Despite extensive research, intraplate earthquakes and required intraplate deformation remain relatively unexplained. To explore this problematic, we tested the possibility that these could derive from the dynamic interaction between the lithosphere and the upper mantle. This was performed by conducting a thorough geophysical exploration of a region with both low plate velocities and clear asthenosphere dynamics, specifically the Gulf of Guinea (GOG) and adjacent Western Africa.

In this work, we developed 3D numerical geodynamic models of the asthenosphere-lithosphere interaction in the GOG, ran with the state-of-the-art LaMEM modelling code. To assess the contribution of individual intraplate deformation sources, we tested various initial/boundary conditions namely: (a) the spreading rate of the individual segments of Central Atlantic mid-ocean ridge, (b) the presence/absence of weak zones, such as the Romanche or Central-African shear zones, as well as (c) the stress contribution by an active mantle plume head with varying width. Seismicity data was utilized as a criterion to assess the validity of the modelled stress/strain localization sites.

Our results suggest that intraplate deformation within the GOG is mostly controlled by the spreading rate of the mid-ocean ridge, with different localization sites deriving from their relative proximity to the shear zones and plume head. This work aims to expand our knowledge of intraplate deformation mechanisms and to contribute towards improving seismic hazard assessment away from plate boundaries.

This work was supported by the European Union and the Instituto Dom Luiz (IDL) Project under Grant UIDB/50019/2020, and it uses computational resources provided by C4G (Collaboratory for Geosciences) (Ref. PINFRA/22151/2016). It was also partly supported by the Fundação para a Ciência e a Tecnologia (FCT) in the content of the Project SHAZAM “Sismicidade e Perigosidade da Margem Atlântica sub-Saariana,” with the reference PTDC/CTA/GEO/31475/2017; POCI-01-0145-FEDER-031475, co-financed by FEDER-COMPETE/POCI 2020.

How to cite: Almeida, J., Riel, N., Neres, M., Mohammadigheymasi, H., Custódio, S., and Dumont, S.: Lithosphere-asthenosphere interaction as the source for intraplate deformation in the Gulf of Guinea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6598, https://doi.org/10.5194/egusphere-egu23-6598, 2023.

Collapsed calderas are circular to elongated large depressions originating from the subsidence induced by depletion and/or migration of magma from a shallow or deep reservoir during eruptions. Despite being distributed in all tectonic settings, they are particularly important in extensional settings where are often associated with rifting processes, e.g., the East African Rift System. Therefore, their structural architecture can be strongly perturbed by extensional faults associated with regional extension or related to earlier stages of caldera formation. Calderas often bear an elongated shape in plain view, and have been considered valuable proxies for the regional stress (e.g., Nakamura, 1977) and regional strain (e.g. Casey et al., 2006). Moreover, other authors have related the elongated calderas to the influence of preexisting structures reactivated during extension (Acocella et al., 2003). We therefore aim to investigate the mechanical interactions between collapsed calderas and regional extension leading to elongated edifices. Analogue models of caldera collapse were performed by placing a circular magma chamber (simulated with poly-glycerine) placed below a sand-mixture package. We induced the collapse by draining out the analogue magma from the base, reproducing the classical fault architecture observed at many collapsed calderas (i.e., early inner outward-dipping reverse faults and late outer inward-dipping normal fault). Once completed, the collapsed depression was stretched such that normal faulting produced caldera elongation and segmentation. Finally, we compared the elongation and the structural pattern deriving from the interacting caldera-related and rift-related structures with natural examples from the East African Rift System. Our results suggest that different interacting factors may contribute to the development of elongated calderas, thereby questioning whether elongated calderas can be considered as a fully reliable proxy for the regional strain.

Acocella, V., Korme, T., Salvini, F., and Funiciello, R. (2003). Elliptic calderas in the Ethiopian Rift: control of pre-existing structures. J. Volcanol. Geotherm. Res., 119, 189–203.

Casey, M., Ebinger, C., Keir, D., Gloaguen, R., and Mohamed, F. (2006). Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments. Geol. Soc. Lond. Spec. Publ., 259(1), 143–163.

Nakamura, K., (1977). Volcanoes as possible indicators of tectonic stress orientation— principle and proposal. Journal of Volcanology and Geothermal Research 2, 1–16

How to cite: Maestrelli, D., Facincani, P., Sani, F., Bonini, M., Montanari, D., Del Ventisette, C., and Corti, G.: Stress-strain relationships at elongated calderas in extensional settings: what analogue models say, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-380, https://doi.org/10.5194/egusphere-egu23-380, 2023.

Dykes are tensile fractures that rapidly transport magma from the hot, ductile asthenosphere across the cold, brittle upper lithosphere. They play an important role in tectonic extension settings by drastically reducing the force needed for rifting (Buck, 2004). Yet the balance of mechanisms that drive dyke propagation and how they promote rift initiation remain unclear. Here we investigate the physics of dyke propagation in a two-phase continuum model that can approximate both faults and dykes in an extensional tectonic setting.  

Dykes are fluid-filled fractures, typically modelled as discrete inclusions in an extended elastic continuum.  These models suggest that dyking is dominated by magma buoyancy and that its direction can be altered according to the competition between tectonic stress and the topographic load (Maccaferri et al., 2014). However, this method assumes a constant background stress field in the lithosphere during dyking. Therefore this method cannot capture the interaction between dykes and the long-term deformation of the lithosphere. To resolve this issue, dyking has been prescribed as a weak material in a continuum, one-phase rifting model in which dyking is included in the conservation of mass, momentum and/or energy (Liu and Buck, 2018). This method respects the scale separation between dyking and long-term dynamics, but still neglects the feedback of dyking on the stress field.

We present a geodynamic model that incorporates a novel poro-viscoelastic–viscoplastic rheological formulation with a hyperbolic yield surface for plasticity. With this model, both dyking and faulting can be simulated consistently (Li et al., in review). We validate our theory by comparing the stress field at the tip of the dyke with that from the linear elastic fracture mechanics theory. We then investigate dynamics of dyking in a geodynamic rifting model. We show that dyking assists rifting and its localisation. First, it reduces the yield strength in the brittle layer as the pore pressure balances the compressive stress; second, it promotes the development of near-surface normal faults localised in a relatively narrow rift region near the rift axis. We investigate the physics of dyke propagation with respect to the balance between buoyancy and tectonic forcing, and the effect of topography.

References

Buck, W .R., (2004). Consequences of asthenospheric variability on continental rifting. In Rheology and deformation of the lithosphere at continental margins, chapter 1, pages 1–30. Columbia University Press. doi: 10.7312/karn12738-002.

Maccaferri, F., Rivalta, E., Keir, D., and Acocella, V., (2014). Off-rift volcanism in rift zones determined by crustal unloading. Nature Geoscience 7, 297–300. doi: 10.1038/ngeo2110.

Liu, Z. and Buck, W. R., (2018). Magmatic controls on axial relief and faulting at mid-ocean ridges. Earth and Planetary Science Letters, 491:226–237. doi: 10.1016/j.epsl.2018.03.045.

Li, Y., Pusok, A., Davis, T., May, D., and Katz, R., Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation, in review of Geophysical Journal International.

How to cite: Li, Y., Pusok, A., Davis, T., May, D., and Katz, R.: Dyke propagation and dynamics during rift initiation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14818, https://doi.org/10.5194/egusphere-egu23-14818, 2023.

The structural style of inverted rift basins is controlled by the inherited structures and stratigraphic elements but also by the presence of salt layers or welded equivalents. Salt acts as a main detachment during extension and, depending on its thickness, different degrees of linkage develop between the basement and overburden. The presence and distribution of salt structures, the linkage between the basement and overburden, and the continuity of salt on these salt-bearing rifted basins have a strong impact on thick- to thin-skinned deformation during inversion. As the weakest rock of the basin infill, salt acts as a contractional detachment and buried diapirs rejuvenate during early inversion. With increasing shortening thick-skinned deformation folds and uplifts the basins while the diapirs are squeezed and welded by thin-skinned deformation.

Using an approach based on systematic analogue models, this work analyses how extensional basins develop above a pre-rift salt layer and how the inherited salt structures evolve during subsequent inversion. A first set of models only affected by extensional deformation was carried out examining how the variation of different parameters such as salt and overburden thicknesses impact the structural style of salt structures developed during thick-skinned extension. Afterwards, some of these models were repeated to understand how pre-existing extensional and salt structures condition the evolution during total inversion tectonics. The experimental apparatus consists of five metal fault blocks simulating a domino basement-fault system that rotate counter-clockwise during extension and clockwise during inversion. Deformation was transferred to the blocks by a motor worm-screw at a constant velocity of 4.6 mm/h until reaching 10 cm of total extension. During the inversion phase, the same velocity was applied until reach total inversion of the basins. A layered unit of sand capped by a uniform-thickness polymer layer and additional layers of sand simulated the pre-kinematic unit. While different sand layers were added during extension, no syn-inversion sedimentation was considered.

The results of this study show that the structural style during inversion is highly conditioned by the inherited extensional configuration but also by the salt thickness that condition the degree of coupling/decoupling of the pre- and syn-kinematic successions. The study also revealed that the thickness of the overburden has a minor impact during the inversion of the basins. Such is the case that in models with either thin or thick overburden succession, the extensional geometry might be preserved if the salt is thick independently of the overburden thickness. Contrary, models with a thin salt layer are characterized by a total inversion of the ramp-syncline basin that as an inversion anticline is developed, crestal collapse extensional faults minimize the developed structural relief. Finally, the analogue modelling allowed to understand how compression caused primary weld reactivation, diapir rejuvenation, salt thickening and/or thrust emplacement. The reactivation of some of these salt-related structures is extremely impacted by the salt thickness distribution that resulted from the extensional phase. Therefore, to characterize structural style and understand the evolution of the basin it is needed an understanding of the inherited salt-related structures.

How to cite: Ferrer, O., Carola, E., and McClay, K.: Experimental approach (analogue modelling) of thin- to thick-skinned inversion of extensional basins with pre-rift salt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11040, https://doi.org/10.5194/egusphere-egu23-11040, 2023.

The inversion of rift basins is commonly associated with the reactivation of normal, basin-bounding faults or shear zones. Analogue models have shown how the reverse reactivation of these pre-existing structures facilitates the uplift of a basin’s sedimentary infill. However, few of these models examine the viscous processes occurring beneath the brittle crust, which may or may not drive basin inversion. In our study, we use lithospheric-scale analogue experiments of orthogonal extension followed by shortening to simulate rifting followed by inversion and orogenesis. Here we explore how the flow behaviours of ductile layers underneath rift basins promote or suppress basin inversion.

In our experiments, we simulate rifting by extending a multi-layer, brittle-ductile lithosphere which floats on a fluid asthenosphere, creating a system of distributed basins. This extension is followed by shortening of the model, during which strain is accommodated by the reactivation of basin-bounding faults and folding or upwelling of the ductile layers. These experiments reveal that the rheology of the ductile lower crust and lithospheric mantle, modulated by the imposed bulk strain rate, determine: (1) how rift basins are distributed during extension and (2) whether all or only some of these basins are inverted during shortening. We interpret that this selective basin inversion is related to the superposition of crustal-scale and lithospheric-scale boudinage during the basin-forming extensional phase. Our findings demonstrate that lithospheric-scale analogue models can be a powerful tool for investigating the interaction between brittle and viscous deformation during basin inversion.

How to cite: Samsu, A., Gorczyk, W., Amirpoorsaeed, F., Schmid, T., Morton, E., Betts, P., and Cruden, A.: Not all basins are created equal: Lithospheric-scale analogue experiments of selective basin inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10149, https://doi.org/10.5194/egusphere-egu23-10149, 2023.

Earlier extended continental margins are frequently involved into late compressive deformation during mountain building (i.e. orogenesis). This process gives rise to positive inversion of previous extensional faults, but these structures may also play different roles during late compressive phases, interacting in various ways with inherited structures from older tectonic stages.

Moreover, different orientation of compression direction related to different phases affecting extended continental margins may give rise to complex structural settings whose evolution is often difficult to reconstruct. To address this problem, we performed an analogue model experimental series aiming at extending a continental margin and then imposing on the same margin differently oriented compressive phases. Models were quantitatively analyzed through particle image velocimetry (PIV) to highlight fault interaction, and by using Digital Elevation Models reconstructed with Structure from Motion (SfM) techniques. Our results show that well developed and favorably oriented normal fault systems drive the location of successive compressive structure, often through inversion processes, but they also condition the final geometrical setting without inversion. Moreover, an important role is also played by the orientation of the direction of compression (obliquity angle a varied from 0° to 90°), which gives rise to different structural patterns when is superimposed to extensional structures as a first compressive phase or is superimposed to already formed compressive structure as second compressive phase. The resultant complex structural patterns show differently oriented structures cutting each other even at high angles, a feature often seen in nature. Therefore, these experiments may be applied to a variety of natural cases, helping to decipher geological evolution of the analyzed areas basing on the geometrical relationships among structures.

How to cite: badraoui, O., Del Ventisette, C., Maestrelli, D., Zaghloul, M. N., and Sani, F.: analogue modelling of multiple compressive phases deforming and extended margin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-432, https://doi.org/10.5194/egusphere-egu23-432, 2023.

Subduction of an oceanic plate beneath either an oceanic, or a continental, overriding plate requires two main conditions to occur in a steady state: i) a high enough subduction rate (~5 cm/yr, Schellart in print); and ii) a weak (efficiently softened/lubricated) subduction channel (Gerya and Meilick, 2011). The first requirement prevents thermal diffusive re-equilibrium of the subducting slab within the asthenospheric ambient mantle, maintaining the slab cold and dense enough to provide the slab-pull subduction driving force. The second condition, is achieved with the contribution of a strong dehydration of the serpentinized oceanic plate, with resulting pervasive fluid circulation in the subduction channel significantly promoting its weakening, thus preventing strong coupling between the subducting and the overriding plate. Avoiding such a coupling has been shown to be key to maintain stable subduction, since it generally leads to a halt in the subduction process and to slab break-off (Duarte et al., 2015). Both these conditions are seemingly not favoured in a continental subduction scenario, since continental lithosphere is positively buoyant and much less, or not al all, serpentinized. Hence, the (geo)dynamics governing continental subduction is still not fully understood.

We thus carried out a set of geodynamic numerical modelling experiments to further understand the first order geodynamic constraints governing continental subduction in the specific scenario that considers the subduction of a continental plate beneath an oceanic one, i.e., upon the arrival of a continental plate at an intra-oceanic subduction zone. The 2D numerical experiments were conceived and constructed using the Underworld code (Moresi et al., 2007), to better understand the influence on continental subduction efficiency, as well as on related synthetic ophiolite obduction, of considering either a scenario of dominant trench retreat (roll-back) or trench advance (roll forward) subduction regime. Roll-back subduction was prescribed in our models by fixing the trailing edge of the overriding plate, whereas roll-forward subduction was favoured (allowed) by leaving it free to move. Our experiments ensure dynamic self consistency in all cases.  

Our preliminary results show that, although synthetic obduction is possible to achieve in both situations, the overall first order (geo)dynamic differences implied by the two different simulated regimes, bear important consequences on the timing, overall kinematic configuration and local stress/strain distribution of the considered continental subduction-exhumation cycle in each case.

Acknowledgments

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL

References

Duarte, J.C., Schellart, W.P., Cruden, A.R., 2015. How weak is the subduction zone interface? Geophysical Research Letters 42, 2664–2673. doi:10.1002/2014GL062876.

Gerya, T.V., Meilick, F., 2011. Geodynamic regimes of subduction under an active margin: effects of rheological weakening by fluids and melts. Journal of Metamorphic Geology 29, 7–31. doi:0.1111/j.1525-1314.2010.00904.x.

Moresi, L., Quenette, S., Lemiale, V., Mériaux, C., Appelbe, B., Muhlhaus, H.B., 2007. Computational approaches to studying non-linear dynamics of the crust and mantle. Physics of the Earth and Planetary Interiors 163, 69–82. doi:10.1016/j.pepi.2007.06.009.

Schellart, W.P., in print. Subduction zones: A short review, in Dynamics of Plate Tectonics and Mantle Convection, Editor: João Duarte, ISBN: 9780323857338.

How to cite: Rosas, F., Gomes, A., Riel, N., Schellart, W., Duarte, J., and Almeida, J.: Geodynamic modelling of continental subduction beneath oceanic lithosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7248, https://doi.org/10.5194/egusphere-egu23-7248, 2023.

Ophiolites are exposed remnants of oceanic lithosphere that are emplaced onto a continental domain, and Tethyan-type ophiolites, specifically, are those that are emplaced within a continental passive margin. The emplacement process for this type of ophiolites occurs when a continental passive margin subducts, and subsequently exhumes, beneath an oceanic overriding plate (future ophiolite). It is the exhumation of the passive margin’s crust that triggers both the separation of the ophiolite from the remaining oceanic overriding plate (OP) and its ensuing emplacement within the continental domain.

Analogue and numerical models have demonstrated the feasibility of this process (Chemenda et al., 1996; Duretz et al., 2016; Porkoláb et al., 2021); however, its specific geodynamic constraints are still poorly understood. For example, the geological record appears to be heavily skewed towards the fast emplacement of very young lithosphere, but it is unclear whether it is possible to emplace older lithosphere via the same process. Here we use 2D numerical models to test the sensitivity of this process to three key parameters: a) overriding plate age (10-60Myr), b) width of ocean-continent transition (OCT, 0-500km), and c) existence/absence of a serpentinization layer in the OP. The models use temperature and strain-rate dependent visco-plastic rheologies, are driven by buoyancy forces (without imposed non-zero velocity conditions), and are run using the Underworld code (Moresi et al., 2003).

Preliminary results show that the continental subduction/exhumation cycle and the ophiolite emplacement process are highly sensitive to variations in initial model conditions. Nevertheless, the emplacement process is physically viable under a somewhat wide range of conditions, being optimized for a narrow OCT and adjacent continental margin subducting beneath a young and serpentinized OP. A 10 Myrs old OP leads to a fast continental subduction-exhumation cycle (15-20 Myrs), while a 60 Myrs old OP induces a slow (>30 Myrs) cycle, but still leads to ophiolite emplacement. A long and tapered margin (OCT, 500km) also promotes a slow (>30 Myrs) cycle, with only a thin melange of exhumed crust, which hinders the formation and emplacement of individual ophiolite klippen; the reverse is true for a very short OCT. The existence of a serpentinization layer greatly facilitates the emplacement of the ophiolite klippe.

Acknowledgments

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia I.P./MCTES through national funds (PIDDAC)–UIDB/50019/2020-IDL and through scholarship SFRH/BD/146726/2019.

References

Chemenda, A., Mattauer, M., Bokun, A. (1996). Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: New modelling and field data from Oman. EPSL, 143, 173–182.

Duretz, T., Agard, P., Yamato, P., Ducassou, C., Burov, E., Gerya, T. (2016). Thermo-mechanical modeling of the obduction process based on the Oman Ophiolite case. GR, 32, 1–10.

Moresi, L., Dufour, F., Mühlhaus, H. B. (2003). A Lagrangian integration point finite element method for large deformation modeling of viscoelastic geomaterials. Journal Comp. Physics, 184, 476–497.

Porkoláb, K., Duretz, T., Yamato, P., Auzemery, A., Willingshofer, E. (2021). Extrusion of subducted crust explains the emplacement of far-travelled ophiolites. Nature Commun., 12, 1499.

How to cite: Gomes, A., Rosas, F., Duarte, J., Riel, N., Schellart, W., and Almeida, J.: 2D numerical modelling of Tethyan-type ophiolite emplacement: The role of overriding plate age, serpentinization, and OCT width., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7370, https://doi.org/10.5194/egusphere-egu23-7370, 2023.

Plates and the convective mantle interact with each other over geological time scales, leading to mantle flow, plate motion, and deformation along plate boundaries.  At convergent boundaries undergoing subduction, the role played by mantle drag remains poorly understood despite its potential impact on subduction dynamics, and in turn on the deformation regime of the overriding plate. Previous studies were generally conducted in two dimensions, limiting their ability to faithfully reproduce processes taking place on Earth. Instead, in this study, we present 11 three-dimensional analog models of subduction at the scale of the upper mantle, including an overriding plate, and in which we control mantle drag at the base of the lower or upper plate by imposing a controlled unidirectional background mantle flow perpendicular to the trench. We systematically vary the velocity and the direction of the imposed horizontal mantle flow and quantify its impact on horizontal and vertical upper plate deformations, plate and subduction velocities, and the geometry of the slab. The geometry of the slab is only marginally affected by the velocity and direction of the mantle flow. In the absence of mantle flow, slab rolls back and deformation is accommodated by trench-orthogonal stretching in the upper plate. Instead, the addition of a background flow dragging the lower or upper plate toward the trench  systematically results either in the absence of upper plate deformation, or in trench-orthogonal shortening with strain rates that increase linearly with increasing mantle flow. We show that the upper plate strain rate is primarily controlled by the velocity of the free plate in the model, which itself results from the drag exerted by the mantle at the base of the plate. Coupling between mantle and plate is larger for models with flow directed toward the upper plate, resulting in strain rates that are about three times larger than for equivalent models with flow directed toward the lower plate. This systematic study provides a better understanding of the effect of mantle drag on plate displacements and deformation along subduction zones, leading to a better understanding of the ingredients required to form Andean-type mountain ranges.

How to cite: Geffroy, T., Benjamin, G., Anne, R., Martine, S., Robin, L., Jean-Jacques, K., and Tania, H.: A systematic study of mantle drag effect on subduction dynamics and overriding plate deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2782, https://doi.org/10.5194/egusphere-egu23-2782, 2023.

We present a new numerical method to simulate the structural patterns emerging from the long-term large-deformation tectonic flows in both two and three spatial dimensions.  The domains of different material properties are each represented by a level set function discretized on a Eulerian mesh with the discontinuous Galerkin method. The level sets are advected by a velocity field provided by a coupled Stokes flow solver. Our method accurately captures the material interface by the adaptive mesh refinement, reduces the computational expenses compared to the traditional particle-in-cell method and offers straightforward handling of geometric splitting and merging.  Under the unified finite element framework, our method promises the flexibility in the choice of mesh geometry as well as the potential for extending to complex rheology.  With passive tracers geat and around areas of interest, the finite strain of the flow field can be integrated through any time interval within the total simulation time.  The strain ellipsoids thus obtained offers the possibility for ground-truthing the simulated deformation patterns with the field structural analysis.  Our results demonstrate identical physical behaviour when compared with established structural geology and geodynamic benchmarks.

The style of the crustal dynamics on the Archean Earth has been subject to controversy on whether a vertical tectonic style in the form of Rayleigh-Taylor instability, induced by an inverted density profile, prevails in the early history of the Earth and if so, how the transition to the present-day plate tectonics, characterized by dominantly horizontal movement, is manifested in the rock record.  Equipped with our modelling scheme, we construct numerical models to simulate the lithological distributions and deformation patterns resulted from a synchronous operation of vertical tectonism and horizontal shearing. The latter can be viewed as a possible result of some far-field tectonic boundary condition (e.g. oblique convergence).  Many aspects of the simulation in terms of the map pattern, foliation/lineation trend and strain distribution compare favorably with the field observations in Neoarchean granitoid-greenstone terranes in the Superior Province as well as worldwide.  Therefore, it is concluded that the vertical and horizontal tectonism are not mutually exclusive tectonic regimes  The symbiosis of both tectonic processes is a viable mechanism for establishing the crustal architecture and the deformation pattern we see today in many Neoarchean terranes and might represent a transition from the former to the latter in the Neoarchean.

How to cite: Wu, Q. and Lin, S.: Modelling tectonic flow with discontinuous Galerkin level set method: Case studies and applications  for the Neoarchean crustal dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10353, https://doi.org/10.5194/egusphere-egu23-10353, 2023.

Flow perturbation can deflect the layering of the host rock around slip surfaces in shear zones resulting in the development of flanking structures. The details of flanking structure geometry can provide important clues about shear sense, flow kinematics, and finite strain, although not without ambiguities. The developing structures share similarities to fault-related folds that play an important role in sedimentary basins.

Mechanical anisotropy has been shown to have a major influence on both the slip rate and flow perturbation. Willis (1964) derived an analytical solution for an elliptical inclusion embedded in a homogeneous anisotropic elastic matrix subject to a uniform load in the far field. The solution can be reduced to the case of an incompressible viscous medium and an arbitrarily oriented inviscid slit (slip line). The reduced solution, which is exact for the initial state of homogeneous planar anisotropy, provides useful insights into the initial stages of deformation and it can be used to approximately study finite strain deformation of a power-law host. However, anisotropic fluids such as ductilely deforming foliated rocks keep a ‘memory’ of deformation due to their evolving microstructure, which affects the flow field. In this study, I will use different numerical modeling techniques to examine the impact of host layering on the perturbing flow and structure development around a slip surface in shear zone.

How to cite: Dabrowski, M.: Numerical modelling of flanking structures in layered viscous media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7357, https://doi.org/10.5194/egusphere-egu23-7357, 2023.

During the last decades, analog models have taken extraordinary advantage of new technologies. High-resolution cameras, analytical methods to extract quantitative data from the experiments (e.g., Digital Image Correlation), and new analog materials are only a few examples of the new improvement. The ease of extraction of quantitative data means that the modeling results can be used to provide new views on natural processes. Reducing unwanted uncertainties is crucial to propose robust new theories. One of the main difficulties for analog modelers is reducing the uncertainties related to the initial setup arrangement. Most of these uncertainties are classically referred to the handmade processes, such as handling analog materials. In the Analog Modeling laboratory of the University of Trieste, we tested the use of a cobot (a cobot is a robot for direct physical interaction with a human user within a shared workspace) to simulate pre-existing faults in wet clay boxes. We present two different sets of experiments. The first set has been designed to evaluate the kinematic efficiency of Riedel shears. The second reproduces differently oriented inherited dip-slip faults in an experimental box reproducing extension. In both cases, we reproduced the same setup more than one time. The collaborative robot reduced the variability of the results, demonstrating the effectiveness of the use of cobots in analog modeling laboratories.

How to cite: Bonini, L. and Bertone, N.: The use of collaborative robots (cobots) in an analog modeling laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6125, https://doi.org/10.5194/egusphere-egu23-6125, 2023.

Continental transform faults transition to a new plate boundary type when strike-slip, transpression or transtension are no longer the most efficient way to accommodate plate motion. In some instances, rather than the transform fault ‘transforming’ plate motion directly to its connecting plate boundary, the continental transform fault can become ‘misaligned’ with its connecting plate boundary. Where a plate boundary misalignment occurs, plate motion that was localised on the transform fault can become distributed over a broad, intervening transition zone between the two major plate boundary faults. In this study we use scaled analogue models to investigate the development of fault networks in regions of localised and distributed simple shear and the transition between the two. We use digital image correlation (DIC) to analyse the surface deformation of the analogue model experiment and present results as incremental shear strain maps of the surface of the analogue models.  The results are compared to natural examples of plate boundary transition zones (e.g., Alpine Fault, New Zealand; North Anatolian Fault, Turkey; San Andreas Fault, USA).  In our previous analogue model experiments, regions of localised and distributed simple shear have been generated in an analogue shear box using a four-way stretchable fabric to adjust the basal boundary conditions. These experiments were limited by the elasticity of the stretchable material, which cannot deform infinitely. Here we will present preliminary results from a new shear box apparatus that uses carbon fibre rods to adjust the basal boundary conditions. This new apparatus has been designed to minimise the boundary effects caused by the limitations of the four-way stretchable fabric in our previous experiments.

How to cite: Withers, M. and Cruden, A.: A new shear box apparatus for investigating distributed deformation at the termination of continental transform faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4555, https://doi.org/10.5194/egusphere-egu23-4555, 2023.

We present and demonstrate our new application of a geophysical seismic technique to acoustically characterise and image layers with different impedance contrast in analogue models. A high-powered pulsed laser in combination with a mirror galvanometer is used to generate a powerful acoustic shockwave at any point of the surface of the analogue model. Reflections, refractions, and diffractions of the acoustic source wave, induced by internal structures inside an analogue model, produce vibrations of the top surface of a model, which are measured by laser vibrometer.

Using our setup, we acquire seismic receiver gathers in less than a minute. Interpretation of the gathers allowed to identify the presence of internal reflecting and refracting material interfaces. In a series of test models, we determined the speed of both P-waves and surface waves in a multitude of brittle analogue materials. In uniform layered models we performed 1D inversion using the gathered waveform data. The results are validated by simulating the test experiments in a finite-difference solver. The novel method will be developed further, aiming to determine stress build-up in the material prior to fault formation or activity.

How to cite: Smits, J., Beekman, F., Willingshofer, E., and Vasconcelos, I.: Laser-based seismic imaging of analogue models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7077, https://doi.org/10.5194/egusphere-egu23-7077, 2023.