TS7.2

The integration of low-temperature thermochronological and thermal maturity analyses constrains the maximum temperatures experienced during burial by the sedimentary fill of the central sector of the Greater Caucasus basin and the timing of its structural inversion. Raman spectroscopy, illite percentage and stacking order in illite-smectite mixed layers, illite crystallinity index, and Rock-Eval Pyrolysis analyses indicate that the maximum paleotemperatures experienced by the Greater Caucasus basin fill increase progressively from about 100 °C in the southern foothills of the central Greater Caucasus to close to 400 °C approaching the axial zone of the orogen. Apatite fission-track and apatite and zircon (U-Th)/He analyses along the same transect yielded ZHe ages between about 137 and 5 Ma, AFT central ages between about 37 and 4 Ma, and AHe ages between about 10 and 2 Ma, with progressively younger ages approaching the axial zone of the Greater Caucasus. Statistical inverse modelling of thermochronological data, integrating thermal maturity results and all other geological and geochronological constraints available, points to episodic exhumation during structural inversion of the central Greater Caucasus basin. Such basin was first partially inverted in Late Cretaceous/Paleocene times following Northern Neotethys closure along the Sevan-Akera suture zone; renewed basin inversion occurred since Middle-Late Miocene times as a consequence of far-field compressional stress transmission from the Arabia-Eurasia hard collision along the Bitlis-Zagros suture zone. It should be emphasised that this sequence of events applies only to the central portion of the Greater Caucasus and by no means should be extended to the other parts of such a large and complex orogenic system.

How to cite: Gusmeo, T., Cavazza, W., Zattin, M., Corrado, S., Schito, A., Alania, V., and Enukidze, O.: Burial and exhumation history of the Georgian sector of the central Greater Caucasus, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-110, https://doi.org/10.5194/egusphere-egu22-110, 2022.

The Carpathians fold-and-thrust belt resulted from the accretion of sediments during subduction of the European slab in the Cenozoic, and the collision of the ALCAPA and Tisza-Dacia blocks with the East European margin in the Early-Middle Miocene. In this study we unravel the history of nappe stacking and the thermal evolution of the Ukrainian Carpathians wedge by combining low-temperature thermochronology and tectono-stratigraphic analysis.

We collected 11 sandstone samples for (U-Th)/He and fission-track dating on detrital apatites and zircons. The resulting thermochronological data were modelled using QTQt to constrain the time-temperature paths independently for several nappes. We compare the results with the burial histories of the respective nappes as derived from their stratigraphy.

All zircon (U-Th)/He (ZHe) ages in our samples are older than the depositional age of the corresponding strata; they are thus non-reset and thought to mark sediment provenance. We identified two groups of ZHe ages; a younger group with ages around 130 Ma to 90 Ma in the inner nappes, and an older group with ZHe ages around 450 Ma to 200 Ma in the outer belt. Potential sources for these zircons are thought to be the Tisza-Dacia basement and its sedimentary cover for the younger ZHe age group, and the East European shield and/or its sedimentary cover for the zircons with older ZHe ages in the external nappes. Apatite (U-Th)/He (AHe) ages are mostly <20 Ma and show a trend of progressive younging toward the outer nappes. Apatite fission-track (AFT) ages display a similar pattern with overall younging of the central age from the inner to the outer nappes, except for the outermost Skyba nappe where AFT central ages are around 10 My older than in the adjacent Krosno nappe.

Modelling the sample time-temperature paths from AHe, ZHe and AFT data permits to unravel the chronology of nappe stacking. Most of the AFT samples are partially reset, allowing to better constrain the burial and exhumation pathways. The inner Magura and Marmarosh nappes started cooling from a peak burial temperature of 100 ± 5°C at 40 to 29 Ma. Our two samples from the following nappes, Burkut and Rakhiv, show younger cooling ages with an onset at 24 ± 2 Ma and at ~10 Ma, respectively, and significantly higher burial temperatures (≈150 °C) than the other nappes, provoking the total resetting of AFT ages. The next nappe, Krosno, started cooling between 21 and 17 Ma from peak burial temperatures of 100 ± 8°C. The Skyba nappe started cooling between 22 and 16 Ma. Whereas the onset of this cooling is similar to the Krosno nappe, the maximum burial temperature of Skyba is higher (130 ± 5°C) and at odds with the minor thickness (<1500 m) of the sedimentary overburden.

These results indicate potential out-of-sequence thrusting and/or significant erosional exhumation in the innermost nappes. For the middle nappes, burial by tectonic overthrusting and/or kilometre-scale syn-tectonic sedimentation is required. The higher temperatures experienced by the outermost nappes can resulted from overthrusting and complete erosion of the middle nappes.

How to cite: Roger, M., de Leeuw, A., van der Beek, P., and Husson, L.: Reconstruction of the Ukrainian Carpathians fold-and-thrust belt from low-temperature thermochronology and tectono-stratigraphic analysis., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7262, https://doi.org/10.5194/egusphere-egu22-7262, 2022.

The transition from the Eastern Cordillera to the Santa Barbara System in NW Argentina is characterized by the inversion of pre-existing Cretaceous and Paleozoic structures. Within this complex fold-and-thrust belt, the Miocene Cianzo basin with its rich sedimentary and structural record tells the tale of Andean reactivation of an extensional fault system and the incorporation of the former Salta rift basin into the orogenic wedge. In the Cretaceous, the intracontinental Salta rift was widely distributed in NW Argentina, with multiple sub-basins radiating from a central high. The Cianzo basin, at that time situated at the northern margin of the ENE-WSW striking Lomas de Olmedo sub-basin, impressively shows the transition from condensed post-rift strata on top of the rift shoulder to thick, proximal syn- and post-rift strata of the Salta Group in the adjacent half-graben. These sediments were buried by up to 3 km of clastics from the approaching orogenic wedge, causing apatite (U-Th-Sm)/He (AHe) and, in part, apatite fission track (AFT) ages to be reset. In the Miocene, deformation reached the Eastern Cordillera, gradually dissecting the foreland basin along pre-existing faults and forming local depocenters such as the Cianzo basin, which is surrounded by inverted normal faults and basement block uplifts. Its unique structural setting and complete sedimentary record provide an excellent natural laboratory to study fold-and-thrust belt formation through reactivation of pre-existing structures. Although the structural and sedimentary characteristics of the Cianzo basin have been studied in detail, low-temperature thermochronology data to quantify deformation processes is lacking. We provide AHe, AFT and zircon (U-Th-Sm)/He (ZHe) cooling ages from 39 samples from the Cianzo basin and adjacent areas. Jurassic ZHe ages from the Aparzo ranges may reflect pre-Salta Group exhumation of the rift shoulder, an event which is also recorded in thick, proximal agglomerates that were shed into restricted depocenters. AHe and AFT data document a Middle-Late Miocene onset of rapid exhumation of the Abra de Zenta, Hornocal and Aparzo ranges that border the Cianzo basin. Furthermore, AHe and AFT ages constrain the sequence of deformation for large folds such as the Cianzo syncline. The new dataset refines the timing of reactivation of pre-existing normal faults that now bound the Cianzo basin and sheds a new light on the propagation of the Eastern Cordillera fold-and-thrust belt in time and space.

How to cite: van Kooten, W. S. M. T., Sobel, E. R., del Papa, C., Payrola, P., Starck, D., and Bande, A.: Constraining the formation of the fault-bound Cianzo basin, NW Argentina using low-temperature thermochronology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7548, https://doi.org/10.5194/egusphere-egu22-7548, 2022.

When orogeny reactivates extensional structures or uplifts pre-existing depocenters in the foreland (inversion), the overall nature, dimension, and geometry of these rheological heterogeneities represent one of the main controlling factors in the spatio-temporal evolution of foreland fold-and-thrust belts. Relationships between inversion structures in the foreland and far-field stresses caused by orogenic fronts have long been identified (e.g., Ziegler, 1989, Geol. Soc. Spec. Publ. 44). However, conditions that facilitate or hinder basin inversion in these settings remain unclear, mainly due to the intrinsic complexity of analysing multiple overprinted geological events.

We use novel laboratory experiments of basin inversion to investigate how compressional stresses are transferred across a heterogeneous crust. More specifically, we determine how the presence of multiple extensional basins in the foreland controls the location, occurrence, and sequencing of foreland thrusts. Quantitative analysis of our experiments allows us to define conceptual models for comparison and application to natural examples where geological interpretation remains partially conjectural due to their intrinsic complexity, such as the permo-carboniferous troughs beneath the Swiss Molasse basin or the inverted Broad Fourteens Basin in the North Sea.

Our experiments are built in a modelling apparatus with a mobile backstop, using quartz sand to model brittle crustal materials and glass microbeads to simulate a weaker basal detachment layer. Velocity discontinuities at the base are created by attaching multiple thin basal sheets to the mobile wall during extensional phases (pulling). The location of each extensional basin is defined by the lengths of the basal sheets. During extension, the resulting graben-like structures are progressively filled with microbeads to create a sedimentary infill that is less competent than the surrounding rock. The basal sheets are completely detached from the mobile wall before the initiation of the shortening phase (pushing). Topography, surface and lateral deformation is quantified employing a high-resolution particle imaging velocimetry (PIV) system.

We present results of shortening multiple extensional basins at fixed distances from the orogenic front. Detailed analysis shows that extensional basin faults are not reactivated during shortening, but instead inversion is characterised by an initial squeezing of the basin fill and subsequent formation of either frontal or back thrusts that localise along the microbead-sand interface, leading to the overall uplift of the basins. This mechanism occurs independent of the distance of the basin to the orogenic front. However, when several grabens are present, the extent of shortening that each extensional structure localises differs greatly between experiments, showing variability according to the number of basins and their distance to the orogenic front.

When compared to reference models with a homogeneous crust, our results show that the presence of multiple extensional basins in the foreland exerts a first-order control on the evolution of propagating fold-and-thrust belts. Thrust location and sequencing evolve differently, with frontal thrusts developing along pre-existing basins boundaries at early stages, and subsequent stages of back thrust formation characterising wedge thickening at the hinterland of the extensional basins.

How to cite: Molnar, N. and Buiter, S.: Analogue modelling of inversion tectonics: investigating the role of multiple extensional basins in foreland fold-and-thrust belts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7027, https://doi.org/10.5194/egusphere-egu22-7027, 2022.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. In this contribution, we focus on the internal deformation of the DI and its eastern continuation towards the Dinarides. Using a series of crustal scale analogue models, we investigate the effect of Jurassic E-W extension on the NW-SE directed shortening of the DI during Alpine orogeny.

The brittle and brittle-ductile analogue experiments can be grouped in two sub-series. In sub-series A, the platform-basin topography has been created by pre-scribing an initial strength contrast between platforms and basins followed by one stage of indentation. In sub-series B, graben structures were developed through an initial extensional phase, subsequently followed by compression. The evolving grabens were syn-kinematically filled up to different thicknesses depending on the material used; either with quartz sand up to a platform/basin thickness ratio of 0,75 or with feldspar sand or glass beads up to the initial, non-stretched crustal thickness. The brittle upper crust of platforms was simulated with quartz sand, the brittle to ductile middle crust by either glass beads or by a mixture of polydimethylsiloxane (PDMS) silicon putty and quartz sand. In both sub-series, variations in the orientation of rheological boundaries with respect to the convergence direction have been modelled. This (oblique) basin inversion allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust.

Modelling results of (oblique) basin inversion confirm the localisation of deformation in areas of lateral strength contrasts (Brun and Nalpas, 1996), as transitions from platforms to basins represent. Spacing of in-sequence thrusts is larger on platforms and smaller in basins, visible in both, models of sub-series A (inversion of strength difference only) and B (inversion of strength difference and actual normal faults). The vergence of in-sequence structures varies from mostly foreland directed using glass beads as basal detachment, to pop-up structures using putty, to a combination of both using quartz sand only. Based on our modelling observations, we propose that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform/basin configuration.

To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out (for details on structural and thermochronological data see the contribution of Klotz et al. in session GD8.4). According to field observations, the shortening direction along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system, external eastern Southern Alps, Italy), changes locally from top SSW to top SSE along strike. We infer that the variability of shortening directions along these thrust faults depends on inherited geometries and is not the result of different deformation phases. One possible conclusion from this observation is that the number of deformation phases in the Southern Alps may have been overestimated so far.

References:

Brun, J.-P., Nalpas, T. (1996). Graben inversion in nature and experiments. Tectonics. v. 15, no. 3, p. 677-687.

How to cite: Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Internal deformation of the Dolomites Indenter, eastern Southern Alps: Orthogonal to oblique basin inversion investigated in crustal scale analogue models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8489, https://doi.org/10.5194/egusphere-egu22-8489, 2022.

Long-term tectonics numerical modelling at accretionary margin scale is a powerful tool to retrieve the influence of many parameters such as the spatial variations of the frictional properties along a simplified interface and its feedback on the deformation. Despite significant analogue and numerical studies on the evolution of accretionary prism, none of them account for heat conservation or temperature-dependent rheological transitions. Since Makran is one of the thickest accretion prisms in the world, the contribution of heat to the rheology of the prism cannot be ignored. Here, we solve for advection-diffusion of heat with imposed constant heat flow at the base of the model domain to allow the temperature to increase with burial. We start with a simple setup of one décollement layer to capture how the brittle-ductile transition affects the structures and geometry of the accretionary prism.

Our results show that a mature brittle-ductile wedge forms four different structural segments that can be distinguished based on topographic slope and deformation. An initial purely frictional segment is characterized by an imbricated zone and active in-sequence thrusts faults at the toe of the wedge. Its topographic slope is controlled by the basal friction of the décollement and is consistent with the critical taper theory prediction. The presence of the smectite-illite transition (dehydration reaction) leads to a flat topographic slope by the drop of friction. This flat segment produces little internal deformation and appears during the early stage of the accretionary prism formation. The third segment is marked by an increase of the topographic slope that begins with the onset of internal distributed viscous deformation in between brittle structures. Viscous deformation appears once the base of the model reaches 180°C while the décollement remains brittle. We refer to that segment as the brittle-ductile transition where both brittle and ductile deformation co-exists within the wedge together with high internal deformation. The last segment of deformation corresponds to the onset of the ductile deformation along the décollement by reaching a temperature of 450°C with an approximate flat zone without effective internal deformation. The topographic slope is again consistent with the critical taper theory, considering that a viscous décollement is equivalent to a brittle décollement of extremely low friction.

Knowing the impact of temperature transitions, we include more complexity in our simulations to increase the relevance of the models with the Makran accretionary prism. We calibrate the basal heat flow from BSR visible along seismic profiles. An intermediate décollement, essential for underthrusting to occur at the rear of the wedge, is added to the simulations. We show that the onset of underthrusting is controlled by the brittle-ductile transition. As tomographic models on land indicate packages with a higher velocity at depth, seamount subduction is another hypothesis tested. We conclude that the subduction of large seamount is accompanied by deep-rooted listric normal faults, whose location migrates through time. Seamount subduction also permits the formation of a large thrust slice zone and lateral variation of basal-erosion which can be followed in seismic profiles of Nankai and Makran subduction zones.

How to cite: Pajang, S., Le Pourhiet, L., Cubas, N., Khatib, M. M., and Heyhat, M.: The brittle-ductile transition signature in accretionary prism, insight from thermomechanical modeling. Application to Makran, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12281, https://doi.org/10.5194/egusphere-egu22-12281, 2022.

The outer wedge of the Indo-Burma wedge (IBW) has resulted due to oblique subduction of the Indian Plate below the Burma. In this study, we will use the analysis of outcrop-scale structures from Tripura-Mizoram fold belt (TMFB) to evaluate the structural evolution of the outer wedge of IBW. TMFB belongs to the widest section of the outer wedge that stretches from east to west for around 270 km (along 23.5° N latitude). The first order structure of the outer wedge is characterized by a series of north-south trending anticlines-and-synclines of varying tightness. Analysis of our field observations provide a detailed understanding on the evolution of the first-order structure of the outer wedge of IBW. Weshow that the style of folding progressively becomes complex towards the hinterland direction of the wedge. The complexity of the fold structure is defined by the development of different geometries of folds, including refolding of earlier structures. Interestingly, different geometries of folds towards the hinterland share a uniform orientation of folds axes, implying pure shear deformation. Our field observations allow us to infer that the outer wedge sediments of IBW have deformed in a ductile manner over a shallow decollement, lying beneath the Neogene sediments of the outer wedge. We attribute the ductile behaviour of the outer wedge sediments to the dominance of weak shale horizons and high pore fluid pressure in the entire Neogene sequence of the outer wedge. To gain a complete understanding on the style of the strain distribution within TMFB, we performed scaled laboratory modelling under oblique convergence. We used Polydimethyl Siloxane (PDMS) to simulate the viscous rheology of the Neogene sediments. Model results show strong consistency not only with the existence of across-strike variations in the tightness of fold patterns from east to west but also provides a strong basis for explaining the occurrence of along-strike variations of deformation intensity in the outer wedge of IBW, which gradually increases southward with narrowing the width of the wedge.

How to cite: Das, A., Roy, S., Dasgupta, S., and Bose, S.: Structural evolution of the outer Indo-Burma Wedge: Insights from field observations and laboratory experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8263, https://doi.org/10.5194/egusphere-egu22-8263, 2022.

The southern Pyrenean Zone shows a classical thin-skinned fold-and-thrust belt. Particularly interesting is the thrust sequence detached in the Upper Triassic low-strength level cropping out in the central-western sector of the chain (Leyre-Orba thrust system). Along its mapped trace, the Leyre thrust cleanly places the Cretaceous units on top of the Eocene (syn-tectonic) marls of the Jaca basin. However, in the footwall of this thrust there is a series of smaller-scale faults related to the main thrust and involving exclusively the marly units.

Understanding the deformation that marls (often lacking structural indicators) have acquired is a subject of interest to many geoscientists, given the role that these rocks play in geological storage systems. Knowing the state of the rock fabric may be essential to understand variations in its expected physical properties. In particular, Anisotropy of Magnetic Susceptibility (AMS) is a technique that can be successfully (although perhaps not easily) applied on these lithologies, provided that clay minerals present on marls are responsible of the magnetic signal of the fabric.

The marls of the Arro-Fiscal formation show fracturing and a pervasive cleavage along a width of hundreds of meters along strike the Leyre fault. The penetrativity of cleavage gradually increases with the proximity to the main fault, as demonstrated by Boiron et al. (2020). However, new data presented in Gracia-Puzo et al. (2021) and this communication show that the deformation gradient is not a single progression, but that there rather are variations in the intensity of deformation, as indicated by the magnetic fabric data of the marls, what can also be correlated with outcrop observations in the field.

A second look at the outcrops, after considering AMS data, has permitted to detect faults that a priori were not observed, since they involve the same monotonous lithology in both walls. Therefore, they are not presented in previous cartographies. In outcrop view, we can detect areas where the marls have undergone significant deformation, with a very penetrative, almost slaty cleavage. These deformation zones are metric in thickness, and the less intense pencil cleavage in the marls can extend several tens of meters in thickness across strike.

For realizing the presence of these faults, the study of the magnetic fabric has revealed as a useful tool, since it gives a very accurate picture of strain conditions (Parés, 1999; Gracia-Puzo, 2021). In this presentation, microscopic data are also added. Together with the AMS dataset, they are aimed to characterize the fabric of the deformed marl, and thus to understand how deformation has developed in the context of the foreland of the Leyre thrust and the Arro-Fiscal Formation, at different scales. Gathering AMS data, thin sections and field observations, we conclude that a symmetrical deformation shadow is observed in these faults, involving both the foot-wall and hanging-wall.

Finally, in this work, we aim to characterize these strained beds, which differ in scale and geometry from other known deformed marls, thus extending our knowledge of shale cleavage formation.

How to cite: Gracia Puzo, F., Aubourg, C., and Casas Sainz, A.: Reverse fault propagation in shales and associated decametric deformation gradients., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5905, https://doi.org/10.5194/egusphere-egu22-5905, 2022.

The Bristol Channel contains major Variscan thrusts juxtaposing distinct tectonostratigraphic terranes:  the Upper Carboniferous Rhenohercynian, Culm (south) and Sub-Variscan Foredeep, Coalfield (north). There is agreement the contrasts across the Channel are not restricted to this, since underlying marine Devonian differs from continental ORS and Lower Carboniferous radiolarian-chert differs from the Main Limestone. The famous basins were mapped intricately over 100+ years by the UK geological survey and by academics across Europe, and questions about their juxtaposition date back to 1895 in the Quarterly Journal of the Geological Society, London.

Our aim, is to use structural styles and shortening to determine an upper limit for displacements upon the major thrusts. We investigate the magnitudes of shortening from south to north through the Culm and north Devon basins, and from west to east across SW Dyfed, central South Wales, Bristol, Mendips, Oxfordshire, and Kent, using an immense legacy of sections drawn by various authors, including the recent basin dynamics group of Wales.

Estimates corrected for Mesozoic negative inversion show 45% shortening due to accommodation-chevron and box folding in the Culm, 40% due to folds, back-thrusts, and fore-thrusts in the north Devon basin, 30% beneath northern parts of the Channel, and 33% along the strike of the foredeep from Wales to Kent. There is also great contrast in deformation style, between the Culm continuous-folds and the foredeep with reactivated faults, rounded folds, and thrusts, related to preferential slip along seams within central parts of the Middle Coal Measures.

Shortening can be 70%, close to underthrusts in the southern Culm; adjacent to regional thrusts along the north Devon coast; and, proximal to disturbances within the foredeep. This intensity of composite deformation would not be out of place close to tectonic-scale thrusts, between these terranes. Additionally, thrusts of this scale are detectable on regional seismic profiles and were the topics of recent studies. Structural inspection reveals significant 1km-scale displacements along NW-SE strike-slip faults common to both terranes and upon WSW-ENE oblique-ramp thrusts local to the Vale of Glamorgan and Severn estuary. WNW-ESE frontal ramps with ~10km-scale displacements are considered candidate ‘stems’ to tectonic-scale thrusts and are found in Gower, Devon, and inner Channel.

Further investigations could elaborate the style of transmission of major thrust displacement from beneath the hinterland into the foredeep, whether by reactivation, decapitation, translation, and rotation of structural fabrics. There are complications of Mesozoic negative and Cenozoic positive inversions to consider in section restoration and adjustments are required to reveal how large displacements were dissipated exactly.

Reservations are that shortenings in hanging-walls can be poor indicators of displacement magnitude upon individual thrusts within sequences. Nevertheless, we conclude there is nothing contrary to the occurrence of a 100km-scale displacement, especially if accounting the tectonic-scale dimension and 300-500km geographic separations of modern terranes analogous to facies equivalent to the Culm and foredeep.

How to cite: Rutter, W. A. J., Miliorizos, M. N., Melis, N. S., and Reiss, N.: Structural styles and shortening estimates for the inverted external British Variscides to determine maximum thrust displacements., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8958, https://doi.org/10.5194/egusphere-egu22-8958, 2022.

The arcuate Jura Mountains Fold-and-Thrust Belt (FTB) is situated in the NW Alpine Foreland and its formation is related to the Alpine orogeny. The western part of the Jura FTB, investigated here, is situated in France to the north of the Geneva Basin (Switzerland). The geothermal project “GEothermie2020”  of the larger Geneva area allowed us to re-assessed the structural geology and the kinematic evolution of the internal part of Western Jura FTB from the Geneva Basin (Switzerland) to the Bienne Valley (France).

Stratigraphic harmonization, new geological and tectonic maps, new seismic interpretation, and a new near top Basement surface were used to construct a kinematic model. This model using forward modelling techniques has been developed in the software Move^tm by Petroleum Experts. The forward model relies on fault-bend fold, trishear, and fault-parallel flow algorithms, and provides a valid and balanced cross-section. The model is constrained by surface, well and seismic data. Therefore, the depth of the near base Mesozoic horizon has been well constrained by seismic depth-converted lines. Thus, we can show, that the top basement under the Jura domain is dipping 1.7° to the SE, whereas under the Geneva Basin it is dipping between 2.7°-3.3° to the SE. The results of our modelling show a shortening of 23.6 km for the western Internal Jura FTB along a basal detachment and a forward stepping deformation accompanied by minor back-stepping thrust sequences. The first deformation is attributed to the thrusting of the Crêt de la Neige anticline followed by the Crêt Chalam thrust and its imbrications. Then, the Tacon thrust and finally the Bienne thrust nucleate. Imbricate fault-bend folding explains the high southern slopes of the anticlines found in this area. In addition to the primary décollement level situated at the base of the Keuper Group evaporites, three other detachment levels are found in marly layers. Using such a multiple thrust horizon approach avoids having to introduce thick unaccounted for evaporitic duplexes in the Keuper units, basement horst, or inverted Permo-Carboniferous grabens. The change in dip of the top basement located under the SE flank of the Crêt de la Neige anticline, at the transition of the Jura FTB to the Molasse Basin, is considered to be linked to a preexisting Paleozoic normal fault and could correspond to the northern edge of a suspected Permo-Carboniferous graben interpreted on seismic lines under the Geneva Basin. This step can be considered as an initiation point for structures developing in the detached cover.

How to cite: Marro, A., Sommaruga, A., Hauvette, L., Borderie, S., Schori, M., and Mosar, J.: Tectonics of the Western Jura Fold-and-Thrust Belt: from the Geneva Basin to the Bienne Valley (France). Mapping and forward modelling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4607, https://doi.org/10.5194/egusphere-egu22-4607, 2022.

The pattern of active deformation of Himalayan frontal structures is complex with out-of-sequence reactivations in the chain and development of scarps associated to earthquake ruptures reaching the surface in the piedmont. We analyze passive seismic records using the Horizontal-to-Vertical Spectral Ratio method along three North-South trending profiles of the Darjeeling Himalayan piedmont, revealing subsurface structures down to 600 meters and imaging the Siwalik sedimentary rocks / recent deposits interface. We find evidence for a thrust fold system hidden beneath the plain correlated to geomorphologic scarps revealed by topographic profiles. These morphological surfaces are incised by large rivers of the piedmont by several tens of meters during phases of low sedimentation, thus slightly emerging in the plain. These scarps are supposedly induced by thrust deformation related to great earthquakes propagating south of the morphological front and inducing subsurface ruptures in the piedmont. This interpretation is comforted by the lateral correlation of the imaged thrusts and associated folds with previously evidenced fault scarps associated to active thrusts of the Darjeeling piedmont. In the piedmont of East/Central Nepal, oil company seismic profiles image similar thrusts and associated folds that can also be correlated to both local incision of small rivers draining the southern flank of the Siwalik hills, and uplift evidenced by a previously analyzed leveling profile.

The long-term kinematic evolution of this hidden thrust fold belt is slow, with a tectonic uplift of the hangingwall lower than the subsidence rate of the foreland basin, i.e., less than ~ half a millimeter per year. The evolution of the hidden structures corresponds to that of an embryonic thrust belt affected by a layer parallel shortening (LPS) acting in the long term with a shortening rate of the order of 5-10% of the shortening rate of the whole Himalayan thrust system. An aseismic deformation associated with the LPS structures that could absorb the entire deformation of the embryonic thrust belt in East/Central Nepal is suggested by the comparison of the long term structural evolution with geodetic and paleoseismological studies. The amplitude of this aseismic deformation is however too limited to significantly reduce the seismic hazard in the Himalayan piedmont.

How to cite: Large, E., Huyghe, P., Mugnier, J.-L., Jouanne, F., Guillier, B., and Chakraborty, T.: Development of a hidden fold and thrust belt in the Himalayan piedmont and distribution of active tectonics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7144, https://doi.org/10.5194/egusphere-egu22-7144, 2022.

Western Sichuan Depression is located in the west of the Sichuan Basin and has shown great gas prospects for many years. Due to the characteristics of multiple stages of tectonic evolution and multiple directions of tectonic distribution, petroleum geological conditions are extremely complex in this area. In this paper, we use geological data, seismic data, well logging, and petroleum geological data to study the tectonic characteristics and controls on hydrocarbon accumulation in the middle section of the Western Sichuan Depression. The middle part of the Western Sichuan Depression is dominated by thrust structures, including thrust structures formed by the combination of thrust faults, fault triangular belt, and thrust imbricate structures, as well as fold deformation related to thrust faults, such as snake-head structure and slippage fold. The study area is characterized by east-west zonation and up-down stratification. In the process of formation and evolution, Western Sichuan Foreland Basin mainly experienced three tectonic periods, namely the Indosinian period, the Yanshan period, and the Himalayan period. Multi-stage tectonics, the change of force source, and principal stress direction also lead to the formation of tectonic series and tectonic belts with different trends. Most of the traps of the Leikoupo Formation in the region had their embryonic form in the late Indochinese period, which was further developed in the Yanshanian period, and basically formed in the early Himalayan period. Therefore, the tectonic conditions and accumulation conditions were well arranged, forming self-generation and self-accumulation or a combination of self-generation and self-accumulation and up-generation and sub-accumulation reservoir formation mode. The superior accumulation conditions and tectonic conditions make the Marine strata of the Leikoupo Formation of the Western Sichuan Depression show more favorable exploration potential. This study reveals structural style of the piedmont belt in the foreland basin and establishes reliable evidences for the further development of regional structural and accumulation models, which are crucial for further oil and gas explorations.

How to cite: Wang, X., Chen, S., Zhang, C., and Li, W.: Tectonic Characteristics and Controls on Hydrocarbon Accumulation in Middle Section of Western Sichuan Depression, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5086, https://doi.org/10.5194/egusphere-egu22-5086, 2022.

The Taiwan thrust-and-fold belt results from the oblique arc-continent collision between the Eurasian Plate and the Luzon Volcanic Arc. In this context, inherited structures from the E-directed underthrusting Eurasian continental margin are being reactivated, causing uplift of crystalline basement rocks and the formation of transverse zones that influence the evolution of the structure, seismicity and topography of the Taiwan thrust-and-fold belt. The depth and geometry of the crystalline basement-cover interface of the continental margin are partly constrained by seismic velocities and the locations of earthquake hypocenters. However, further constraints are needed in order to obtain a better resolved location and geometry of this interface since this would improve the understanding of the deep structure of the thrust belt.

In this work, we investigate the geometry and position of first-order discontinuities of the underthrusting Eurasian continental margin to understand i) the role of the crystalline basement and the inherited structures in the deformation and, ii) the overall crustal geometry resulting from this collision. The approach we use is based on FFT and derivative based analyses, and 2D modelling of absolute gravity and magnetic anomalies. Techniques such as the calculation of the averaged power spectra have allowed us to infer the depth to the top of the most important discontinuities through the analysis of their gravity and magnetic wavelength signature. Results, obtained over different datasets show an upper interface at ~6 km depth and a lower at ~13 km depth. These are average values that we have better constrained through modelling, integration with other structural studies and comparison with tomography and seismic data. Results have helped us to improve the comprehension of the crustal structure of Taiwan and the Eurasian continental margin.

This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency of the Ministry of Science and Innovation of Spain. Olivia Lozano acknowledges funding from the same agency through contract PRE2019-091431.

How to cite: Lozano Blanco, O., Ayarza, P., Álvarez-Marrón, J., and Brown, D.: Depth to main crustal interfaces calculated through potential field data analysis and modelling: Implications for the study of inherited structures in Southern Taiwan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9529, https://doi.org/10.5194/egusphere-egu22-9529, 2022.

Taiwan orogen records a singular geological context where different stages of convergence are preserved. From south to north, the Taiwan region records the transition from oceanic and continental subduction of the SE China passive margin to its collision with the Luzon Magmatic Arc.

This study aims to define the thermal, compositional, and structural inheritance of the Chinese SE passive margin onto the processes of continental subduction and early collision. We combined geological and geophysical data (e.g., crustal thickness, seismicity, gravimetry, and magnetic anomaly maps) to propose a structural rift domain map of the SE China margin and its inversion. Open access seismic data is currently being interpreted to identify key tectonostratigraphic sequences, showing the crustal architecture and the tectonic-sedimentary evolution of the region. By building these offshore-onshore transects, we aim to capture the along-strike variations of the trench morphology. Initial results suggest a northward thickening of the accretionary prism in relation to the change from oceanic to continental subduction.

This work is part of an ongoing Ph.D. thesis in which the analysis of the results will not only focus on establishing new first-order tectonic models for the early collision but also on better constraining the control of former rifted margin on the locus of the deformation in this tectonically active zone.

How to cite: Rodrigues de Vargas, M., Mohn, G., and Tugend, J.: The role of the SE China passive margin for the formation of Taiwan orogen, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4911, https://doi.org/10.5194/egusphere-egu22-4911, 2022.

Andean orogenesis is expressed in the diverse deformational records of crustal structures and sedimentary basins in western South America.  Here we summarize retroarc structural styles within the Andean orogenic belt and foreland basin system through consideration of regional contractional fault geometries, their kinematic interactions with other structures, and the comparative involvement of crystalline basement and sedimentary cover rocks.  In assessing the controls on structural style, we emphasize the importance of precursor conditions and employ the concept of tectonic inheritance to identify four factors that influence Andean deformation.  (1) Structural inheritance involves the reactivation of preexisting faults or basement fabrics and accompanying inversion of sedimentary basins.  (2) Stratigraphic inheritance is exemplified by the preferential localization of interconnected thin-skinned structures above regional décollements developed in wedge-shaped stratigraphic packages versus isolated basement-involved thick-skinned fault structures formed in provinces with limited cover strata.  (3) Rheological properties guide the activation of new structures by means of the integrated strength, rock and mineral composition, fluid content and pressure, and associated mechanical heterogeneities and anisotropies that define crustal and lithospheric architecture.  (4) Thermal structure in the form of initial thermal conditions and later thermal perturbations (such as cooling/heating episodes related to arc magmatism, subducting slab dynamics, or lithospheric removal) can promote inboard advance or outboard retreat of deformation.  Spatial and temporal variations in the relative importance of these four inherited attributes likely resulted in a complex evolution of structural styles during Andean shortening. 

The major styles include: (1) thin-skinned fold-thrust systems affecting principally cover strata with ramp-flat structures above gently dipping regional décollements that ultimately root in middle to upper crustal levels; (2) thick-skinned basement-involved block uplifts delineated by isolated high-angle reverse fault structures that penetrate deeply and may root in the lower crust; (3) pre-Andean (preorogenic) and (4) Andean (synorogenic) extensional basins that have been inverted by fault reactivation during later shortening; (5) upper-crustal backthrust belts linked to deeper foreland-directed structures; and (6) salt-involved contractional structures with weak décollement horizons that facilitate lateral flow of evaporite facies.  These structural styles are not mutually exclusive and may overlap in time and space.  We propose that evaluation of the contrasting roles of structural, stratigraphic, rheological, and thermal inheritance will help explain how numerous Andean structures do not bear simple relationships to the history of plate convergence, subduction, and magmatism along the western margin of South America.

How to cite: Horton, B. K. and Folguera, A.: Structural styles and tectonic inheritance in the Andean fold-thrust belt and foreland basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6798, https://doi.org/10.5194/egusphere-egu22-6798, 2022.

GPS data suggest that the NW South America corner forms a semi-rigid and distinct tectonic block (Northern Andean Block) drifting at 0.6 cm/yr NE-ward along regional dextral strike-slip faults that bound an oceanic terrane accreted in Late Cretaceous times to western Ecuador and Colombia. This is consistent with an average 0.76 cm/yr Quaternary slip rate obtained from field investigation along the main strike-slip faults. Nevertheless, pure thrust tectonics characterize the external (eastern) Northern Andes deformation front from Ecuador to Colombia. Thus, the relevance of strike-slip versus thrust tectonics during Cenozoic times and their relation with oceanic terrane accretion are unclear. 

The incertitude on the magnitude of a hypothetical Cenozoic strike-slip deformation is reflected by the variable interpretations of the tectonic regime that generated the Ecuadorian Interandean Valley. This tectonic depression, blanketing the eastern side of the Cordillera Occidental, has been variably considered as due to extensional, thrust, or strike-slip tectonics.

Paleomagnetism may represent an important tool to unravel the Cenozoic tectonic history of the Northern Andean Block, as peculiar patterns of vertical axis rotations arise from strike-slip and thrust tectonics.

Here we report on the paleomagnetism of 31 mid-upper Eocene to upper Miocene mainly volcanic sites from the Cordilleras Occidental and Real of southern Ecuador. Eleven sites show that the western Cordillera Occidental underwent a 24°±10° clockwise (CW) rotation with respect to South America after late Miocene, while no rotation occurred further east. We relate the regional CW rotation to the emplacement of the Cordillera Occidental nappe onto the continental sediments of the Interandean Valley. As rotation and continental sedimentation onset ages are similar, we interpret such tectonic depression as a narrow flexural basin formed ahead of the advancing nappe front.

Previous authors find a post-Cretaceous 28°±9° CW rotation of the Coastal forearc that is statistically indistinguishable from the 24°±10° Neogene CW rotation documented by us in the Cordillera Occidental and Interandean Valley, implying that the whole W Ecuador Andean chain was detached and rotated over a mid-crustal detachment during the last 10 Ma. Eocene-Miocene paleomagnetic inclination values are systematically consistent with those expected for South America, thus excluding latitudinal terrane drift. Our results show that thrust tectonics prevailed over strike-slip displacement in the southern Ecuadorian Andes during the late Cenozoic.

Finally, we note that the orogenic reentrant-salient sequence of the Nazca trench / Andean chain from northern Chile to Ecuador mimics closely the margin of the Archean–Paleoproterozoic Amazonian Craton and other minor cratons of South America. Considering our results on a continental scale and in combination with previous paleomagnetic data from the Andean belt we infer that the stiff crust of the Amazonian Craton behaved as a foreland indenter, hampered inland deformation propagation, and caused the formation of what we call the “Ecuadorian Orocline”, arisen by opposite-sign nappe rotations around the Craton apex.

How to cite: Siravo, G., Speranza, F., Mulas, M., and Costanzo Alvarez, V.: Significance of the northern Andean Block extrusion and genesis of the Interandean Valley: Paleomagnetic evidence from the “Ecuadorian Orocline”, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3681, https://doi.org/10.5194/egusphere-egu22-3681, 2022.