TS7.2
Subduction zones evolution and slip styles: advances from tectonics, metamorphism and rheology

TS7.2

EDI
Subduction zones evolution and slip styles: advances from tectonics, metamorphism and rheology
Convener: Mathieu SoretECSECS | Co-conveners: Francesca Meneghini, Guillaume BonnetECSECS, Ake Fagereng, Francesca PiccoliECSECS, Kohtaro Ujiie, Matthijs Smit, Martijn van den EndeECSECS
vPICO presentations
| Mon, 26 Apr, 13:30–17:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Mathieu Soret, Francesca Piccoli, Matthijs Smit
13:30–13:35
Subduction zones: from start to finish
13:35–13:40
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EGU21-10097
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solicited
Stefan Markus Schmalholz, Lorenzo Candioti, Joshua Vaughan-Hammon, and Thibault Duretz

Subduction zones are one of the main features of plate tectonics, they are essential for geochemical cycling and are often a key player during mountain building. However, several processes related to subduction zones remain elusive, such as the initiation of subduction or the exhumation of (ultra)high-pressure rocks.

Collision orogens, such as the European Alps or Himalayas, provide valuable insight into long-term subduction zone processes and the larger geodynamic cycles of plate extension and subsequent convergence. For the Alps, geological reconstructions suggest a horizontally forced subduction initiation caused by the onset of convergence between the Adriatic and European plates. During Alpine orogeny, the Piemont-Liguria basin and the European passive magma-poor margin (including the Briançonnais domain) were subducted below Adria. The petrological rock record indicates burial and subsequent exhumation of both continental and oceanic crustal rocks that were exposed to (ultra)high-pressure metamorphic conditions during their Alpine burial-exhumation cycle. Moreover, estimates of exhumation velocities yield magnitudes in the range of several mm/yr to several cm/yr. However, published estimates of exhumation velocities, ages of peak metamorphic conditions and estimates for peak pressure and peak temperature vary partly significantly, even for the same tectonic unit. Consequently, many, partly significantly, contrasting conceptual models for subduction initiation (convergence versus buoyancy driven) or rock exhumation (channel-flow, diapirism, episodic regional extension, erosion etc.) have been proposed for the Alps. 

Complementary to the data-driven approach, mathematical models of the lithosphere and upper mantle system are useful tools to investigate geodynamic processes. These mathematical models integrate observational and experimental data with the fundamental laws of physics (e.g. conservation of mass, momentum and energy) and are useful to test conceptual models of subduction initiation and rock exhumation. Here, we present numerical solutions of two-dimensional petrological-thermo-mechanical models. The initial model configuration consists of an isostatically and thermally equilibrated lithosphere, which includes mechanical heterogeneities in the form of elliptical regions with different effective viscosity. We model a continuous geodynamic cycle of subsequent extension, no far-field deformation and convergence. During extension, the continental crust is necked, separated and mantle is exhumed, forming a marine basin bounded by passive margins. During the subsequent stage with no far-field deformation, the thermal field of the lithosphere is re-equilibrated above a convecting mantle. During convergence, subduction is initiated at one passive margin and the mantle lithosphere below the marine basin as well as the other passive margin are subducted. During progressive subduction, parts of the subducted continental upper crust are sheared-off the subducting plate and are exhumed to the surface, ultimately forming an orogenic wedge. For the convergence, we test the impact of serpentinites at the top of the exhumed mantle on orogenic wedge formation. We compare the model results with observational and experimental constraints, discuss the involved processes and driving forces and propose a model for subduction initiation and (ultra)high-pressure rock exhumation for the Alps.

How to cite: Schmalholz, S. M., Candioti, L., Vaughan-Hammon, J., and Duretz, T.: Subduction initiation and subsequent burial-exhumation of (ultra)high-pressure rock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10097, https://doi.org/10.5194/egusphere-egu21-10097, 2021.

13:40–13:42
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EGU21-11068
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ECS
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Attila Balazs, Claudio Faccenna, Taras Gerya, Kosuke Ueda, and Francesca Funiciello

The dynamics of oceanic and continental subduction zones is linked to the rise and demise of forearc and backarc basins in the overriding plate. Subsidence and uplift rates of these distinct sedimentary basins are controlled by variations in plate convergence and subduction velocities and determined by lithospheric rheological structure and different lithospheric thicknesses.

In this study we conducted a series of high-resolution 2D numerical models applying the thermo-mechanical code 2DELVIS (Gerya and Yuen, 2007). The model, based on finite differences and marker-in-cell techniques, solves the mass, momentum, and energy conservation equations for incompressible media; assumes elasto-visco-plastic rheologies and involves erosion, sedimentation and hydration processes.

The models show the evolution of wedge-top basins lying on top of the accretionary wedge and retro-forearc basins in the continental overriding plate, separated by a forearc high. These forearc regions are affected by repeated compression and extension phases. Higher subsidence rates are recorded in the syncline structure of the retro-forearc basin when the slab dip angle is higher and the subduction interface is stronger and before the slab reaches the 660 km discontinuity. This implies the importance of the slab suction force as the main forcing factor creating up to 3-4 km negative dynamics topographic signals.

Extensional back-arc basins are either localized along inherited crustal or lithospheric weak zones at large distance from the forearc region or are initiated just above the hydrated mantle wedge. During trench retreat and slab roll-back the older volcanic arc area becomes part of the back-arc region. Back-arc subsidence is primarily governed by crustal and lithospheric thinning controlled by slab roll-back. Onset of continental subduction and soft collision is linked to the rapid uplift of the forearc basins; however, the back-arc region records ongoing extension. Finally, during hard collision the forarc and back-arc basins are ultimately under compression.

Our results are compared with the evolution of the Mediterranean and based on the reconstructed plate kinematics, subsidence and heat flow evolution we classify the Western and Eastern Alboran, Paola and Tyrrhenian, Transylvanian and Pannonian Basins to be genetically similar forearc–backarc basins, respectively.

How to cite: Balazs, A., Faccenna, C., Gerya, T., Ueda, K., and Funiciello, F.: Subduction dynamics and rheology control on forearc and backarc subsidence: Numerical models and observations from the Mediterranean , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11068, https://doi.org/10.5194/egusphere-egu21-11068, 2021.

13:42–13:44
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EGU21-3047
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ECS
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Kristóf Porkoláb, Thibault Duretz, Philippe Yamato, Antoine Auzemery, and Ernst Willingshofer

Continental subduction below oceanic plates and associated emplacement of ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit a far-travelled ophiolite sheet that is separated from its oceanic root by tectonic windows exposing continental crust, which experienced subduction-related high pressure-low temperature (HP-LT) metamorphism during obduction. However, the link between continental subduction-exhumation dynamics and far-travelled ophiolite emplacement remains poorly understood. We combine data collected from ophiolite belts worldwide with thermo-mechanical simulations of continental subduction dynamics to show the causal link between the extrusion of subducted continental crust and the emplacement of far-travelled ophiolite sheets. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. The broken-off piece of oceanic lithosphere is then transported on top of the continent along a flat thrust segment and becomes a far-travelled ophiolite sheet separated from its root by the extruded continental crust. Our results indicate that the extrusion of the subducted continental crust and the emplacement of far-travelled ophiolite sheets are inseparable processes.

How to cite: Porkoláb, K., Duretz, T., Yamato, P., Auzemery, A., and Willingshofer, E.: Extrusion of subducted crust explains the emplacement of far-travelled ophiolites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3047, https://doi.org/10.5194/egusphere-egu21-3047, 2021.

13:44–13:46
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EGU21-8587
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ECS
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Zoe Braden, Jonas B. Ruh, and Whitney M. Behr

Observations of several active shallow subduction megathrusts suggest that they are localized as décollements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.  Exhumed accretionary complexes from a range of subduction depths, however, preserve underplated mafic slivers, which indicate that megathrust faults can occasionally develop within the mafic oceanic crustal column. The incorporation of mafic rocks into the subduction interface shear zone has the potential to influence both long-term subduction dynamics and short-term seismic and transient slip behaviour, but the processes and conditions that favour localisation of the megathrust into deeper oceanic crustal levels are poorly understood.

In this work, we use visco-elasto-plastic numerical modelling to explore the long-term (million year) factors influencing the incorporation of mafic volcanic rocks into the subduction interface and accretionary wedge through underplating. We focus on the potential importance of oceanic seafloor alteration in facilitating oceanic crustal weakening, which is implemented through a temperature-dependent pore-fluid pressure ratio (lambda = 0.90-0.99 between 160 and 300oC). We then examine the underplating response to changes in sediment thickness, geothermal gradient, sediment fluid pressure, and surface erosion rates. Our results indicate that a thinner incoming sediment package and a lower geothermal gradient cause oceanic crustal underplating to initiate deeper beneath the backstop (overriding plate) compared to thicker incoming sediment and a higher geothermal gradient. Relative pore fluid pressure differences between sediments and altered oceanic crust control the amount of altered oceanic crust that is underplated, as well as the location of underplating beneath the backstop or accretionary wedge. When sediments on top of the altered oceanic crust have the same fluid pressure as the altered oceanic crust, no oceanic crustal underplating occurs. Modelling results are also compared to exhumed subduction complexes to examine the amount and distribution of underplated mafic rocks.

How to cite: Braden, Z., Ruh, J. B., and Behr, W. M.: Underplating altered oceanic crust: Insights from numerical modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8587, https://doi.org/10.5194/egusphere-egu21-8587, 2021.

13:46–13:51
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EGU21-2019
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ECS
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solicited
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Ismay Vénice Akker, Christoph E. Schrank, Michael W.M. Jones, Cameron M. Kewish, Alfons Berger, and Marco Herwegh

During the accretion of foreland basin sediments into an accretionary or orogenic wedge, the sediments dehydrate and deform. Both dehydration and deformation intensity increase from the outer to the inner wedge and are a function of metamorphic processes and strain. Here, we study the microstructural evolution of slates from the exhumed Flysch units making up a paleo accretionary wedge in the European Alps. With classic SEM imaging and synchrotron X-ray fluorescence microscopy, we document the evolution of slate fabrics and calcite veins and aim at understanding the role of the evolving slate fabrics for strain localisation and fluid flow at the micro-scale.

The investigated slate samples are from a NW-SE transect covering the outer and inner wedge from 200 to 330 °C. The metamorphic gradient directly correlates with an increasing background strain gradient. With the use of the autocorrelation function, we quantify the evolution of the microfabrics along the metamorphic gradient and document deformation stages from a weakly deformed slate without foliation in the outer wedge to a strongly deformed slate with a dense spaced foliation in the inner wedge. The foliation mainly forms by dissolution-precipitation processes, which increase with increasing metamorphic gradient.

The slate matrix reveals two main sets of veins. The first vein set includes micron-scaled calcite veinlets with very high spatial densities. The second vein set includes layer parallel calcite veins that form vein-arrays (couple of metres thick) in the inner wedge. Both vein sets could have moved large amounts of fluids through the wedge. The spatial distribution of the micron-veinlets reveals that fluids were moved pervasively. In the case of the layer parallel veins forming vein-arrays, fluid flow was localized, supported by the dense spaced foliation formed in the slate matrix in the inner wedge. This way we now establish a direct link between the microstructural evolution in the slate matrix and associated dehydration, where fluids become increasingly channelled towards the inner wedge. Knowing that the vein-arrays have length dimensions in the order or hundreds of metres to kilometres, these structures are important for larger-scale fluid flow, the feeding of fluids into megathrusts and for related seismic activity in the wedge.

How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Kewish, C. M., Berger, A., and Herwegh, M.: What slates can tell us about strain localisation and fluid flow in accretionary wedges: a microstructural analysis of deforming foreland basin sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2019, https://doi.org/10.5194/egusphere-egu21-2019, 2021.

13:51–13:53
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EGU21-2391
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ECS
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Carolyn Tewksbury-Christle, Alissa Kotowski, and Whitney Behr

The strength, or viscosity, of the subduction interface is a key parameter in subduction dynamics, influencing both long-term subduction plate speeds and short-term transient deformation styles. Fossil subduction interfaces exhumed from downdip of the megathrust record ductile deformation accommodated by diverse lithologies, including metasedimentary and metamafic rocks. Existing flow laws for quartz-rich rocks predict relatively low viscosities, in contrast to high viscosities predicted for basalt and eclogite, but the rheological properties of blueschists representative of metamorphosed oceanic crust of the down-going slab are poorly constrained. Two key questions remain: 1) are there significant viscosity contrasts between blueschists and quartz- or mica-rich metasedimentary rocks, and 2) what are the microscale mechanisms for creep in naturally deformed blueschists and how do they vary with pressure and temperature? To address these questions, we characterized deformation in natural samples from the Condrey Mountain Schist (CMS) in northern California, USA, and the Cycladic Blueschist Unit (CBU) on Syros Island, Cyclades, Greece, using outcrop-scale structural observations, optical microscopy, and Electron Backscatter Diffraction. The CMS and CBU record pressure-temperature conditions of 0.8-1.1 GPa, 350-450°C and 1.4-1.8 GPa, 450-550°C, respectively. 

In the field, blueschists form m- to km-scale lenses that are interfolded with quartz schists, ultramafics, and, in the CBU, eclogites and marbles. At the outcrop scale in both localities, quartz-rich schists and blueschists each exhibit strong foliations and lineations and planar contacts at lithological boundaries. At the thin section scale, the prograde foliation and mineral lineation in blueschists are commonly defined by Na-amphiboles elongated in the lineation direction. Crystallographic preferred orientations in Na-amphibole in all samples have c-axes parallel to lineation and a-axes predominantly defining point-maxima perpendicular to the foliation, suggesting some component of dislocation activity for all temperature conditions in our sample suite. Microtextures in lower temperature CMS samples suggest strain accommodation primarily by dislocation glide and kinking in Na-amphibole, with extremely high-aspect-ratio grains and limited evidence for climb-controlled dynamic recrystallization. Some higher temperature CBU samples show large porphyroclasts with apparent ‘core-and-mantle’-type recrystallization textures and subgrain orientation analyses consistent with the (hk0)[001] slip systems. In contrast, epidote grains accommodate less strain than Na-amphibole, via some combination of rigid rotation, brittle boudinage, and minor intracrystalline plasticity.

Observations of evenly-distributed strain, despite lithological heterogeneity, suggest low viscosity contrasts and comparable bulk strengths of quartz schists and blueschists. Our microstructural observations suggest that Na-amphibole was the weakest phase and accommodated the majority of strain in mafic blueschists. Dislocation activity, and not just rigid-body-rotation or diffusional processes, accommodated some component of strain and possibly transitioned with increasing temperature from glide- to climb-controlled. Although effective viscosities appear to be similar, subduction interface shear zones dominated by blueschists may exhibit a power-law rheology consistent with dislocation activity, in contrast to the common inference of Newtonian creep in metasediments. Complementary experimental work on CMS and CBU rocks will also be presented at this meeting (see Tokle et al. and Hufford et al.).

How to cite: Tewksbury-Christle, C., Kotowski, A., and Behr, W.: Deformation mechanisms in naturally-deformed blueschist facies metabasalts: constraints from exhumed subduction complexes in Greece and California, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2391, https://doi.org/10.5194/egusphere-egu21-2391, 2021.

13:53–13:55
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EGU21-5897
Armel Menant, Onno Oncken, Johannes Glodny, Samuel Angiboust, Laurent Jolivet, Romain Augier, Eloïse Bessière, and Taras Gerya

Subduction margins are the loci of a wide range of deformation processes occurring at different timescales along the plate interface and in the overriding forearc crust. Whereas long-term deformation is usually considered as stable over Myr-long periods, this vision is challenged by an increasing number of observations suggesting a long-term pulsing evolution of active margins. To appraise this emerging view of a highly dynamic subduction system and identify the driving mechanisms, detailed studies on high pressure-low temperature (HP-LT) exhumed accretionary complexes are crucial as they open a window on the deformation history affecting the whole forearc region.

In this study, we combine structural and petrological observations, Raman spectroscopy on carbonaceous material, Rb/Sr multi-mineral geochronology and thermo-mechanical numerical models to unravel with an unprecedented resolution the tectono-metamorphic evolution of the Late-Cenozoic HP-LT nappe stack cropping out in western Crete (Hellenic subduction zone). A consistent decrease of peak temperatures and deformation ages toward the base of the nappe pile allows us to identify a minimum of three basal accretion episodes between ca. 24 Ma and ca. 15 Ma. On the basis of structural evidences and pressure-temperature-time-strain predictions from numerical modeling, we argue that each of these mass-flux events triggered a pulse in the strain rate, sometimes associated with a switch of the stress regime (i.e., compressional/extensional). Such accretion-controlled transient deformation episodes last at most ca. 1-2 Myr and may explain the poly-phased structural records of exhumed rocks without involving changes in far-field stress conditions. This long-term background tectonic signal controlled by deep accretionary processes plays a part in active deformations monitored at subduction margins, though it may remain blind to most of geodetic methods because of superimposed shorter-timescale transients, such as seismic-cycle-related events.

How to cite: Menant, A., Oncken, O., Glodny, J., Angiboust, S., Jolivet, L., Augier, R., Bessière, E., and Gerya, T.: Accretion-controlled forearc deformation pulses recorded by high-pressure paleo-accretionary wedges: the example of the Hellenic subduction zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5897, https://doi.org/10.5194/egusphere-egu21-5897, 2021.

13:55–13:57
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EGU21-13868
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ECS
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Rilla C. McKeegan, Victor E. Guevara, Adam F. Holt, and Cailey B. Condit

The dominant mechanisms that control the exhumation of subducted rocks and how these mechanisms evolve through time in a subduction zone remain unclear. Dynamic models of subduction zones suggest that their thermal structures evolve from subduction initiation to maturity. The series of metamorphic reactions that occur within the slab, resultant density, and buoyancy with respect to the mantle wedge will co-evolve with the thermal structure. We combine dynamic models of subduction zone thermal structure with phase equilibria modeling to place constraints on the dominant controls on the depth limits of exhumation. This is done across the temporal evolution of a subduction zone for various endmember lithologic associations observed in exhumed high-pressure terranes: sedimentary and serpentinite mélanges, and oceanic tectonic slices.

Initial modeling suggests that both serpentinite and sedimentary mélanges remain positively buoyant with respect to the mantle wedge throughout all stages of subduction (up to 65 Myr), and for the spectrum of naturally constrained ratios of mafic blocks to serpentinite/sedimentary matrix. In these settings, exhumation depth limits and the “point of no return” (c. 2.3 GPa) are not directly limited by buoyancy, but potentially rheological changes in the slab at the blueschist-eclogite transition stemming from: the switch from amphibole-dominated to pyroxene-dominated rheology and/or dehydration embrittlement. These mechanisms may increase the possibility of brittle failure and hence promote detachment of the slab top into the subduction channel. For the range of temperatures recorded by exhumed serpentinite mélanges, the locus of dehydration for altered MORB at the slab top coincides with the point of no return (2.3 GPa) between 35 and 40 Myr, suggesting a strong temporal dependence on deep exhumation in the subduction channel. 

Tectonic slices composed of 50% mafic rocks and 50% serpentinized slab mantle show a temporal dependence on the depth limits of positive buoyancy. For the range of temperatures recorded by exhumed tectonic slices, the upper pressure limit of positive buoyancy is ~2 GPa, and is only crossed between ~30 and 40 Myr after subduction initiation. Some exhumed tectonic slices record much higher pressures (2.5 GPa); thus, other mechanisms or lithologic combinations may also play a significant role in determining the exhumation limits of tectonic slices. 

Future work includes constraining how the loci of dehydration vary through time for different degrees of oceanic crust alteration, how exhumation limits and mechanisms may change with different subducting plate ages, and calculating how initial exhumation velocities may vary through time. Further comparison with the rock record will constrain the parameters that control the timing and limits of exhumation in subduction zones.

How to cite: McKeegan, R. C., Guevara, V. E., Holt, A. F., and Condit, C. B.: Exhumation of subducted mafic rocks in a dynamically evolving thermal structure: constraints from phase equilibria modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13868, https://doi.org/10.5194/egusphere-egu21-13868, 2021.

13:57–13:59
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EGU21-12967
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ECS
Daniel Rutte, Joshua Garber, Andrew Kylander-Clark, and Paul Renne

We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40Ar/39Ar dates of amphibole and phengite and U‐Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu‐Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole‐eclogite facies conditions occurred as a single episode at 165–157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to ~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one‐time event led to exhumation of amphibole‐eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist‐facies metamorphism of the Red Ant schist of the Sierra Nevada foothills.

How to cite: Rutte, D., Garber, J., Kylander-Clark, A., and Renne, P.: An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12967, https://doi.org/10.5194/egusphere-egu21-12967, 2021.

13:59–14:01
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EGU21-15318
Guillaume Bonnet, Christian Chopin, Andrew Kylander-Clark, and Bradley Hacker

The Dora-Maira massif (Western Alps) is among the most studied subducted continental terranes in the world. It consists of a tectonic stack of <km-thick units metamorphosed at different grades, from blueschist to Ultra-High-Pressure (UHP) eclogite. While the UHP unit has been extensively studied, little is known about the chronology of subduction and exhumation of other units.

We here present new petrological observations and U-Pb geochronology on rutile and titanite to constrain the prograde, peak and retrograde evolution of distinct units.

 Rutile U-Pb geochronology on peak UHP assemblages is compared to existing results and is examined in light of closure temperatures for Pb diffusion. The results confirm peak metamorphism at ~35 Ma and fast cooling rates. This method is then applied to colder units (where closure temperatures are higher than peak temperatures, as shown by the preservation of the Permian age of pre-Alpine rutiles) and yields peak metamorphic ages of the different units between 39 and 32 Ma.

Rutile and titanite U-Pb geochronology help constrain the age of pervasive retrogression in the Dora Maira massif which is likely synchronous with the early exhumation of the lowermost continental unit and the transition from subduction to collision at 32-31 Ma.

We finally examine the possibility and potential consequences of melt circulation in the UHP unit during subduction.

How to cite: Bonnet, G., Chopin, C., Kylander-Clark, A., and Hacker, B.: Constraints on continental subduction in the Dora-Maira massif from rutile and titanite U-Pb geochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15318, https://doi.org/10.5194/egusphere-egu21-15318, 2021.

14:01–14:03
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EGU21-3543
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ECS
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Diego Rojo, Mauricio Calderón, Matias Ghiglione, Rodrigo Javier Suárez, Paulo Quezada, Francisco Hervé, Marly Babinski, and C. Mark Fanning

The Eastern Andean Metamorphic Complex (EAMC) in southwestern Patagonia (4°-52°S) is a 450 km long belt mainly composed by low-grade metasedimentary rocks of Upper Devonian-lower Carboniferous, and Permian-lower Triassic ages. Previous works have suggested a passive margin environment for the deposition of the protolith.  The EAMC comprise scarce interleaved tectonic slices of marbles, metabasites, and exceptional serpentinite bodies. At Lago O´Higgins-San Martin (48°30’S-49°00’S) the metasedimentary sucessions are tectonically juxtaposed with lenses of pillowed metabasalts and greenschists having OIB, N-MORB, BABB and IAT geochemical affinities. The Nd-isotopic composition of metabasalts is characterized by εNd(t=350 Ma) of +6 and +7. The metabasalts show no signal of crustal contamination, instead, the mantle source was probably modified by subduction components. New and already published provenance data based on mineralogy, geochemistry and zircon geochronology indicate that the quartz-rich protolith of metasandstones were deposited during late Devonian-early Carboniferous times (youngest single zircon ages around of latest Devonian-earliest Carboniferous times) sourced from igneous and/or sedimentary rocks located in the interior of Gondwana, as the Deseado Massif, for instance. Noticeable, the detrital age patterns of all samples reveal a prominent population of late Neoproterozoic zircons, probably directly derived from igneous and/or metaigneous rocks of the Brasiliano/Pan-African orogen or from reworked material from variably metamorphosed sedimentary units that crops out at the same latitudes in the extra-Andean region of Patagonia. We propose that the protolith of metabasites formed part of the upper part of an oceanic-like lithosphere generated in a marginal basin above a supra-subduction zone, where plume-related oceanic island volcanoes were generated. The closure of the marginal basin, probably in mid-Carboniferous times, or soon after. The oceanic lithosphere was likely underthrusted within an east-to-northeast-dipping subduction zone, where ophiolitic rocks and metasedimentary sequences were tectonically interleaved at the base of an accretionary wedge.

How to cite: Rojo, D., Calderón, M., Ghiglione, M., Suárez, R. J., Quezada, P., Hervé, F., Babinski, M., and Fanning, C. M.: Petrotectonic implications of metabasites of the Eastern Andean Metamorphic Complex at Lago O´Higgins-San Martin, southern Patagonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3543, https://doi.org/10.5194/egusphere-egu21-3543, 2021.

14:03–14:05
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EGU21-3833
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Wenbin Ning, Timothy Kusky, Junpeng Wang, Lu Wang, Hao Deng, Ali Polat, Bo Huang, Hongtao Peng, and Peng Feng

Subduction initiation and arc–polarity reversal have rarely been recognized in the Archean rock record. We document Neoarchean subduction initiation, fore-arc magmatism, and an arc–polarity reversal event from the Zunhua structural belt along the eastern margin of the Central Orogenic Belt (COB) of the North China Craton (NCC). The Zunhua ophiolitic mélange within the Zunhua structural belt is a mappable unit characterized by blocks of metamorphosed harzburgite/lherzolite, podiform chromite –bearing dunite, pyroxenite, amphibolite, metabasites (basalt and diabase) with rare intermediate volcanics, chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. New geochronological and geochemical analyses of magmatic blocks within the ophiolitic mélange show that the crustal magmatic rocks were produced in a fore-arc region at 2.55–2.52 Ga from depletion of the harzburgitic–lherzolitic mantle tectonites. Chemical, petrological, and temporal links between the depleted mantle blocks, and the suite of magmatic blocks derived from partial melting and metasomatism of these depleted mantle blocks, unequivocally shows that they represent part of the same original Neoarchean fore-arc ophiolite suite. After formation and accretion in the oceanic realm, the mélange was emplaced on the continental margin of the Eastern Block between 2.52–2.50 Ga, and underwent two stages of metamorphism at ca. 2.48–2.46 Ga and 1.81 Ga. Metamorphosed intermediate–mafic volcanic blocks exhibit systematic successive geochemical variations, from MORB-like to volcanic arc-like, and the N-MORB-like meta-basalts show remarkable similarity with the subduction initiation-related Izu–Bonin–Mariana (IBM) fore-arc basalts. We suggest that the Zunhua fore-arc complex records continuous geodynamic processes from subduction initiation to arc magmatism. The Zunhua ophiolitic mélange is part of a ca. 2.5 Ga suture between an oceanic arc of the COB and Eastern Block of the NCC. After the arc–continent collision, an arc–polarity reversal event has been proposed to initiate a new eastward–dipping subduction zone on the western side of the COB. This arc–polarity reversal can be traced for more than 1,600 km along the length of the orogen, similar in scale, geometry, and duration between collision and polarity flip to the present-day arc–polarity reversal of the Sunda–Banda arc during its ongoing collision with the Australia continent. This indicates that a life cycle of an Archean subduction zone, including birth (subduction initiation), maturity (arc magmatism), death (arc-continent collision) and re-birth (arc–polarity reversal), is recorded in the Zunhua ophiolitic mélange, and the geodynamics of plate tectonics at the end of the Archean was similar to that of today.

 

How to cite: Ning, W., Kusky, T., Wang, J., Wang, L., Deng, H., Polat, A., Huang, B., Peng, H., and Feng, P.: Life cycle of an Archean subduction zone from initiation to arc–polarity reversal: Insights from the Zunhua ophiolitic mélange, North China Craton, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3833, https://doi.org/10.5194/egusphere-egu21-3833, 2021.

14:05–14:07
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EGU21-9155
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Hongyuan Zhang, Zhibin Lei, Bo Yang, Qing Liu, Haijun Zhang, Yongquan Li, and Chenhang Lv

A 1:50000 regional survey, covering an area of about 2000 km2, was carried out in the Shangrimuce area of Qilian Mountain in Northwest China. The results show that during Caledonian, the northern margin of the Central Qilian block experienced collision with mature island arcs and subsequently northward expansion. In the Shangrimuce study area, five geological units have been identified; they are, form south to north, back-arc basin, early Ordovician island arc, inter arc basin, middle Late Ordovician island arc, and fore-arc and oceanic lithosphere amalgamation zone. 

(1) back-arc basin. In the Yangyuchi- Shule River- Cuorigang- Wawusi area, there may be a back-arc spreading basin, and there should be spreading basins in this area. It is speculated that there was a northward reverse subduction in the late Ordovician, accompanied by a syenite body, a broad spectrum dyke swarms and an accretionary wedge zone in the whole area.

(2) early Ordovician island arc. In the Shangrimuce-Dander area, the Proterozoic basement granitic gneiss, the early Ordovician island arc block and the high-pressure geological body all occur in the form of thrust horses, forming a double metamorphic belt, which reveals the existence of ocean subduction to south in the early Ordovician. 

(3) inter arc basin. On both banks of Tuolai River to the east of Yanglong Township, there are early Middle Ordovician inter-arc basins with oceanic crust. 

(4) middle Late Ordovician island arc. To the north of Tuolai River, there is a middle Late Ordovician island arc belt. Both sides of the island arc zone experienced strong ductile shear deformation, which recorded a complex arc-continent collision. 

(5) fore-arc and oceanic lithosphere amalgamation zone (Fig.1). The Yushigou area has developed a fore-arc and oceanic lithospheric amalgamation zone, with weakly deformed fore-arc flysch basin, strongly deformed siliceous rocks, pillow Basalt, diabase, gabbro, peridotite and other rock assemblages.

Combined with the characteristics of arc-continent collision zone in the Western Pacific, there are two stages of shear zone series (Fig.2). One is ductile shear zones formed by the South dipping gneissic belt, revealing the existence of oceanic subduction accretion wedge and emplacement of high-pressure rocks. Another superimposed one is north dipping. This indicates that the arc-continent collision caused by back-arc reverse subduction, which ultimately controls the overall geometric and kinematic characteristics of the shear zones in the region.

Figure 1 United sections showing a Caledonian trench-arc system in the Qilian Mountain, NW China.

Figure 2 Structural analysis at Hongyahuo, indicating two stages of deformation.

The research has been supported by projects from the Ministry of Land and Resources (No.201211024-04; 1212011121188) and the 2020 undergraduate class construction project from China University of Geosciences (Beijing) (No. HHSKE202003).

 

How to cite: Zhang, H., Lei, Z., Yang, B., Liu, Q., Zhang, H., Li, Y., and Lv, C.: Composition, structure and tectonic analysis of the Shangrimuce arc-continent collision zone on the northern margin of the Central Qilian, NW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9155, https://doi.org/10.5194/egusphere-egu21-9155, 2021.

14:07–14:09
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EGU21-14473
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ECS
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Mathieu Soret, Guillaume Bonnet, Philippe Agard, Kyle Larson, John Cottle, Benoit Dubacq, Alexander Kylander-Clark, Mark Button, and Nicolas Rividi

Subduction zones are crucial features of Earth’s plate tectonics, yet subduction initiation remains enigmatic and controversial. Herein, we reappraise the timing of formation of the first fragments detached from the leading edge of the downgoing slab during subduction initiation (i.e., the Semail metamorphic sole; Oman–United Arab Emirates). Based on geochronology and phase equilibrium modeling, we demonstrate that subduction initiated prior to 105 Ma and at a slow pace (< cm/yr). Subduction stagnated at relatively warm conditions (15–20°C/km) for at least 10 Myr before evolving into a faster (≥ 2–5 cm/yr) and colder (~7°C/km) self-sustained regime.  Subduction unlocking at 95-96 Ma, through the progressive change of the interplate thermo-mechanical structure, triggered the onset of slab retreat, large-scale corner flow and fast ocean spreading in the overriding plate. These results reconcile conflicting analogue and numerical subduction initiation models and reveal the thermal, mechanical and kinematic complexity of early subduction steps.

How to cite: Soret, M., Bonnet, G., Agard, P., Larson, K., Cottle, J., Dubacq, B., Kylander-Clark, A., Button, M., and Rividi, N.: Slow subduction initiation drives fast mantle upwelling and lithosphere formation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14473, https://doi.org/10.5194/egusphere-egu21-14473, 2021.

14:09–14:11
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EGU21-6388
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ECS
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Brandon Shuck, Sean Gulick, Harm Van Avendonk, Michael Gurnis, Rupert Sutherland, Joann Stock, Erin Hightower, and Jiten Patel

Subduction zones are fundamental to Earth’s plate tectonic history yet details of how they initiate remain enigmatic. Geodynamic models suggest that early stages of subduction depend on whether underthrusting is driven by horizontal or vertical forces. If horizontal forces dominate, the upper plate experiences compression and uplift followed by extension and subsidence, whereas vertically-forced subduction involves only extension. Geologic evidence from the Izu-Bonin-Mariana forearc supports a ~1 Myr rapid transition, whereas observations from Oman indicate a >8 Myr time lag between initial underthrusting and the onset of upper plate extension. We present seismic images of the incipient Puysegur subduction zone south of New Zealand. Our data show evidence for a stress signal (compression followed by extension) that spread from north to south as the trench initiated and propagated along the plate boundary. Both the magnitude and duration of the compressional phase diminish from ~8 Myrs long in the north to ~5 Myrs in the south. This timing indicates that the transition to self-sustaining subduction is more rapid when an adjacent downgoing slab contributes a driving force that aids subduction initiation. We therefore argue for a new framework in which horizontal forces dominate at sites of subduction nucleation and vertical forces gradually strengthen during later propagation as the developing plate boundary weakens and the slab-pull force intensifies. Our findings corroborate evidence for ancient horizontally-forced subduction initiation events and suggest that the geologic record may be biased, since vertically-forced scenarios of subduction propagation are more likely to be preserved than destructive subduction nucleation events.

How to cite: Shuck, B., Gulick, S., Van Avendonk, H., Gurnis, M., Sutherland, R., Stock, J., Hightower, E., and Patel, J.: 4D stress signals in the upper plate record subduction nucleation and lateral propagation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6388, https://doi.org/10.5194/egusphere-egu21-6388, 2021.

14:11–14:13
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EGU21-10467
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ECS
Sambuddha Dhar, Jun Muto, Yoshiaki Ito, Satoshi Miura, James D. P. Moore, Yusaku Ohta, and Takeshi Iinuma

During the 2011 Tohoku-oki earthquake the pacific coast of northeast Japan experienced significant subsidence, while in the years after it has undergone a continuous phase of uplift during the post-seismic period. The dense geodetic network deployed by GEONET and Tohoku university between 2011 and 2016 have captured variations in surface deformation along the coast, highlighting rapid uplift rates of ~7 cm/year on the Miyagi coast (Muto et al., 2019, Sci.Adv.) and ~3-4 cm/year on the Fukushima and Iwate coasts. Previous studies in the last decade have revealed the post-seismic deformation is due to a combination of both rapid viscoelastic flow and stress-driven afterslip, explaining the post-seismic vertical deformation pattern over northeast Japan as well as unravel its associated rheological complexity (e.g., Agata et al., 2019, Nat. Commun; Freed et al., 2017, EPSL; Hu et al., 2016, JGR; Muto et al., 2019, Sci.Adv.). Furthermore, continuous coastal uplift has had societal consequences, where the piers at the port are no longer suited to conduct many activities, particularly those for the fish industry. The large co-seismic subsidence of coastal areas caused the submersion of port piers, with rapid rebuilding to return the now submerged piers to sea-level. Nevertheless, the continuous uplift in the post-seismic period has now raised these rebuild piers above sea level and necessitates reduction in height back to sea level again (Iinuma, 2018, JDR). In this presentation, we employ forward modeling to improve estimates of future uplift and the time required for full recovery of coastal regions to their pre-event relative sea level.

 

We present a numerical model using laboratory-derived constitutive laws and compare our modeled displacement with the geodetic observations (Ozawa et al., 2012, JGR; Tomita et al., 2017, Sci.Adv.; Watanabe et al., 2014, GRL). The model is constrained by terrestrial and seafloor geodetic observations in both horizontal and vertical components and incorporates a three-dimensional heterogeneous viscoelastic rheology fully coupled with stress-driven afterslip on the plate interface.

 

Our model exhibits good agreement with the cumulative displacements, both in magnitude and azimuthal direction. We extend the time-series simulation for a further 20 years and estimate the recovery time to pre-event levels for the GNSS sites along the coastal areas. Our results show a recovery period of ~18 years after the mainshock for Ishinomaki site in Miyagi prefecture, which had the largest coseismic subsidence (up to ~1.2 m). We also estimate a recovery period of ~14-16 years for the coastal areas of Iwate and Fukushima prefectures, which experienced coseismic subsidence of ~0.5 m. The model adds an improvement to the previous estimates (Iinuma, 2018, JDR) by incorporating consideration of the coupling of viscoelastic relaxation and stress-driven afterslip.

How to cite: Dhar, S., Muto, J., Ito, Y., Miura, S., Moore, J. D. P., Ohta, Y., and Iinuma, T.: Post-seismic recovery of subsided coastal northeast Japan after the 2011 Tohoku-oki earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10467, https://doi.org/10.5194/egusphere-egu21-10467, 2021.

14:13–15:00
Chairpersons: Francesca Meneghini, Ake Fagereng, Martijn van den Ende
The role of fluids in subduction processes
15:30–15:35
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EGU21-13926
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ECS
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solicited
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Cailey Condit, Victor Guevara, Melodie French, Adam Holt, and Jonathan Delph

Feedbacks amongst petrologic and mechanical processes along the subduction plate boundary play a central role influencing slip behaviors and deformation styles. Metamorphic reactions, resultant fluid production, deformation mechanisms, and strength are strongly temperature dependent, making the thermal structure of these zones a key control on slip behaviors.

 

Firstly, we investigate the role of metamorphic devolatilization reactions in the production of Episodic Tremor and Slip (ETS) in warm subduction zones. Geophysical and geologic observations of ETS hosting subduction zones suggest the plate interface is fluid-rich and critically stressed, which together, suggests that this area is a zone of near lithostatic pore fluid pressure.  Fluids and high pore fluid pressures have been invoked in many models for ETS. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or via up-dip transport from greater depths remains an open question. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface along predicted pressure–temperature (P-T) paths for the plate boundary along Cascadia, Nankai, and Mexico which all exhibit ETS at depths between 25-65 km. Our models suggest that 1-2 wt% H2O is released at the depths of ETS along these subduction segments due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across this narrow P-T range to cause high pore fluid pressures. Punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction margins, and up-dip migration of fluids from deeper in the subduction zone is not required to produce ETS-facilitating high pore fluid pressures. These dehydration reactions not only produce metamorphic fluids at these depths, but also result in an increased strength of viscous deformation through the breakdown of weak hydrous phases (e.g., chlorite, glaucophane) and the growth of stronger minerals (e.g., garnet, omphacite, Ca-amphibole). Lastly, we present preliminary data on viscosity along warm subduction paths showing the locations of these dehydration pulses correlate with viscosity increases in mafic lithologies along the shallow forarc.

How to cite: Condit, C., Guevara, V., French, M., Holt, A., and Delph, J.: Warm thermal structures in subduction zones lead to ample dehydration at the depths of deep slow slip and tremor and resultant transformations in viscous rheology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13926, https://doi.org/10.5194/egusphere-egu21-13926, 2021.

15:35–15:40
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EGU21-15157
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solicited
Catriona D. Menzies, Olivier Sissmann, Jeffrey G. Ryan, C. Geoff Wheat, Adrian J. Boyce, Thomas B. Chalk, Gavin L. Foster, and Damon A. H. Teagle

Geological processes in subduction zones strongly influence seismicity, igneous activity, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and overlying mantle wedge. Direct study of active slab evolution is inhibited by the great depths at which these processes occur and there is a dearth of physical samples to assess the state of water-rock-sediment reactions, thermal and pressure conditions, and physical properties of materials within the subduction channel.

The drilling of serpentinite mud volcanoes in the Mariana forearc provides a telescope into these deep processes and allows us to sample fluids and xenoliths from the subducting slab and forearc mantle.  Fluid-laden serpentinite is transported along active extensional faults in the upper plate and seeps out at mud volcano edifices. There is widespread evidence for episodic voluminous serpentine eruptions, likely related to seismic events. Mud volcanoes are found across the forearc and sample the slab interface from 13 to 19 km depth. Samples obtained over three Scientific ocean drilling legs (ODP Legs 125 and 195; IODP Leg 366) and additional ROV expeditions elucidate the evolution of fluid production, reaction and exchange, during the progressive subduction of the down-going plate.

Fluid analyses show clear trends in pore water chemical and isotopic composition with progressive subduction. These parameters can be used to assess the thermal state of the subduction channel at different depths, identify the reactions controlling fluid releases, and to estimate fluid fluxes. Pore waters from the shallowest depths-to-slab (13-16 km) are Ca and Sr-enriched compared to seawater, but otherwise solute poor, low alkalinity fluids of pH ~11. In contrast, more deeply derived fluids (>18 km) have higher pH (12.5), reduced concentrations of Ca and Sr and elevated DIC, Na and Cl, as well as B and K compared to seawater – these changes are associated with the breakdown of slab sheet silicate phases. These waters also have higher δD and δ11B values than shallower waters (δD values up to +16 ‰; δ11B ~ 14-15 ‰ cf. δD < 0‰; δ11B ~ 12-13 ‰). PHREEQC modelling indicates pore water chemical evolution reflects mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate; however, a component from sediment sources is a likely contributor, especially for those mud volcanoes near the trench.

Our new data indicate that the lawsonite-epidote mineral transformation boundary (~250 °C, >18 km depth) is an important source of devolatilization waters and may also drive slab carbonate breakdown, despite its apparent thermodynamic stability at such temperatures and pressures. At shallower depths, the main reactions controlling fluid liberation are sediment compaction (<13 km) followed by clay diagenesis and desorbed water release (>13 km depth). This study thus provides direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate.

How to cite: Menzies, C. D., Sissmann, O., Ryan, J. G., Wheat, C. G., Boyce, A. J., Chalk, T. B., Foster, G. L., and Teagle, D. A. H.: Tracing the Evolution of Slab Fluids during Progressive Subduction: Insights from Serpentinite Mud Volcanoes in the Mariana Forearc, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15157, https://doi.org/10.5194/egusphere-egu21-15157, 2021.

15:40–15:42
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EGU21-13728
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ECS
Luca Dal Zilio and Taras Gerya

A major goal in earthquake physics is to derive a constitutive framework for fault slip that captures the dependence of friction on lithology, sliding velocity, temperature, and pore fluid pressure. Here, we present a newly-developed two-phase flow numerical model — which couples solid rock deformation and pervasive fluid flow — to show how crustal stresses and fluid pressures within subducting megathrust evolve before and during slow slip and fast events. This unified 2D numerical framework couples inertial mechanical deformation and fluid flow by using finite difference methods, marker-in-cell technique, and poro-visco-elasto-plastic rheology. An adaptive time stepping allows the correct resolution of both long- and short-time scales, ranging from years to milliseconds during the dynamic propagation of dynamic rupture.

We investigate how permeability and its spatial distribution control the interseismic coupling along the megathrust interface, the interplay between seismic and aseismic slip, and the nucleation of large earthquakes. While a constant permeability leads to more regular seismic cycles, a depth dependent permeability contributes substantially to the development of two distinct megathrust zones: a shallow, locked seismogenic zone and a deep, narrow aseismic segment characterized by slow-slip events. Furthermore, we show that without requiring any specific friction law, our models reveal that permeability, episodic stress transfer and fluid pressure cycling control the predominant slip mode along the subduction megathrust. Furthermore, we analyze how rate dependent strength and dilatation affect rupture propagation and arrest. Our preliminary results show that fluid-solid poro-visco-elasto-plastic coupling behaves similarly to rate- and state-dependent friction. In this context, fluid pressure plays the role of state parameter whose time evolution is governed by: (i) the short-term elasto-plastic collapse of pores inside faults during the rupture (coseismic self-pressurization of faults) and (ii) the long-term pore-pressure diffusion from the faults into surrounding rocks (post- and interseismic relaxation of fluid pressure). This newly-developed numerical framework contributes to improve our understanding of the physical mechanisms underlying large megathrust earthquakes, and demonstrate that fluid play a key role in controlling the interplay between seismic and aseismic slip.

How to cite: Dal Zilio, L. and Gerya, T.: Episodic fluid pressure cycling controls earthquake sequences on subduction megathrusts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13728, https://doi.org/10.5194/egusphere-egu21-13728, 2021.

15:42–15:44
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EGU21-3954
Whitney Behr and Taras Gerya

The deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tremor and low-frequency earthquakes. Observations from exhumed rocks suggest that the deep subduction interface is a shear zone in which frictional lenses are embedded in a weaker, distributed viscous matrix deformed under high fluid pressures and low stresses. Here we use numerical models to explore the transient slip characteristics of finite-width frictional-viscous shear zones. Our model formulation utilizes an invariant form of rate- and state-dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface) with inclusions (clasts) of varying sizes, aspect ratios, distributions and viscosity contrasts with respect to the surrounding matrix. The entire shear zone exhibits the same velocity-weakening RSF parameters, but the low viscosity matrix in the shear zone has the capacity to switch between RSF and linear viscous creep as a function of its local viscosity and stress state. Results show that for a range of matrix viscosities near a threshold viscosity (representative of the frictional-viscous transition), viscous damping and stress heterogeneity in these shear zones both 1) sets the ‘speed limit’ for earthquake ruptures that nucleate in clasts such that they propagate at velocities similar to observed slow slip events; and 2) simultaneously permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. For reasonable input parameters, modeled events have moment-duration statistics, stress drops, and rupture propagation rates that match natural slow slip events. Events resembling very low-frequency earthquakes appear to be favored at high clast densities and low matrix viscosities, whereas longer duration, higher-magnitude slow slip events are favored at intermediate clast densities and near-threshold viscosities. These model results have potential to reconcile geophysical constraints on slow slip phenomena with the exhumed geological record of the slow slip environment.

How to cite: Behr, W. and Gerya, T.: Seismic and transient slip characteristics of frictional-viscous shear zones in subduction environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3954, https://doi.org/10.5194/egusphere-egu21-3954, 2021.

15:44–15:46
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EGU21-1885
Kohtaro Ujiie, Kazuya Noro, Norio Shigematsu, Åke Fagereng, Naoki Nishiyama, Christopher Tulley, and Haruna Masuyama

Aseismic megathrust slip down-dip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. Here, we show that subduction mélanges deformed at ~370–500 °C in warm-slab environments record fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions induced albitization of metapelite, resulting in depth-dependent rheological behavior. In a mélange deformed at ~370 °C, near the down-dip limit of the seismogenic zone, very fine-grained metasomatic albite facilitated grain boundary diffusion creep at stresses less than those in the surrounding metapelite and metabasalt, contributing to an overall decreased megathrust strength. In a mélange deformed at ~500 °C, near the mantle wedge corner, metasomatic reactions led to brittle fracturing, albite grain growth, and incorporation of strengthened albitized metapelite blocks into a chlorite-actinolite matrix deforming at locally elevated strain rate of ~10-10 s-1. We suggest that metasomatic reactions facilitate localized changes in megathrust slip mode with depth, potentially providing a mechanism for change from viscous creep to SSEs with tremor.

How to cite: Ujiie, K., Noro, K., Shigematsu, N., Fagereng, Å., Nishiyama, N., Tulley, C., and Masuyama, H.: Localized megathrust slip controlled by metasomatic reactions in subduction mélanges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1885, https://doi.org/10.5194/egusphere-egu21-1885, 2021.

15:46–15:48
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EGU21-3326
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ECS
Fabián Gutiérrez-Aguilar, David Hernández-Uribe, Robert M. Holder, and Cailey B. Condit

Subduction controls key geological processes at convergent margins including seismicity and resultant seismic hazard. The September 19th 2017 Mw7.1 Mexican earthquake nucleated ~250 km from the trench within the Cocos plate near its Moho, ~57 km below Earth’s surface. The prevailing hypothesis suggests that this earthquake resulted from bending stresses occurring at the flat-to-steep subduction transition. Here, we present an alternative, but not mutually exclusive, hypothesis: the dehydration reaction brucite + antigorite = olivine + H2O in the slab mantle controls intermediate-depth seismicity along the flat portion of the subducted Cocos plate. This reaction releases a substantial amount of H2O, resulting in a large positive volume change, and thus in an increase in pore fluid pressure at the appropriate depth–temperature conditions to cause the Puebla-Morelos and other intraslab earthquakes in Mexico. The amount of H2O released by this reaction depends on the degree of serpentinization of the oceanic mantle prior to subduction. Only oceanic mantle with > 60% serpentinization—as expected along abundant deep extensional faults at the mid-ocean-ridge or where the plate bends at the outer rise—will stabilize brucite, and thus, will experience this reaction at the same depths where the September 19th 2017 earthquake nucleated.

How to cite: Gutiérrez-Aguilar, F., Hernández-Uribe, D., M. Holder, R., and B. Condit, C.: The Mw7.1 September 19th Puebla-Morelos (Mexico) earthquake triggered by brucite and antigorite dewatering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3326, https://doi.org/10.5194/egusphere-egu21-3326, 2021.

15:48–15:50
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EGU21-7741
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ECS
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Jesús Muñoz-Montecinos, Samuel Angiboust, Antonio Garcia-Casco, Johannes Glodny, and Gray Bebout

Devolatilization and fluid-rock interaction processes along subduction interfaces, in particular at depths where episodic tremor and slip events (ETS) are inferred, are evidenced by the occurrence of metamorphic veins in exhumed metamorphic terranes. We investigate the late Cretaceous lawsonite blueschist-facies Seghin complex, part of the Zagros suture zone (Iran), a well-preserved paleo-subduction mélange composed of an antigorite-rich matrix wrapping foliated metatuffs and minor carbonate-bearing metasediments. We first focus on characterizing the relative chronology, conditions of deformation and potential fluid source(s) of Lws+Cpx+Gln veins and aragonite-filled explosive hydraulic breccias. Petrological, geochemical as well as O-C and Sr-Nd isotopic systematics of silicate-rich veins suggest formation mostly from internal devolatilization. This stage is followed at near peak burial conditions by pervasive, externally-derived fluid influx events, with fluids characterized by REE enrichments, and geochemical signatures indicating mixing between metasedimentary-derived fluids and far-traveled mafic-ultramafic-derived fluids. Our geochemical and petrological observations suggest that a host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix.

The high pore fluid pressures that enabled the formation of these deep veins also enabled the formation of shallower fault-related rocks including breccias, foliated cataclasites and fluidized ultracataclasites, intimately associated with extensional Gln-bearing veins and Lws+Gln+Ph+Ab fluid-filled pockets. Mineral assemblages reveal that this faulting occurred upon exhumation throughout the lawsonite blueschist-facies (i.e. 35 to 20 km depth). Crosscutting relationships among multiple generations of fluidized ultracataclasites and extensional veins show that episodic seismic faulting and hydrofracturing were contemporaneous processes. Mechanical modelling confirms that the studied fault-related features can only form under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promotes recurrent seismic faulting. We propose a large-scale tectonic model in which deeply produced H2O-rich fluids are transported as highly pressurized “pulses” over tens of km parallel to the subduction interface, triggering episodic hydrofracturing and host rock-buffered isotopic homogenization within the ETS region. The mechanical consequence of these events is the triggering of unstable slip within the seismogenic window, as deduced in this unique record of blueschist-facies crustal paleo-earthquakes. These results shed a new light on the physical nature of the numerous moderate magnitude events (Mw=3-6) that are extensively recorded nowadays in Mariana-type plate boundary systems.

How to cite: Muñoz-Montecinos, J., Angiboust, S., Garcia-Casco, A., Glodny, J., and Bebout, G.: Large-scale fluid circulation in deep subduction interfaces: implications on fast and slow earthquake-related processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7741, https://doi.org/10.5194/egusphere-egu21-7741, 2021.

15:50–15:52
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EGU21-7760
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Makoto Otsubo, Kohtaro Ujiie, Hanae Saishu, Ayumu Miyakawa, and Asuka Yamaguchi

Pore fluid pressure (Pf) is of great importance to understand slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélange eastern Kyushu, SW Japan. The mélange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: a few tens of microns) may represent episodic tremor and slow slip (Ujiie et al., 2018). The foliation-parallel extension cracks can function as the fluid pathway in the mélange. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel extension vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal σ1-axis trending NNW–SSE and a sub-vertical σ3-axis, and the driving pore fluid pressure ratio P* (P* = (Pf – σ3) / (σ1 – σ3)) is ~0.1. When the pore fluid pressure exceeds σ3, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds σ3 is the pore fluid overpressure ΔPf (ΔPf = Pf – σ3). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). Pf and ΔPf in the case of the Makimine mélange are ~280 MPa and 80–160 kPa (assuming depth = 10 km, density = 2800 kg/m3, tensile strength = 1 MPa and Young’s modulus = 7.5–15 GPa). When the pore fluid overpressure is released, the cracks are closed and the reduction of pore fluid pressure is stopped (Otsubo et al., 2020). After the pore fluid overpressure is reduced, the normalized pore pressure ratio λ* (λ* = (Pf – Ph) / (Pl – Ph), Pl: lithostatic pressure; Ph: hydrostatic pressure) is ~1.01 (Pf > Pl). The results indicate that the pore fluid pressure constantly maintains the lithostatic pressure during the extension cracking along the foliation.

References: Gudmundsson (1999) Geophys. Res. Lett., 26, 115–118; Jolly and Sanderson (1997) Jour. Struct. Geol., 19, 887–892; Otsubo et al. (2020) Sci. Rep., 10:12281; Palazzin et al. (2016) Tectonophysics, 687, 28–43; Sato et al. (2013) Tectonophysics, 588, 69–81; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371–5379, https://doi.org/10.1029/2018GL078374.

How to cite: Otsubo, M., Ujiie, K., Saishu, H., Miyakawa, A., and Yamaguchi, A.: Temporal changes in pore fluid pressure during slow earthquake cycle estimated from foliation-parallel extension cracking, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7760, https://doi.org/10.5194/egusphere-egu21-7760, 2021.

15:52–15:54
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EGU21-10988
Audrey Galve, Andreas Rietbrock, Philippe Charvis, Giorgio De la Torre, Sandro Vaca, Monica Segovia, Anne Meltzer, and Susan Beck and the HIPER Team

Identifying the circulation of fluids in subduction zone system and understanding their role on the megathrust fault slip modes remains one of the outstanding challenges in Earth Sciences. As these faults have the capacity to generate mega-earthquakes, the associated hazard to the society is significant.

The Ecuadorian subduction zone is one of the places in the world where very large earthquakes can occur, as shown by the Mw 8.8 earthquake in 1906. In April 2016, a Mw 7.8 earthquake broke the southern part of the 1906 earthquake rupture zone, causing hundreds of deaths and millions of dollars in damages along an increasingly populated coastline. The seismological and geodetic network in place since several years and a dense post-seismic deployment, contributed to observe and define the rupture zone and areas affected by aseismic slip on the shallowest portion of the megathrust fault. Those hints of transient slip behaviors, for which fluids have been invoked to explain their occurrence, bring Ecuador to the forefront of natural laboratories to study the link between fluids and slip mode.

The HIPER marine campaign in March/April 2020 on board R/V Atalante was designed to acquire a dense active/passive, 2D/3D, onshore/offshore dataset, and in particular to derive the role of fluids in slip modes on the Ecuadorian margin. Thanks to an international consortium (Ecuador, Germany, France, United States) we had access to a large number of OBS (47) and land stations (~700) to record both R/V Atalante’s shots and the seismic activity.

The large-N experiment allowed a high density onshore/offshore deployment to perform shots and earthquakes FWI (Full Waveform Inversion) and obtain sufficient resolution to tackle the role of fluids with respect to interplate roughness, the nature of sediments, upper plate and lower plate’s structural heterogeneity in seismic/aseismic slip behavior.

A few days after starting the marine campaign, countries closed their frontiers due to the Covid-19 health crisis. The HIPER marine campaign was stopped and scientists on board were repatriated home. During the 10 days out of the 42 days planned, we managed to acquire the planed multichannel seismic reflection lines (abstract by L. Schenini - TS12.1). However, we collected only one of the three planned OBS wide-angle seismic lines (abstract by A. Skrubej - GD4.3), and no OBSs have been deployed for seismic activity monitoring.

The unique joint reflection/refraction line is perpendicular to the trench, sampling the megathrust fault where aseismic slip occurs, north of Pedernales. On our tomographic inversion, iso-velocity contours characterizing the oceanic crust entering the subduction, are downwards deflected 15 km before the trench. Such observation could be related to fluids affecting the crust and the upper mantle. On MCS image, we observe within the trench a rough oceanic basement, with a horst-like topographic high which outcrops at sea-bottom. Such structure could facilitate fluids infiltrating the crust before the trench in addition to bending faults, and possibly explain low Vp anomaly obtained on our coincident tomographic image.

A new marine campaign HIPER 2.0 is rescheduled in March/April 2022 to acquire the missing data.

How to cite: Galve, A., Rietbrock, A., Charvis, P., De la Torre, G., Vaca, S., Segovia, M., Meltzer, A., and Beck, S. and the HIPER Team: How fluids impact seismic/aseismic slip in the Ecuadorian subduction zone? The HIPER marine project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10988, https://doi.org/10.5194/egusphere-egu21-10988, 2021.

15:54–15:56
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EGU21-9339
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Naoki Nishiyama, Kohtaro Ujiie, and Masayuki Kano

Repeated slow earthquakes downdip of the seismogenic zones may trigger megathrust earthquakes by transferring stress to the seismogenic zones. Geodetic observations have suggested that the recurrence intervals of slow earthquakes decrease toward a next megathrust earthquake. However, the temporal variation in recurrence intervals of slow earthquakes during megathrust earthquake cycles remains poorly understood due to the limited duration of geodetic and seismological monitoring of slow earthquakes. The quartz-filled, crack-seal shear veins in the subduction mélange deformed near the downdip limit of seismogenic zone in warm-slab environments record the cyclic changes in the inclusion band spacing in the range of 5–65 μm. The two-phase primary fluid inclusions in quartz between inclusion bands show various vapor/liquid ratios regardless of inclusion band spacing, suggesting a common occurrence of fast quartz sealing due to a rapid decrease in quartz solubility associated with a large fluid pressure reduction. A kinetic model of quartz precipitation, considering a large fluid pressure change and inclusion band spacings, indicates that the sealing time during a single crack-seal event cyclically decreased and increased in the range of 0.2–2.7 years, with minimum one cycle duration estimated to be 31–93 years. The ranges of sealing time and one cycle duration may be comparable to the recurrence intervals of slow earthquakes and megathrust earthquakes, respectively. We suggest that the spatial change in the inclusion band spacing is a potential geological indicator of the temporal changes in slow earthquake recurrence intervals, particularly when large fluid pressure reduction occurred by brittle fracturing.

How to cite: Nishiyama, N., Ujiie, K., and Kano, M.: Spatial changes in inclusion band spacing as an indicator of temporal changes in slow earthquake recurrence intervals, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9339, https://doi.org/10.5194/egusphere-egu21-9339, 2021.

15:56–15:58
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EGU21-320
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ECS
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Francesco Giuntoli and Giulio Viola

The geological record of deep seismic activity in subduction zones is generally limited due to common rock overprinting during exhumation and only a few regions allow studying well-preserved exhumed deep structures. The Northern Apennines (Italy) are one such area, granting access to continental units (Tuscan Metamorphic Units) that were subducted to high-pressure conditions, were affected by brittle-ductile deformation while accommodating deep tremor and slip and then exhumed back to surface, with only minor retrogression.

Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal a metamorphosed broken formation composed of boudinaged metaconglomerate levels enveloped by metapelite displaying a pervasive mylonitic foliation. Shear veins occur in both lithologies, but are more common and laterally continuous in the metapelite. They are mostly parallel to the foliation and composed of iso-oriented stretched quartz and Mg-carpholite (XMg>0.5) fibres, which are single-grains up to several centimetres long. These fibres define a stretching direction coherent with that observed in the metaconglomerate and metapelite, which is marked by K-white mica and quartz. Thermodynamic modeling constrains the formation of the high-pressure veins and the mylonitic foliation to ~ 1 GPa and 350°C, corresponding to c. 30-40 km depth in the subduction channel.

Shear veins developed in subducted (meta)sediments are a key indicator of episodic tremor and slip (e.g. 1). We propose that these structures reflect the repeated alternation of localised brittle failure, with shear veins development, and more diffuse viscous deformation. These cycles were probably related to the fluctuation of pore pressure that repeatedly reached lithostatic values. Concluding, these structures can be considered the geological record of episodic tremors and slip occurring at >30 km of depth in the Apenninic subduction channel.

1. Fagereng, Å., Remitti, F. & Sibson, R. H. Incrementally developed slickenfibers — Geological record of repeating low stress-drop seismic events? Tectonophysics 510, 381–386 (2011).

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 839779.

How to cite: Giuntoli, F. and Viola, G.: Brittle-ductile deformation in high-pressure continental units and deep episodic tremor and slip, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-320, https://doi.org/10.5194/egusphere-egu21-320, 2021.

15:58–16:00
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EGU21-9856
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ECS
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Kristijan Rajic, Hugues Raimbourg, Vincent Famin, Donald Fisher, and Kristin Morell

The Kodiak archipelago (Southwest Alaska) represents a well exposed paleo-accretionary prism with its modern equivalent to the modern Alaskan Trench further southeast. The complex consists of metasedimentary and magmatic rocks, whose age span from the Triassic-Jurassic units on the northwestern side of the archipelago towards the Miocene units on the southeast. The complex dominantly consists of trench sediments, in which the sedimentary stratification is still visible. In addition, two tectonic mélanges, composed of lenses of metabasites embedded in sheared metasediments, are intercalated between the coherent formations. We carried out an extensive field survey to describe the kinematics and temperature conditions of deformation across the whole subduction complex.

Mélange terrains are characterized by subduction-related deformation in the form of a pervasive network of top-to-the-trench shear zones. In contrast, we observed wider range of deformation geometries in coherent units: The Kodiak Landward belt is characterized by top-to-the-trench simple shear. In the Kodiak Central belt, strain geometry varies spatially from dominant top-to-the-trench simple shear to horizontal extension evidenced by conjugate sets of extensional shear bands. Further to southeast, the Kodiak Seaward belt and the Ghost Rocks Formation are characterized by horizontal shortening with conjugate thrust faults and symmetric folds. Post-Paleocene deformation includes strike-slip faulting in the southeastern part as well as in the Kodiak granite, which was previously described as completely undeformed. The main tectonic contact in the area is the Uganik Thrust, delimiting the Uyak Complex and the Kodiak Formation. The thrust consists of a meter-thick mylonitic zone of the hanging wall material (Uyak Complex), with significantly deformed foot wall (Kodiak Formation). Finally, extension can be observed in the Narrow Cape Formation, unconformably overlying the Ghost Rocks mélange in the SE margin of the belt. Such extension predates the very recent-to-present deformation, characterized by normal faulting and block tilting within the SE margin.

Preliminary results of Raman spectroscopy of carbonaceous material (RSCM) provide essential information as to the large-scale thermal structure of the accretionary prism. In the investigated profile, running from southeastern margin towards the northwest, the temperature does not increase monotonically towards the inner part of the wedge. Indeed, the highest temperatures (>300 ℃) are found within the central part of the complex, in very thick turbiditic series accreted in a short period of time in the Paleocene. The thermal gap at the unconformity between the Ghost Rocks and Narrow Cape formations indicates fast uplift after accretion, followed by erosion, subsidence and sedimentation of Narrow Cape sediments. On the other side, no thermal gap is found around the Uganik Thrust like described at other OOST thrusts, which suggests that its activity predates exposure to the peak temperature.

 

How to cite: Rajic, K., Raimbourg, H., Famin, V., Fisher, D., and Morell, K.: Deformation structure and peak-metamorphic temperature in Kodiak accretionary complex, Alaska, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9856, https://doi.org/10.5194/egusphere-egu21-9856, 2021.

16:00–16:02
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EGU21-10074
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Hugues Raimbourg, Vincent Famin, Kristijan Rajic, Saskia Erdmann, Benjamin Moris-Muttoni, Donald Fisher, and Kristin Morell

Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary Complex in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, southern nappes of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks that have experienced the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks that have experienced the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire.

Based on laser ablation analysis and electron microprobe mapping, variations in CL colors appear correlated with the trace element content of quartz. The highly luminescent quartz contains high concentrations of aluminum (Al) and lithium (Li), up to 3000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for SiàAl substitution.

Due to their ubiquitous presence in various settings, the variations in CL colors in the lower temperature range reflect a common, general process. We interpret these cyclic growth structures as a result of deformation/fracturing events, which triggered transient changes in fluid pressure. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at lower pace after fluid pressure evolved to equilibrium conditions, leading to the formation of CL-brown quartz with fewer substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.

The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. 

We conclude that quartz trace element content might be a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure are smaller.

How to cite: Raimbourg, H., Famin, V., Rajic, K., Erdmann, S., Moris-Muttoni, B., Fisher, D., and Morell, K.: Fluid pressure variations recorded by quartz vein geochemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10074, https://doi.org/10.5194/egusphere-egu21-10074, 2021.

16:02–17:00