Displays

For long time,3D tectonic modelling was reserved to analog methods and many practitioners spent a lot of time and energy developing methods and materials to make their naturally 3D "simulations" as cylindrical as possible.

Fighting with so-called boundary effects, they actually obtained a lot of interesting structures and dynamics related to "border effects" . In the last 5 years, 3D numerical simulations have really emerged thanks to new numerical technics and increase in available 'computational power. The two methods are now competing and sooner or later, with the emergence of exa-scale and quantum technology, it is quite certain that numerical simulations will dominate the field because it is much better suited to tackle multi- physics problems arising in long term tectonics.

However, before entering an era of mass production, it is interesting to re-think how we introduce 3 dimensionality in numerical models. Numerical models can easily produce perfectly free slip boundary conditions, and it has therefore never been a problem to simulate a perfectly cylindrical situation. Is it useful ? Not really since we can run 2D simulations.

However, many models introduce the 3 dimensionality by imposing inherited structures in simulations that use perfectly cylindrical boundary conditions. Technically this corresponds to imposing free slip boundaries in the third dimensions. Nobody question it, and in a way, we numerical modellers, are just mimicking traditional analogue model set ups and emphazing on the multi-physics aspect of our simulations.

Yet, comparing to analogue models, we some time reach different solutions and sometimes, analogue models with their boundary effects produce tectonic structures that are much more realistic than models with perfectly free slip boundaries.

In this pico presentation, I will show exemples of free slip boundaries that introduce biased in continental break-up propagation models and discuss in which conditions free slips are acceptable and in which conditions are should be carefull in our interpretations of simulation results.

How to cite: Le Pourhiet, L., Jourdon, A., Watremez, L., and Vendeville, B.: Free slip conditions in 3D, what does it actually mean ?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13667, https://doi.org/10.5194/egusphere-egu2020-13667, 2020.

Strike-slip systems can accommodate hundreds to thousands of kilometres of horizontal displacement by simple shear.  These systems are prone to high earthquake risk and understanding their structural geology will assist with hazard mapping and future risk mitigation. Deformation by simple shear can be concentrated on a single fault or distributed over tens to hundreds of kilometres.  It is usually challenging to understand the complex geometries that form in strike-slip systems by analysing finite strain in simple horizontal and vertical sections observed in the field.  To understand the fundamental processes that form such system, geologists use analogue experiments to test the development and evolution of structures through time.  The internal 3D evolution of deformation within analogue models of simple shear is often inferred by changes in topography and by using Particle Image Velocimetry (PIV) to analyse changes in incremental and finite strain on the model surface, similar to horizontal outcrop and map patterns, except showing the evolution of these features through time.  Cutting vertical cross sections through a simple shear experiment at specific time steps to reveal its 3D geometry is not an option when using granular materials to represent upper crustal deformation.  In this study, we use X-Ray Computed Tomography (CT) scanning to analyse the 3D evolution of strike-slip fault systems in granular materials without disruption to the analogue experiments.  We present results of the 3D evolution of localized and distributed simple shear zones by CT scanning analogue experiments at regular intervals.  Localized and distributed strike-slip shear zones are generated in an analogue shear box by using stretchable fabric to adjust the basal boundary conditions.    The results are compared to the Marlborough Fault System; a system of strike-slip faults that form the Australian – Pacific plate boundary in northeast South Island, New Zealand. 

How to cite: Withers, M. and Cruden, A.: The 3D evolution of localised and distributed strike-slip shear zones, visualised by X-Ray CT scanning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4371, https://doi.org/10.5194/egusphere-egu2020-4371, 2020.

Sedimentary basins in tectonically active settings, such as rift basins, are characterised by complex, dynamic depositional environments, with the interplay between sedimentation and tectonic processes controlling basin architecture and resource distribution. Scaled 3D analogue sandbox experiments with high-resolution digital 3D deformation monitoring, constrained by geological and geophysical data, can realistically simulate upper-crustal brittle deformation on crustal to basin-scale and allow kinematic and mechanical analysis of complex 3D fault systems. First-order syn-kinematic sedimentation can be conceptually applied to the surface of evolving experiments, permitting investigation of its effect on fault localisation, linkage and displacement and resulting tectonic basin subsidence. However, to date, first-order syn-kinematic sedimentation onto analogue models has been done manually; depositing incremental, homogeneous sand layers on top of the evolving experiment surface to simulate tectonic loading. Consequently, current syn-kinematic sedimentation methods are not capable of simulating complex stratal architectures or incorporating depositional controls like eustasy and climate variations. Conversely, numerical stratigraphic-forward modellers are able to produce these more complex stratal geometries, including their controlling parameters, however they currently lack the ability to simulate the complex tectonic subsidence of basins realistically, or in sufficient spatial resolution.

This work presents a new integrated experimental method; applying cellular numerical stratigraphic forward modelling to dynamically scaled analogue sandbox experiments, permitting realistic, incremental deposition of syn-tectonic sediments. Surface topography and displacement components (e.g. subsidence) of the analogue experiment are derived by 3D-Stereo Digital Image Correlation (DIC) and yield scaled inputs for the cellular carbonate stratigraphic forward modelling software (SFM - CarboCAT). These are then run in combination with suitable production parameters (production rate, surface light intensity, extinction coefficient etc.) as a numerical model, to generate a realistic spatial distribution of sediment facies to be incrementally deposited back onto the surface of the evolving sandbox experiment. Deposition of volumes onto the analogue sandbox is achieved using a cellular sieving device which utilises an array of tubes to maintain the spatially heterogeneous material volumes within their corresponding analogue surface locations. This apparatus has been shown to be capable of repeatedly depositing heterogeneous sandpacks with locally controlled volumes and homogeneous mechanical properties.

The novel integrated analogue and numerical workflow is systematically tested in a series of static (depositional ramp) and dynamic (asymmetric half-graben) analogue experiments with varying initial parameters for both the analogue and numerical models. Results demonstrate that model evolution is purely deterministic, producing diverse final architectures solely as a result of initial parameters and ongoing feedback between the analogue tectonic subsidence history and the SFM-derived sediment loading.

Deposition of SFM-calculated sediment volumes onto the analogue model produces more realistic syn-tectonic depositional patterns and facies distributions than current methods can achieve. If applied to larger-scale experiments, this workflow would be capable of simulating more complex, tectonically-controlled settings like segmented rift basins or passive margin sedimentary basins affected by gravity-driven deformation, as well as investigating the role of climatic impacts on basin evolution. Findings have potential to improve understanding of basin evolution and subsequent facies distribution, with implications for resource exploration.

How to cite: Hughes, A., Adam, J., and Burgess, P.: Integrating analogue and numerical modelling techniques for improved simulation of coupled regional tectonic processes and syn-depositional systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5059, https://doi.org/10.5194/egusphere-egu2020-5059, 2020.

Monitoring the surface evolution on physical analogue models is important for quantification of the model deformation. We present the application of LiDAR (Light Detection and Ranging) 3D scanning for monitoring the surface evolution of physical analogue models, complemented with digital imagery acquired during scanning.

Our previous work tested this approach for the first time on sandbox analogue models of geological systems with two model configurations, and sizes, representing specific tectonic settings: a convergent tectonic setting and a strike-slip tectonic setting.

Perspex parallelepiped boxes were used with four fixed walls (one basal and three laterals) and one mobile that worked as a vertical piston on the first model, or with two parallel basal plates with a step contact, where one remained fixed and the other was mobile attached to the back vertical wall, creating a strike-slip displacement with a restraining bend (second model). Initial sizes of model surfaces were 50 × 10 cm to 70 × 50 cm (length × width), respectively. On both models, the mobile walls were pushed by a computer controlled stepping motor at steady velocity, so deforming the models. Fine dry natural quartz sand from Fontainebleau was used as the analogue of brittle rocks. Sequential scanning of the models surface was performed during the models deformation and complemented with digital time-lapse image acquisition synchronized with LiDAR scanning, using an 18 MP camera orthogonally positioned to the models surface (top view), in order to monitor in-plane displacements and timing of the structures development. The previous work highlighted the results obtained for the smaller surface model, whereas here we highlight the results obtained for the large surface model (strike-slip model).

For each scanning, 3D point clouds were obtained and processed into 3D digital surface models (DSM) with high surface accuracy and resolution. With this set of DSMs, a time series of digital elevation models (DEM) was obtained for each analogue model allowing the quantification of the topography with high resolution and to analyse its evolution. Also, in-plane deformation quantification was obtained from the top view digital images and through the correlation of both sets of data, the timing of geomorphological expression and evolution during model deformation.

This work confirm that the LiDAR 3D scanning technique can be applied in laboratory to measure surface topography of physical analogue models with very good results regardless of their sizes and to monitor the topography evolution during deformation.

It also confirm that this combined monitoring method, the synchronized LiDAR 3D scanning and time-step digital image acquisition, can be used to measure the surface deformation of analogue models both vertical (topography) and horizontal (in-plane displacements).

Finally, this work shows new indoor employment possibilities for this technical equipment (LiDAR terrain 3D laser scanners), often available on Earth research institutions, which are generally used for outdoor measurements.

Acknowledgements:

The author CRN would like to acknowledge the financial support of FCT through grant SFRH/BD/71005/2010 and project UIDB/50019/2020 – IDL.

How to cite: R. Nogueira, C. and O. Marques, F.: Monitoring 3D Surface Deformation of Physical Analogue Models using LiDAR Scanning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21182, https://doi.org/10.5194/egusphere-egu2020-21182, 2020.

Here we use dynamically scaled analogue experiments to investigate the influence of tectonic loading on continental rifting. Analogue models consist of a two-layer brittle-viscous set up overlying a foam base, which expands homogeneously when extension is being applied perpendicular to the rift axis trend. A layer of quartz sand on top of a viscous silicone/corundum sand mixture layer is used as an analogue for an upper brittle crust and a ductile lower part of the crust, respectively. An additional package of sand on one part of the model simulates tectonic loading.

The aim of this work is to investigate in detail dynamic rift propagation in such a setting by means of a fully quantitative monitoring of surface and internal deformation, focusing on rift propagation velocity, growth rate and orientation. The evolution of the surface topography (DEM) and deformation (3D displacement field) is monitored and quantified using 3D Digital Image Correlation (3D stereo DIC). Furthermore, we apply an automated fault segment tracer on the surface deformation data to characterize rift evolution. Model internal deformation is investigated by digital volume correlation (DVC) techniques applied on X-ray computed tomography data of the time-series experiment volumes. With the use of such techniques we are able to visualize, quantify and link deep-seated internal flow and surface deformation over time.

Preliminary results from these experiments suggest that rift propagation in our analogue models is directly influenced by load-induced deep-seated deformation resulting in a horizontal lower-crustal flow opposing rift propagation.

How to cite: Schmid, T., Schreurs, G., Adam, J., and Hollis, D.: Effect of tectonic loading on continental rifting: Imaging, quantification and linkage of deep-seated flow and surface deformation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5935, https://doi.org/10.5194/egusphere-egu2020-5935, 2020.

Magnetic fabric is used as strain indicator to provide further insights into different tectonic settings. Applying anisotropy of magnetic susceptibility (AMS) analysis on analogue models has shown to be a useful approach to understand details of deformation. Here we use this technique on shortened sandbox models to illustrate the relationship between rotation of grains and the influence of décollement friction in fold-and-thrust belts. Layers of sand were scraped to a thickness of 2.5 cm on top of high-friction sandpaper on one side and on low-friction fibreglass on the other side of the sandbox model. After shortening the model by 26%, samples were taken at the surface and at depth for measuring AMS. During shortening, above the high-friction décollement, a stack of imbricates was formed, which shows distinct clustering of the main principal magnetic susceptibility axes (k1 ≥ k2 ≥ k3) around the dip of the forethrusts. In contrast, AMS data above the low-friction décollement show a more heterogeneous AMS pattern due to complex structure development with box folds and fault bending. In general, the magnetic fabric can be differentiated between the initial model fabric in the foreland and a tectonic overprint within the hinterland. The AMS analysis show that strain increases with the development of structures towards the hinterland and additionally with depth, but differs between the two frictional décollements. At the transition zone between the two different frictional environments, a deflection zone developed where the trace of thrusts change trend causing additional rotation of sand grains within this zone perpendicular to main shortening direction, as reflected by the orientation of the k1 and k3 axes. Overall, the orientation of the AMS axes and shape of anisotropy depend on the structure geometry and movement, which are determined by the friction of the individual décollement beneath. Consequently, AMS in models indicates and describes the development of structures and reflects strain above different basal friction.

How to cite: Schöfisch, T., Koyi, H., and Almqvist, B.: Anisotropy of magnetic susceptibility as strain indicator in a fold-and-thrust belt sandbox model above décollements with frictional contrast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15507, https://doi.org/10.5194/egusphere-egu2020-15507, 2020.

Records of thermal maturities in boreholes have led to a better understanding of the formation of geological structures, especially the duration of thrusting during the evolution of accretionary wedges. The temporal extent of thrusting is controlled by a host of factors such as the nature of sedimentation, the topography of the incoming plate and so on. As a result, estimating the peak heating through the thermal maturity of organic material can help elucidate which mechanism has played a prominent role in wedge evolution. However, the thermal maturity value expressed as the distribution of vitrinite reflectance is the combined effect of two factors: the geothermal gradient and the time the sediments were exposed to different temperatures. Thus, the distribution of vitrinite reflectance in accretionary wedges does not necessarily reveal the deformational pathway of individual thrusts. Moreover, since the conductivity of the sediments close to the surface (<10 km) is most accessible in borehole data and predominantly controlled by porosity, models of accretionary wedge simulating thermal maturity ought to incorporate the impact of porosity on thermal conductivity. Additionally, phase transitions of the sediments in the wedge, such as smectite-illite transition and the formation of zeolite facies, that lead to increased thermal conductivity and internal angle of friction for sediments at structurally deeper locations within the wedge, must be accounted for in modeling studies. Therefore, we use a 2D thermomechanical model of subduction with empirical porosity values form the Nankai subduction margin and incorporate the effect of phase transitions to simulate the formation of the accretionary wedge under several sedimentary conditions and track the evolution of the vitrinite reflectance. As a result, we gain a holistic picture of deformation in accretionary wedges exploring different scenarios using geodynamic modeling alongside field data.

How to cite: Mannu, U., Fernández-Blanco, D., Miyakawa, A., Gerya, T., and Kinoshita, M.: Thermal maturity of the accretionary wedge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6446, https://doi.org/10.5194/egusphere-egu2020-6446, 2020.

The Trans-Mexican Volcanic Belt (TMVB) is a large-scale, NW to SE trending volcano-tectonic feature extending through central Mexico for a length of more than 1000 km. In some models, its genesis is related to the interaction between the subducting Rivera and Cocos plates and the North America plate, with the eastward propagation of volcanism being associated with slab detachment and consequent asthenospheric upwelling (e.g., Ferrari, 2004). Progressive SE-directed slab tearing has been causing crustal extension and the emplacement of large-scale volcano and caldera edifices. In the frame of the GEMex Europe-Mexico cooperation project (Horizon 2020 Programme, grant agreement No. 727550), we aim to investigate the interplay between continental extension and inherited crustal fabrics. Particularly, in the easternmost part of the TMVB, where the GEMex Project is focusing geothermal investigation on two calderas (Los Humeros and Acoculco), the inherited fabric is represented by ca. NE-SW and NW-SE regional faults (Campos-Enriquez & Garduño-Monroy, 1987). This fabric may have localized volcanic centres, thereby bearing significant implications for geothermal investigation. We aim to evaluate if and how the inherited structures may have interacted with continental-scale rift propagation through analogue modelling. In the models, the upper continental crust was simulated by a Qz- and K-feldspar sand mixture (80%-20% proportion in weight), while a PDMS-corundum mixture reproduced the lower crust. Continental rift propagation was simulated using a deformation apparatus represented by two basal moving plates hinged at their topmost side, allowing rotational opening. Extensional deformation was distributed using a basal rubber sheet. Artificial dilation zones (simulating the inherited fabrics) have been introduced within the analogue brittle crust at various angles to the rift axis. Our modelling highlights that a propagating rift may reactivate the inherited fabrics as extensional structures or transfer zones (depending on their orientation) for angles ≤45° to the rift axis. Numerical analysis of slip and dilation tendency evaluated for the reactivated fabrics corroborate the modelling results, and suggest that they may represent favourable sites for magma emplacement, and ultimately for geothermal exploration.

Campos-Enriquez, J., & Garduño-Monroy, V. H. (1987). The shallow structure of Los Humeros and Las Derrumbadas geothermal fields, Mexico. Geothermics, 16(5-6), 539-554.

Ferrari, L. (2004). Slab detachment control on mafic volcanic pulse and mantle heterogeneity in central Mexico. Geology, 32(1), 77-80.

How to cite: Maestrelli, D., Bonini, M., Corti, G., Montanari, D., and Moratti, G.: Rift propagation vs inherited crustal fabrics in the Trans-Mexican Volcanic Belt (Mexico): insights into geothermal investigations from analogue models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-666, https://doi.org/10.5194/egusphere-egu2020-666, 2020.

The evolution of fault-bounded basins and the concomitant migration of hydrocarbons and fluids are strongly influenced by fault activity and, in the case of an extensional tectonic setting, by the interaction of fault planes in relay zones. Fault linkage is a process that develops at relays between sufficiently closely spaced fault planes during their propagation. Fault interaction depends on several factors, such as the degree of under- or overlapping fault arrays, the similar or opposed polarity of fault planes and a separation that should not exceed a critical distance.

Motivated by observations at a km-scale fault relay of a major normal fault in the Magdalena Valley, Northern Andes of Colombia, we designed an analogous sandbox model, in which we simulated the linkage of rift zones separated at distances equivalent to two to four times the dimension of the height of a uniform sand layer. Fault nucleation took place at pre-designed seeds or at the velocity discontinuity of a moving sheet along the base of the sandbox and gave rise to two offset graben structures. Early fault linkage took place by means of two sub-vertical faults, which formed a shortcut between an inner and an adjacent outer border fault of the offset graben structures, enclosing a small horst in between.

The kinematic meaning of these short-cut faults became evident by the subsequent growth pattern of the faults opposite to the linked strands. On approaching the relay zone, these faults turned into an attitude almost perpendicular to their imposed trend. According to the displacement senses set up parallel to the axes of the offset graben structures, the displacement transfer on the two short-cut faults accommodated a strike-slip component. Particle analysis by means of the MATLAB’s PIVlab © tool and photogrammetric processes corroborated these findings. Displacement transfer on the short-cut faults set in at the very onset of the formation of the two graben structures. During successive deformation stages two distinct velocity fields parallel to the graben axes became established, each one pointing away from the structural high of the relay zone.

Although our boundary conditions are restricted to a uniform layer and orthogonal extension, this experimental scenario may form a starting point for testing new questions about the propagation of bounding faults at the termination of graben structures, such as those found at the East Africa Rift. Here, rifting evolved within a lithospheric high, impeding the accumulation of fine-grained or “soft” sedimentary sequences in precursor basins.

How to cite: Avila Paez, M. A., Quintana Gomez, R., Kammer, U. A., and Saavedra Daza, F.: Fault linkage and its controls on fault growth and basin evolution: Insights from analogue experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1213, https://doi.org/10.5194/egusphere-egu2020-1213, 2020.

The interference between two offset propagating rift systems creates fractures, with a sigmoid shape in map view and previously referred to as accommodation zones (Mc Clay et al, 2002). This peculiar kinematics may be observed in the Southeastern Brazilian margin in the Santos Basin, developed between the tips of two propagating, offset rifts. In this region, northward propagating rift was aborted during the southward propagation of another rift further to the east leading eventually to the opening of this segment of the South Atlantic. Could this structural setting explain the geometry and the position of the fracture zones in this basin?

To answer this question, we explore a range of geometrical and kinematic parameters with sandbox experiments to observe the deformation between these two propagating rift systems. The basement of the rift zones were modelled with rubber strips glued to rigid metal plates, following the setup of McClay et al, 2002. However, this setup suffers from the lateral contraction of the rubber due to its elastic extension (the Poisson’s effect). This introduces a spurious kinematics, and in particular an unrealistic opening at the contact between the two rift parts. A new device, whereby thin metallic strips are glued to the sides of the rubber sheet reduces very substantially the Poisson effect and therefore improves the simulation of the overall extension. 

Two main parameters are varied: the offset between the two rifts (D) and the relative velocity of extension of each rift. Narrowly spaced cross –sections of two experiments are interpreted to build 3D patterns.

The main results from the sandbox experiments are:

- Major and minor faults with the rifting zone localized by the rubber base present dips approximately equal to 75°.

- To obtain sigmoid fault array in map view best resembling the structural interpretation of Lebreton (2012), the rifts must be offsets (D>0) and the extension must be synchronous.

- The 3D fault patterns reveal that fault planes are not continuous in the accommodation zone, between the two rifts.  If these major faults are not connected in the central zone as shown by the physical models, then the fluid flow will be certainly influenced. This central relay zone could also be considered as a diffuse strain zone.

Numerical models will be helpful to introduce further material heterogeneities in this key area. The experimental results provide the data to validate the numerical modeling and to guide in the selection of the boundary conditions.

How to cite: Souloumiac, P., Robert, R., Maillot, B., Mohn, G., Leroy, Y.-M., Gauthier, B., and Gomez, J.-P.: Evolution of extensional faults between two rift systems: insights from sandbox experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8915, https://doi.org/10.5194/egusphere-egu2020-8915, 2020.

Development of normal fault that are affected by inherited extensional tectonic settings can be observed in many rift basins and is highly related to the some parameters, such as mechanical contrast between layers in different successive extensional tectonics, extensional ratio and post-rift stratal thickness of the inherited rift, etc. The South Depression of Tainan Basin (SD-TB), which consists of several half-grabens and went through two phases of rifting during the Paleogene and Neogene respectively, is one of a series of E-W to NE-SW trending Cenozoic rift basins in NE South China Sea. The main purpose of this study is, based on detailed description of normal fault structures on seismic sections and numerical PFC models, to investigate the sequential development of normal faults during the successive rifting and the effects of inherited tectonics on time-spatial distribution of the younger normal faults in the depression.

The normal faults in SD-TB can be grouped into three types. Type 1 normal faults cut downward through the pre-, syn- and the lowest part of post-rift strata of the Paleogene rift, Type 2 normal faults only cut off the Neogene strata, and Type 3 normal faults cut off both the Paleogene and Neogene strata and down to the basement. There is distinct distribution for the Type 2 normal faults; for the thinner post-rift strata of the Paleogene rift, the Type 2 normal faults would widely distribute in the area over the Paleogene grabens during the Neogene rifting, or rather concentrate on the margin the older graben if the post-rift strata are thick. As for Type 3 normal faults, the first type are the upward extended part of Type 1 normal faults that are characterized by significant displacement during the Paleogene rifting and the second type are located outside of the older grabens.

Such spatial distribution of normal faults can be demonstrated by numerical PFC models as set with different thickness of post-rift strata of the Paleogene rift before the initiation of the Neogene rifting. The models also demonstrate that the second type of Type 3 normal faults outside of the older graben initially were Type 2 normal faults but further cut downward to become Type 3 normal faults. While the second type of Type 3 normal faults have developed at variable thickness of post-rift strata, the first type did formed in the cases that thicker post-rift strata were deposited.

We propose that the thick post-rift strata of the Paleogene rift are related with the greater displacement along the main boundary fault of the graben, which not only created thick syn-rift strata but also induced significant post-rift subsidence as indicated by the estimated extension ratio. Also for the thicker post-rift strata, the induced stress during the Neogene rifting was more focusing over the inherited main boundary faults and caused the localized Type 2 normal fault and the first type of Type 3 normal faults.

Key words: Normal fault, Inherited structure, Numerical PFC model, South China Sea

How to cite: Wang, C.-H., Wu, P.-R., Yang, K.-M., Yang, C.-C. B., and Lin, C.-W.: Development of normal faults affected by inherited extensional tectonic settings and their succeeding strata in the South Depression of Tainan Basin, NE South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13241, https://doi.org/10.5194/egusphere-egu2020-13241, 2020.

Mobile belts are generally characterized by deformational structures of multiple generations, indicating complex spatial and temporal evolution of the strain fields. These deformed terrains show interference patterns indicating superposition of structures striking transverse to the orogenic trend which leads to the development of cross folds in mobile belts. Despite significant work on cross-folding, it is still not well understood how horizontal shortening can develop regionally along the trend of an orthogonal convergent belts. Our present work deals with the spectacular cross-folds in the eastern flank of the Singhbhum Proterozoic mobile belt.

This study uses three-dimensional continuum models to address the long-standing question: what is the tectonics of regional scale cross-folds with axial planes transecting the orogenic trend? Physical experiments were conducted with PDMS (Poly dimethyl siloxane), a Newtonian viscous material under lower strain rate of deformation. We propose that the belt underwent orogen-parallel flow during tectonic relaxation, developing orogen-parallel shortening, as observed in analogue models. This gravity-driven flow appears to be potential factor for cross folding in orogenic belts. In order to substantiate the deformation of analogue models, the horizontal shear stress was mapped in FE models. This reveals a distinct zone of shear stress localization in the eastern flank. Model results suggest that the arcuate belt is likely to show deformations by large horizontal shear at the flank of the model. This prediction agrees to the observations from analogue models. In order to study the large scale three-dimensional flow pattern, velocity vectors are plotted in the model. The vector diagram shows that the material flow does not take place orthogonally to the orogenic trend, while at the NE margin the flow direction is parallel to orogenic trend, resulting in the development of cross folds in Singhbhum mobile belts.

How to cite: Saha, P., Mitra, A. K., and Mandal, N.: Tectonic relaxation and the development of cross fold in the Singhbhum Proterozoic mobile belt: Insights from physical and numerical model experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1047, https://doi.org/10.5194/egusphere-egu2020-1047, 2020.

            In Central Mediterranean, the Sicilian Fold and Thrust Belt (SFTB) and Calabrian Arc, as well as the whole Apennine-Maghrebian belt, result from the subduction and collision with drifted micro-continental terranes. These terranes detached from the European margin and migrated southeastward in response to Neogene slab roll-back and associated back-arc extension. From N to S, the SFBT is divided in 4 main tectono-stratigraphic domains: (1) the Calabro-Peloritani terrane, drifted from the European margin and detached from the Corso-Sarde block since the back-arc opening of the Tyrrhenian basin, (2) the Neotethyan pelagic cover, constituting the remnants of the Alpine Tethys oceanic accretionary wedge, (3) the folded and thrusted platform (Panormide) and basinal (Imerese-Sicanian) series of the down-going African margin, and (4) the undeformed african margin foreland (Hyblean).

            The scarce good quality outcrops of key tectono-stratigraphic units and crustal scale seismic lines makes the structural architecture of the SFTB very controversial, as testified by the wide variety of tectonic interpretations (Bianchi et al., 1987; Roure et al., 1990; Bello et al., 2000; Catalano et al., 2013). Major outstanding issues particularly concern: (1) the occurence of Alpine Tethys units far from the region where the remnants of the Tethyan accretionary wedge outcrop (Nebrodi range); in a forearc position above the Peloritani block north of the SFTB and in an active foreland context along the southern front of SFTB; (2) the diverging suggested tectonic styles, from stacked large-scale tectonic nappes to foreland imbricated thrust systems rooted into a main basal décollement; and (3), the deposition environnement of substantial units such as the widespread Numidian Flyschs, from syntectonic foreland basin to wedge-top sedimentation.

            We used 2D analogue models to investigate the mechanical processes involved in the formation of the SFTB starting from the Oligocene Tethys subduction to the Middle Miocene - Late Pliocene continental collision with the African paleo-margin. Based on a detailed tectono-stratigraphic synthesis, complemented by field observations, we reproduce the first-order mechanical stratigraphy of the sedimentary and basement units involved in the SFTB as well as the structural inheritance of the African margin. Our models also include: syntectonic erosion and sedimentation, syn-orogenic flexure and adjustable material output via a “subduction channel“.  

            The analog models succeed in reproducing the general structure of the SFTB and main tectono-stratigraphic correlations. For instance, the Panormide platform is underthrusted beneath the Alpine Tethys accretionary wedge, then stacked above the Imerese basinal units and belatedly exhumed in response to basement anticlinal stack. Our results also suggest that the Alpine Tethys units couldn’t overthrust the whole African foreland in the Middle Miocene, nor be back-thrusted over the forearc basin during the Burdigalian. We rather favor a gravity-induced sedimentation process inducing reworking of the tethysian sediments at specific building stages of the accretionary wedge. The structural architecture of the modeled orogenic wedge is also consistent with a SFTB growing by frontal accretion and basal underplating of mechanically resistant stratigraphic units rather than by large-scale nappe overthrusting.  

How to cite: Henriquet, M., Dominguez, S., Barreca, G., Malavieille, J., and Monaco, C.: Analogue modeling and tectono-stratigraphic evolution of the eastern Sicilian fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5573, https://doi.org/10.5194/egusphere-egu2020-5573, 2020.

Although analogue models have successfully simulated many different types of arcuate fold-and-thrust belts, we were able to design a backstop whose curvature ratio diminished and its protrusion grade increased during experiments reproducing several kinematic features of progressive arcs never seen before 2016. General models were made up of an homogeneous silicone layer, where detachments tend to localize, overlain by a sand layer. They accomplished to simulate the overall structure and kinematics of fold-and-thrust belts of Mediterranean Arcs, especially that of the Gibraltar arc: (1) highly divergent thrust transport directions, (2) arc-perpendicular normal and strike-slip faults accommodating arc-lengthening, (3) transpressive and transtensional bands oblique to the main trend located in the lateral zones, (4) vertical axis-rotations up to 70º and (5) block individualization that rotated independently clockwise and counterclockwise in the left and right arc limbs, respectively.

However, the ductile layer is neither continuous nor homogeneous in natural cases, such that pinch-outs and diapirs previous to deformation are frequently found across and along strike. Thus, we have modified our original set-up including silicone pinch-outs and different sizes of silicone diapirs. Where silicone pinch-outs were subparallel to the apex movement, differences in the structural style along the foreland thrust-belt occurred. A forward thrust system over frictional detachments (no silicone), or wide, double verging thrust-systems over ductile detachments (with silicone) developed. Differential displacement between both types of thrust-belts was accommodated by transfer zones. Where silicone pinch-outs were perpendicular to the apex movement, the deformation front propagated up to the pinch-out, where it stopped and the thrust-system thickened up to its subsequent collapse. In models with pre-existing diapirs, first thrust and strike-slip faults nucleated close to diapirs and linked them. When deformation proceeded, all diapirs were added and deformed within the fold-and-thrust belts.

We also made experiments to analyze the ductile deformation and the influence of the brittle layer (sand) thickness. In only silicone models, a homogeneous deformation was observed at the grid scale, where each square was deformed by mostly simple shear in the lateral parts whilst by mostly pure shear in its most frontal part of the models. When a sand layer was sieved on top of the silicone layer, discrete structures developed. Although all models showed strain partitioning between arc-perpendicular shortening and arc-parallel stretching, as the brittle layer thickness increased, fold wavelength increased.

All these models show the high complexity derived from the different strain partitioning modes and the strain localization along and across-strike fold-and-thrust belts in progressive arcs. They can be extremely helpful to better understand this kind of arcuate orogens that are also the most frequent in nature. Even though these models were previously carried out to simulate the evolution of fold-and-thrust belts of Mediterranean arcs, they can also shed lights for the evolution of many others progressive arcs.

How to cite: Jiménez-Bonilla, A., Crespo, A., Expósito, I., Balanyá, J. C., Díaz-Azpíroz, M., and Soriano, M. T.: Analogue models of progressive arcs: strain partitioning and localization in fold-and-thrust belts developed over ductile layer of different geometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7358, https://doi.org/10.5194/egusphere-egu2020-7358, 2020.