic fractures that form during pulsed fluid release which occurs on much shorter time scales. To get a better understanding of the multi-scale formation of a channelized fluid network a mathematically and thermodynamically valid model is needed that describes the process of rock dehydration crossing a range of scales over several orders of magnitudes. The project is therefore in collaboration with mathematicians as part of the DFG funded CRC 1114. As a first step we want to extend the model of Plümper et al. (2016) by considering chemical transport during reactive fluid flow and upscale it by one order of magnitude, from the mm- to the cm-scale. 
In order to understand the chemical and structural heterogeneities in non-deformed serpentinites we mapped and sampled an outcrop in the Mirdita ophiolite in Albania. These serpentinites are still fully hydrated due to ocean floor serpentinization and show information about intrinsic heterogeneities of serpentinized mantle as it enters the subduction zone. We will use these data as input for the extended model that aims to simulate serpentinite dehydration in the downgoing slab.
We present the results of field work in the Mirdita ophiolite and the results of the preliminary extended and upscaled model. Geological mapping has been done on an outcrop scale as well as detailed mapping of representative units in order to get information about structural and lithological heterogeneities that might influence the formation of the dehydration vein network on large scales. The samples were then further studied by EDX mapping, XRF and detailed electron microscopy. Of special interest will be the coupling of the chemical and mechanical processes on different scales and what controls the transition from a chemically to a mechanically dominated system.

Reference

Plümper, Oliver et al. (Dec. 2016). "Fluid escape from subduction zones controlled by channel-forming reactive porosity".
In: Nature Geoscience 10.2, pp. 150-156. doi:10.1038/ngeo2865.

How to cite: Huber, K., John, T., and Vrijmoed, J. C.: Modeling serpentinite dehydration on multiple scales constrained by field observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18242, https://doi.org/10.5194/egusphere-egu2020-18242, 2020.

We present a set of MATLAB codes that can be used to perform equilibrium and non-equilibrium thermodynamic calculations. This will be of general use in geomaterial research and education, from the calculation of equilibrium phase diagrams to the development of dynamic models of reaction, deformation, mass and heat transport processes. The main MATLAB function calculates Gibbs energies of pure substances and mixtures using internally consistent thermodynamic databases, for rocks, minerals, melts and fluids. A general formulation of calculating Gibbs energy of mixtures based on linear algebra allows users to add custom solution models in an easy manner. The main Gibbs energy function can also be further extended, updated and customized, for example to involve other thermodynamic databases and equations of state.

We show three methods on how these Gibbs energies can be used to calculate chemical equilibrium based on optimization techniques and linear programming: 1) A brute-force method in which Gibbs energies of all possible phases and solutions are generated as a set of discrete phases. 2) A method of refining and restricting the Gibbs energies of solution phases to save computational resources and 3) A method that further saves computational resources by using system composition to generate Gibbs energies of solutions in a subset of the compositional space.

Finally, we demonstrate how these codes can be used in non-equilibrium thermodynamic processes such as reactive-fluid flow involving density and porosity changes.

How to cite: Vrijmoed, J. C. and Podladchikov, Y. Y.: Introducing Thermolab: a toolbox for Thermodynamics in MATLAB, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20069, https://doi.org/10.5194/egusphere-egu2020-20069, 2020.

One of the most important  problems for reservoir simulation is the computation of a multicomponent flow of compressible fluids in porous media with mass exchange between phases. Phase equilibrium ratios (K-values) play a fundamental role in such calculating. Current work proposes the new analytical formulas for K-values. The theory takes into account not only the dependence on pressure, temperature and composition, but also takes into account the conditions formation of real fluid in a porous medium. Such accounting is performed with application of the integral fluid parameters, rather than with application individual characteristics of each component. For calculation of these parameters it is necessary to know dependence volumes of gas and liquid phases in some pressure range (in two phase region) and values of compositions at one pressure.

If combine a compositional model and this K-values approach, it is possible to create an effective model for numerically modeling the complex phase state of solutions. The technique of thermodynamic potentials makes it possible to construct a thermodynamically consistent model of a real solution in an analytical form. The proposed formulas properly describe phase behavior of real solutions in some practically important pressure range for volatile and black oil. The approach can be used for several phases (not only for two phase). Newly developed methods will be added to open source thermo-hydromechanical matlab codes.

How to cite: Koldoba, E.: NEW ANALYTICAL FORMULAS FOR PHASE EQUILIBRIUM RATIOS (K-values), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19790, https://doi.org/10.5194/egusphere-egu2020-19790, 2020.

Geodynamic processes of formation of mobile belts operate during entire tectonic cycle since sedimentation up to recent uplift and erosion. In general, we can expect that some quantitative parameters of tectonic events will be associated with such processes, so they can be used to solve the inverse problem of recognizing quantity and nature of geodynamic processes.

The Greater Caucasus is a well-studied Alpine structure, within which the sedimentary cover (total thickness of 10-15 km) has a thin layering, deformed in small and moderate-sized folds. The folded structure was described in 24 detailed profiles with a total length about 500 km. Using a special method of sections balancing, models of the sedimentary cover were compiled, based on the balance of the sediments volume and the shortening values. By the method, profiles were divided into 505 "folded domains", for which their pre-folded states were restored. Then, the pre-folded domains were combined into 78 "structural cells", for which their shortening values were estimated.

For calculations, a three-stage’s conditional model of the development of the Greater Caucasus was adopted: 1) sedimentation (Jurassic-Eocene), 2) shortening and folding formation (Oligocene), 3) uplift and erosion (post-Oligocene). Six parameters were digitized in the structural cells: the depth of the basement top for development stages (1, 3, 4), the shortening value (2), the amplitude of uplift and erosion (5), the difference between the depths of the basement top in stages 3 and 1 (6). Obviously, these parameters are directly related to geodynamic processes of the Greater Caucasus formation. The calculation of the correlation matrix showed the presence of such strong correlations between a numbers of parameters, which may have a genetic sense. Factor analysis was used to clarify all these relationships. It showed the presence of two well-defined factors that explain the main dispersion of the six parameters. Factor (process) F1 (named ISOSTASY) has a weight of 46.6%, the loads on parameters 1-6 were 0.790, -0.195, 0.665, 0.982, 0.005 and 0.853. Process F1 showed the dependence of the actual depth of the basement top (4) on its first value (1), which is clearly associated with isostasy and necessarily indicates an increase of the density of the crust rocks up to mantle values. The F2 factor (named SHORTENING) has a weight of 40.2%, the loads amounted to 0.022, 0.938, -0.736, -0.158, 0.957, -0.219. Factor (process) F2 indicated the dependence of the uplift amplitude (5) on the shortening value (2), which can also be associated with isostasy and changes in the density of the crust and mantle rocks.

The calculation of the crust layer thicknesses for a part of the structure during the development, in which it has an isostatic equilibrium, showed its gradual degradation from 40 km (before a sedimentation) to 14 km after sedimentation and to present 19 km after folding and uplift (9.5 km without shortening influence).

Yakovlev F.L., Gorbatov E.S., 2018. On using the factor analysis to study the geodynamic processes of formation of the Greater Caucasus. Geodynamics & Tectonophysics 9 (3), 909–926.

How to cite: Gorbatov, E. and Yakovlev, F.: Balanced model of the folded sedimentary cover of the Greater Caucasus as a source of information about geodynamic processes on the scale of the lithosphere - statistical approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5051, https://doi.org/10.5194/egusphere-egu2020-5051, 2020.

One of the most unstable and unpredictable process in sedimentary basin is salt diapir movement. It changes the structure of strata and can break its integrity and make trap structures for hydrocarbons. The movement of salt diapir through geologic timescale can be described in viscous terms, elastic terms were used to predict the geomechanical response of sediment surroundings.

This work describes the workflow of visco-elastic flow modeling of salt diapirism process. Salt has different geomechanical property such as much lower viscosity comparing to typical sediments. Mixed rheology make different geomechanical response such as stress, which cannot be solved in the same timescale.  To solve the problem of different timescales of viscous and elastic flow there was used a pseudo-transient method of solving the system of equations. Used equations calculate full stress tensors and pressure over time which can help in understanding of stress evolution around salt diapir. Maximizing time step during each calculation was accomplished with density scaling, which assumes that inertial forces are negligible.

The used approach allows taking into account the loading history and easily can be supplemented with sedimentation mechanisms.

How to cite: Iskander, I., Podladchikov, Y., and Myasnikov, A.: Loading history dependence of stress field around salt diapirs due to path dependence of visco-elasto-plastic rheology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10534, https://doi.org/10.5194/egusphere-egu2020-10534, 2020.

Today, the problem of global climate change is the most exciting challenge for the world community of scientists. One of the most recommended technology for decreasing carbon dioxide concentration in the atmosphere is its injection into natural geological reservoirs. The most significant attention is paid to this issue in Norway offshore. Such operations must be conducted with extreme caution since, in the petroleum systems of north European seas, such a phenomenon as a gas chimney is widespread. The most straightforward indicator for detecting them is pockmarking at the bottom of the sea. Nevertheless, it does not provide information about the depth of the gas formation zone. Thus, we cannot identify the genesis of the chimney, the instability of the gas hydrate zone or reservoir gas leakage. Identification of the chimney root also can be determined using seismic monitoring, but this is an expensive study.

In this work, we suggest the new method to identify the potential zones of reservoir chimney based on basin modelling data interpretation. We compare the anomalies of physical fields calculated in the simulator with detected acoustic noises on the seismic profile associated with the chimney being at a depth of ~ 2 km to the surface of the seabed. The pattern of the presence of the chimneys is determined. The study is conducted on a 2D basin model along with the PETROBAR-07 profile of the south-west part of the Barents Sea.

How to cite: Peshkov, G., Ibragimov, I., Yarushina, V., and Myasnikov, A.: Basin modelling as a predictive tool for potential zones of chimney presence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18689, https://doi.org/10.5194/egusphere-egu2020-18689, 2020.

Understanding of instantaneous and long-term compaction of porous [1, 2] rocks is important for reservoir engineering and Earth sciences in general. Reservoir depletion due to petroleum extraction or reservoir expansion due to prolonged injection of large volumes of fluids as in geological CCS operations lead to non-hydrostatic changes in stress conditions in the reservoir and surrounding rocks inducing noticeable shear stress components. The phenomenon of mutual influence of compaction and shear deformation was repeatedly reported in the literature. Dilatancy and shear-enhanced compaction of porous rocks were experimentally observed during both rate-independent (plastic) and rate-dependent (viscous) inelastic deformation. Dilatancy and shear-enhanced compaction can alter the transport properties of rocks through their influence on permeability and compaction length scale.

Effective bulk viscosity is commonly used to describe compaction driven fluid flow in porous rocks. Several effective media models were proposed to model its dependence on porosity, stress state and material parameters of the solid rock grains. They are based on the averaging of a solution obtained for a single pore in a solid matrix. Thus, interaction between pores is ignored and such models are applicable strictly speaking only to very small porosities of a few percent. In high porosity rocks, pore interaction is rather significant and can lead not only to non-linear effective rheological behavior but also to formation of zones of localized deformation such as shear bands. To address these phenomena, we develop new effective media model based on Representative Volume Element [3, 4] consisting of multiple interacting pores. To resolve stress and strain field interactions caused by the presence of multiple pores in elastoplastic matrix we use numerical simulator CAE Fidesys [5], where classical associated plastic flow law with von Mises and Tresca yield criteria are implemented. For viscoplastic rocks, correspondence principle is used. We derive 3D effective stress-strain relations for porous viscoelastoplastic rocks in a general non-hydrostatic stress field.

1. Levin, V.A., Lokhin, V.V., Zingerman, K.M. Effective elastic properties of porous materials with randomly dispersed pores: Finite deformation (2000) Journal of Applied Mechanics, Transactions ASME, 67 (4), pp. 667-670.
2. Levin, V.A., Zingermann, K.M. Effective Constitutive Equations for Porous Elastic Materials at Finite Strains and Superimposed Finite Strains (2003) Journal of Applied Mechanics, Transactions ASME, 70 (6), pp. 809-816.
3. Levin, V.A., Zingerman, K.M., Vershinin, A.V., Yakovlev, M. Numerical analysis of effective mechanical properties of rubber-cord composites under finite strains (2015) Composite Structures, 131, pp. 25-36.
4. Vershinin, A.V., Levin, V.A., Zingerman, K.M., Sboychakov, A.M., Yakovlev, M.Y.Software for estimation of second order effective material properties of porous samples with geometrical and physical nonlinearity accounted for (2015) Advances in Engineering Software, 86, pp. 80-84.
5. http://cae-fidesys.com

How to cite: Yakovlev, M. and Yarushina, V.: Prediction of effective viscoelastoplastic rheology of porous rocks using numerical averaging with CAE Fidesys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18019, https://doi.org/10.5194/egusphere-egu2020-18019, 2020.

Reservoir processes and systems cover wide spatial range of scales from nanoscale physical and chemical transport in the pore to fluid migration in reservoir systems during formation of sedimentary basins. Thorough analysis of physico-chemical properties on each scale allows us to conclude that for adequate consideration of the majority of multiscale features it is necessary to solve a finite number of fundamental problems, which include:

- creation and development of a new concept of Representative Elementary Volume (REV), which takes into account the specificity of multiporous and multi-permeable multiscale cracked environment;

- development of a new approach to solving the problem on phase equilibrium of fluids and solid phase in pores and micropores;

- nano-chemical-mechanical determination of quantitative strength characteristics of rocks due to phase transformations of various inhomogeneities that make up a given rock.

These problems are interrelated [1,2]. The REV problem is of primary importance, both from conceptual and practical points of view.  Success of modeling depends on correct selection of REV for different spatial scales. For example, instead of development of double porosity models for fractured rocks, it is possible to grind REV up to its homogeneity in relation to heterogeneities of interest. We support and develop the second approach. We believe, that the future belongs to the ability to describe multiscale processes using the same set of defining relations, in which the coefficients depend on the selected scale. When choosing the second approach, we put great attention to the development of new approaches to solving the problem of phase equilibrium of fluids and solid phase in pores and micro-nanopores. And, if in the first case we are talking about methods based on thermodynamically consistent systems of equations and numerical methods, intensively developed at present and based on minimization of basic thermodynamic potentials, for nanopores there is still a question of expanding the concept of thermodynamic equilibrium, where in the pore may be no more than 1-3-10 molecules [3].

Experiment on the nano-scale acquires a special meaning. Filtration, rock elastic and strength parameters play a desizive role for uch formations. And they may be changed due to field dtvelopment.  Such works are currently in progress, however, we believe they are of an exquisitely fundamental nature and are still far from practical oil and gas applications.

 [1].Мясников А.В. О моделировании экологически безопасной закачки флюидов в пласт // EAGE/SPE Joint Workshop 2015. Exploration of shale oil resources and reserves

[2]Yarushina V., Podladchikov Yu (De) compacion of porous viscoelastoplastic media, JGR, 2015, 120(6), 4146.

[3].Stroev N., Myasnikov A.V. Review of Current Results in Computational Studies of Hydrocarbon Phase and Transport Properties in Nanoporous Structures AIP Conference Proceedings, 2017, 1909, 020213-1–020213-6

How to cite: Myasnikov, A.: Analysis of physical, chemical and mechanical rock properties for effective multiscale modelling of reservoir processes and systems , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22615, https://doi.org/10.5194/egusphere-egu2020-22615, 2020.

Development of the homogenization algorithms for the heterogeneous periodic and non-periodic materials has applications in different domains and considers different types of upscaling techniques (Fish, 2008, Bagheri, Settari, 2005, Kachanov et al. 1994, Levin et al. 2003).

The current presentation discusses an algorithm implemented in CAE Fidesys (Levin, Zingerman, Vershinin 2015, 2017) for calculating the effective mechanical characteristics of a porous-fractured medium (Myasnikov et al., 2016) at the scale of a periodicity cell dissected by a group of plane-parallel cracks modeled by elastic bonds with specified stiffnesses in the normal and tangential directions in accordance with the method of modeling cracks based on elastic bonds (Bagheri, Settari, 2005, 2006) In this case, the relationship between the components of the displacement vector and the force vector (normal stresses at the fracture’s boundaries) in the normal and tangential directions will be diagonal, neglecting the effects of dilatancy and shear deformations as a result of normal stresses.
The presentation also considers the general case of the relationship between displacements and forces along the fracture’s boundaries, taking into account shear deformations (which leads to an increase in the effective Young's modulus by 30%), and additionally a cell’s geometrical model is generalized by the presence of pores in the matrix’s material. The results of numerical studies on mesh convergence, the influence of periodicity cell sizes and fracture’s thicknesses on the computed effective properties are presented. A comparison between analytical (Kachanov, Tsukrov 1994, 2000) and numerical results obtained in CAE Fidesys for the effective elastic moduli estimation for particular cases of geometrical models of the periodicity cell is shown.
The developed algorithm is used to evaluate the effective mechanical properties of a digital core model obtained by the results of CT-scan data interpretation. A comparison is made with the results of laboratory physical core tests. Additionaly an algorithm implemented in CAE Fidesys and the results for the effective thermal conductivity and the effective coefficient of thermal expansion estimation are given for the considered test rock specimen.

The reported study was funded by Russian Science Foundation project № 19-77-10062. 

1. Bagheri, M., Settari, A. Effects of fractures on reservoir deformation and flow modeling // Can. Geotech. J. 43: 574–586 (2006) doi:10.1139/T06-024
2. Bagheri, M., Settari, A. Modeling of Geomechanics in Naturally Fractured Reservoirs – SPE-93083-MS, SPE Reservoir Simulation Symposium, Houston, USA, 2005.
3. Fish J., Fan R. Mathematical homogenization of nonperiodic heterogeneous media subjected to large deformation transient loading // International Journal for Numerical Methods in Engineering. 2008. V. 76. – P. 1044–1064.
4. Kachanov M., Tsukrov I., Shafiro B. Effective moduli of a solid with holes and cavities of various shapes// Appl. Mech. Reviews. 1994. V. 47, № 1, Part 2. P. S151-S174.

How to cite: Levin, V.: Use of "digital core" module in SAE Fidesis to determine effective parameters of fractured porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22623, https://doi.org/10.5194/egusphere-egu2020-22623, 2020.