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TS7.2

Foreland basins archive the evolution of mountain belts, and fold-thrust belts are the linking elements between orogens and their forelands. One of the major challenges for understanding the dynamics of mountain belts is untangling the different driving mechanisms that can be responsible for exhumation of mountain belts and foreland basin deformation. In particular, the signals of plate convergence (i.e. tectonic processes), deep seated (mantle-related) processes, or climate differ with respect to their timing and spatial extent. Ensuing foreland deformation is also influenced by heterogeneity of the deforming material. For instance, stratigraphic variations of the foreland basin fill or its substrate or inherited structures add complexity to the system.

In this session we invite contributions focusing on linking mantle and crustal tectonic processes with foreland basin dynamics. This includes addressing the interplay between plate boundary forces and of inherited structures, sediment production, transport and deposition (source to sink studies), and studies constraining timing of orogen processes at different scales (ranging from short term deformation rates to longer term rates based on cosmogenic nuclides or thermochronometry). We particularly invite contributions linking geophysical with geological data including 3-D models and addressing their respective uncertainties. We encourage the presentation of field-based studies as well as analog and numerical models highlighting the link between foreland basin deformation and mountain building processes including deformation of fold-thrust belts.

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Co-organized by GD5/SSP2
Convener: Christoph von Hagke | Co-conveners: Pablo Granado, Matthias Rosenau, Stefano Tavani
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| Attendance Wed, 06 May, 14:00–18:00 (CEST)

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Chat time: Wednesday, 6 May 2020, 14:00–15:45

D1257 |
EGU2020-5607
Jonas B. Ruh

Accretionary wedge systems result from scraping clastic sedimentary sequences off a descending oceanic plate at subduction zones. Sediments covering the incoming, subducting oceanic plate may be accreted at the wedge front forming a typical imbricate fan. Buried parts of the stratigraphic sequence may underthrust the wedge body and get subsequently accreted at its base (underplating) or even descend further into the mantle. Tectonic underplating requires a step-down of the major décollement horizon representing in fact the subduction plate interface. Intense underplating at the rear of accretionary wedges ultimately leads to antiformal stacking and, in combination of surface erosion, to the uplift of sediments formerly accreted at the base of wedges. In the present study, I try to determine and quantify the main features leading to tectonic underplating and subsequent uplift of underthrust incoming sediments at subduction zones. To do so, a numerical model is set up defined by a downgoing rigid plate, an overlying sedimentary sequence and a rigid backstop resulting in sediment accretion. The implementation of two weak décollement layers allows for the potential development of tectonic underplating. Tested parameters for the tectonic evolution of such systems include décollement strength, surface erosion, elastic stiffness of the downgoing plate and geometry of the rigid backstop. The applied numerical code is based on the finite difference marker-in-cell technique with a visco-elasto-plastic rheology. Results indicate that underplating is intensified when i) the stratigraphically lower décollement is strengthened with respect to the upper one, ii) surface erosion is increased, or iii) the downgoing plate becomes stiffer. Modelling results are compared to natural cases of accrretionary wedges where sediments have been underplated (and eventually exhumed) as in the Makran, the Franciscan, or the Appalachians.

How to cite: Ruh, J. B.: Numerical modelling of tectonic underplating in accretionary wedges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5607, https://doi.org/10.5194/egusphere-egu2020-5607, 2020.

D1258 |
EGU2020-5256
Sinah Kilian, Hugo Ortner, and Barbara Schneider-Muntau

We studied the Karwendel mountains of the Northern Calcareous Alps fold-and-thrust belt, that formed during the Upper Cretaceous. The study area exposes one of the principal thrusts that emplaces Triassic sediments on the top of Cretaceous rocks. Based on a detailed structural analysis (Kilian and Ortner, 2019) it has been demonstrated that the folds above the thrust are buckle folds.

With the finite element modelling, we aimed to create folds with wavelengths comparable to those observed in the field. The model set up is a simple layer cake model based on the stratigraphy of the western Northern Calcareous Alps. We used ABAQUS a finite element software to create the model. The material model is linear elasticity. During modelling, we tested different material characteristics and layer thickness. In all model runs a very weak décollement (possibly salt) is necessary to produce folds. We further tested the influence of erosion and re-sedimentation on the development of structures. We concluded that the growth of folds having wavelengths comparable to the field examples depends on the thickness of the competent layer, whereas the thickness of the incompetent layer has negligible influence.

We suggest that fold development in this part of the Northern Calcareous Alps is dependent on the interplay between the growth of evaporite-cored anticlines and surface erosion.

Reference

Kilian, S., Ortner, H.: Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps. Austrian Journal of Earth Sciences, Vienna, 2019, V.112/1, p.62-83, DOI: 10.17738/ajes.2019.0005.

How to cite: Kilian, S., Ortner, H., and Schneider-Muntau, B.: Buckle fold growth in the western Northern Calcareous Alps fold-and-thrust belt of Austria– finite element modelling and field observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5256, https://doi.org/10.5194/egusphere-egu2020-5256, 2020.

D1259 |
EGU2020-21901
Charles Aubourg, Gracia-Puzo Francho, Casas-Sainz Antonio, Izquierdo-Llavall Esther, Boiron Tiphaine, and Saur Hugo

The Sigues fold (Aragon, Spain) presents an exceptional outcrop where 1) the footwall is largely exposed, 2) it is constituted of homogenous shales, 3) the strain varies at distance from the emergent thrust, with all steps of cleavage development. The best model to explain the strain distribution is the trishear propagation of a thrust with a P/S ratio of 1.  However, from East to West, the thrust geometry is changing progressively from blind thrust to flat ramp. The topographic surface as well as the position of the emerging part of the thrust determine the geometry of the structure. This is, therefore, a place with variable geometries, which allow us to describe the different geometric stages of the ramp-and-flat model that we are used to find in major orogenic thrusts.

To map the strain, we measured the magnetic fabric of hundreds of shale fragments (weighting a few grams) in dozens of localities. The magnetic fabric is governed primarily by illite. Hence, the magnetic fabric represents a 3D view of illite organization, i.e. the matrix of those shales (see Gracia-Puzo et al., EGU, EMRP3.8). The measurement of 3D fabric of illite takes about a minute per fragment and is non-destructive.

Magnetic fabric of shale fragments provides three useful parameters, the degree of anisotropy, the shape parameter from oblate to prolate, and the length of the confidence angle of the minimum axis of tensor. We show that all these three parameters are highly sensitive to strain. While each locality provides homogenous results from ~15 fragments (covering few square meters each), it is statistically different from one site to the other, with trends consistent with distance to the main thrust. Assuming rigid rotation of illite particles, we calculate the strain using Eigen values of magnetic fabric tensor.

Our preliminary maps shows: 1) that the strain increases considerably (from units to tens in %) when approaching the main thrust, 2) at a distance of more than 1 km, several strain gradients are detected, suggesting that blind thrusts propagate in the footwall. Serial N-S cross-sections are expected to describe the lateral variability on the structure, the deformation accumulated on the footwall and also establishing the portion of the hanging wall which is being affected and the décollement of the thrust. Our approach is thus promising to map strain in shales from deformed regions, both from natural outcrops, or from boreholes.

How to cite: Aubourg, C., Francho, G.-P., Antonio, C.-S., Esther, I.-L., Tiphaine, B., and Hugo, S.: Mapping strain in the footwall of a thrust: Preliminary results from 3D bulk fabric of illite. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21901, https://doi.org/10.5194/egusphere-egu2020-21901, 2020.

D1260 |
EGU2020-1916
Wen-Xin Yang, Dan-Ping Yan, Liang Qiu, Michael. L Wells, Jian-Meng Dong, Tian Gao, and Zhi Zhang

Nanpanjiang Basin (also called the Youjiang Basin or Dian-Qian-Gui Basin in literatures), the foreland basin of the Indosinian orogenic belt, is located on the boundary belt between the South China and Indochina Blocks. This foreland basin is characterized by a transition from the Early Triassic shallow-marine carbonate platforms to Middle and Upper Triassic continental facies clastic rocks and reworked by the subsequent Indosinian foreland thrusting and deformations. The development of the Indosinian foreland fold-and-thrust belt remains underappreciated in part because of the loose constraints of the transition from basin deposition to deformation and erosion. In this study, we present two geological cross-sections that synthesized field geological investigations, together with the structural interpretation of three seismic profiles, and LA-ICP-MS detrital zircon age constraints. The results reveal that the thrust belt is characterized by fault-related folds with duplex and imbricate thrusts, which yield the NNE-trending regional shortening estimate of ~36%. The new constraints indicate that the Nanpanjiang foreland basin formed before 237 Ma (D11) was overridden by the following NNE-ward progressive deformations, including 237-225 Ma thick-skinned thrusts (D12), 223-183 Ma thin-skinned thrusts (D13), and after that entire basin-involved deformation (D14). Subsequently, D1 was re-deformed and superimposed by the Middle to Late Jurassic NNE-striking fault-related fold system (D2). D11-4 reveals a NNE-verging propagation in-sequence foreland thrusting which overrode the foreland basin and the corresponded NNE-ward progressive foreland basin during the Indosinian.

How to cite: Yang, W.-X., Yan, D.-P., Qiu, L., Wells, M. L., Dong, J.-M., Gao, T., and Zhang, Z.: Formation and forward propagation of the Indosinian Nanpanjiang foreland basin and foreland thrust belt in SW China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1916, https://doi.org/10.5194/egusphere-egu2020-1916, 2020.

D1261 |
EGU2020-11748
Matthieu Branellec, Nuria Carrera, Josep Anton Munoz, Jean-Claude Ringenbach, and Jean-Paul Callot

The Central Andes (12°S-36°S) stretches over more than 2400km. They are characterized by strong longitudinal and latitudinal segmentation (Sierra Pampeanas, Precordillera, Cordillera Frontal, Cordillera Principal from east to west), each domain having distinctive basement involvement and showing different structural styles. The Argentinian Precordillera, located at 30°S, has long been interpreted as a thin-skinned wedge detached below into the lower part of Paleozoic succession. It makes up a typical Coulomb foreland thrust belt system. However, the impact of the Paleozoic inheritance derived from the various orogenic stages on the current structural style has been overlooked. The Chanic structures that developed in Silurian / Devonian times have been reactivated by the Andean deformation that took place from Oligocene to Plio-Pleistocene times. The current structure of the Precordillera has been the subject of numerous studies in the last decades. Thanks to compilation of this literature and fieldwork, we present a new cross-section considering these 2 superimposed events. This cross-section can be divided into 2 different zones depending on the dominant structures. The western Precordillera involves an Ordovician succession characterized by Chanic superimposed folding phases with cleavage development. On the contrary, in the eastern part, most of the observed structures were developed during Andean orogeny. The structural style is characterized by thrusts faults and penetrative deformation is absent. The Sierras Pampeanas in the East are a Miocene thick-skinned system that makes up a typical broken foreland system. The association of both systems of Precordillera and Sierras Pampeanas delineate an inheritance-controlled original orogenic thin-skinned system that turns to the east into a broad thick-skinned system involving up to Precambrian rocks.

How to cite: Branellec, M., Carrera, N., Munoz, J. A., Ringenbach, J.-C., and Callot, J.-P.: Contractional inheritance and rheology controls in a FTB: the Argentinian Precordillera, Central Andes (30°S), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11748, https://doi.org/10.5194/egusphere-egu2020-11748, 2020.

D1262 |
EGU2020-22109
Thomas Voigt, Jonas Kley, and Christoph Wehner

Triangle zones are thrust sheets or stacks of thrust sheets underlain by foreland-directed thrusts and overlain by a kinematically linked “passive” roof thrust - a backthrust - directed towards the hinterland. They are not uncommon in thin-skinned fold-and-thrust belts. Most triangle zones are known from seismic data and drilling. We describe a km-scale example exposed on a flank of the Altyn Dara valley near the thrust front of the Pamir mountains in Kyrgyzstan. The External Pamir is a high-level thrust belt built from non-metamorphic strata of Permian to Neogene age. It is bounded on its internal, southern side by the Main Pamir thrust with metamorphic rocks in its hanging-wall and in the north by the Pamir Frontal thrust which juxtaposes it with undeformed foreland strata of the Alai valley.

The triangle zone has formed where the basal detachment of the External Pamir ramps up from Lower Cretaceous redbeds into a succession of Upper Cretaceous marine pelites with a few intercalated limestone horizons. The strongly deformed Upper Cretaceous strata are contained between a north-directed thrust and a south-directed backthrust, both of which carry Lower Cretaceous rocks in their hanging-walls. In stark contrast to classical models, the core of the triangle zone is occupied by a bundle of essentially unfaulted, isoclinal upright folds. The subvertical axial planes diverge slightly upwards and changing elevations of the synclinal troughs suggest an anticlinorium. This structure is exposed over a vertical distance of 1 km in the steep flank of Pik Sverdlova. The folds involve four shaly packages and three limestone horizons. The initial total thickness of this succession was about 500 m. A strong slaty cleavage is developed in the shales, but the limestones do not show marked thickness variations between the long, straight fold limbs and the tight but rounded hinges. Assuming negligible penetrative strain in the limestones, unfolding the sinuous bed length suggests 10 km of horizontal shortening accommodated by folding.

Its overall geometry suggests that the triangle zone originated as a wide zone of detachment folding above a thrust fault propagating at the base of the weak Upper Cretaceous shales. The strong contraction may indicate some kind of buttress towards the foreland such as a syndepositional fault against which the Cenomanian-Turonian succession thinned or terminated, or the backthrust itself if it initiated early on. At any rate, the highly shortened bundle of folds was at some point bypassed along a deeper detachment in Lower Cretaceous strata into which the backthrust merges.

The internal structure of the Pik Sverdlova triangle zone would be difficult to image by conventional seismic techniques. Vertical drilling would also be unlikely to fully reveal the folded architecture. We speculate that in many triangle zones folding may be a more important mechanism than incorporated in the prevailing thrust-stacking models.

 

How to cite: Voigt, T., Kley, J., and Wehner, C.: A mountain of folds: triangle zone in the fold-and-thrust belt of the External Pamir, Kyrgyzstan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22109, https://doi.org/10.5194/egusphere-egu2020-22109, 2020.

D1263 |
EGU2020-13310
charlie kergaravat, jean-claude ringenbach, and Jaume Verges

The New Guinea Orogen evolved by the accretion of volcanic arcs onto the northern Australian margin during the Cenozoic arc-continent collision. Since that time, the northern Australian margin has undergone oblique convergence with Pacific plate involving volcanic arcs and intra-oceanic basin in between. The resulting FTBs are the Papuan FTB, the Mobile Belt and the Aure-Moresby FTB belonging to the curved shape New Guinea Highlands.

Previous regional structural studies were focus to the Central Papuan FTB. Concerning the Aure-Moresby FTB, few studies based mainly on field work describe a highly deformed Neogene underfilled foreland basin with mixed carbonate-siliciclastic deposits. One regional cross-section through the onshore Aure-Moresby FTB is proposed by Kugler during his PhD in 1967. In this regard, some lack of consistency about the regional structural style can be highlighted such as the different timing and amount of shortening between the Papuan and Aure-Moresby FTBs and the large N-S positive flower structure to explain the uplift of the Aure FTB.

The main goals of this study on the Aure-Moresby Foreland FTB are: (i) to discuss the impact of the mechanical stratigraphy on structural style, (ii) to estimate the significance of basement involvement and its morphology, (iii) to determine the shortening by comparing the regional balanced and restored cross sections, (iv) to estimate the relative ages of tectonic deformation and to propose a 2D kinematic model illustrating the evolution of the orogenic system since the Late Cretaceous.

For this purpose, 2D seismic profiles, chronostratigraphic synthesis, remote sensing mapping, wells and gravimetric data have been integrated in order to construct a consistent structural evolutionary model of the Aure FTB. This study is mainly focused on the interpretation of NE-SW trending 2D seismic lines in Move software to build a balanced cross-section from the hinterland to the foreland Aure foredeep.

This study shows that the Aure-Moresby FTB structure is the result of thin-skinned deformation along Late Cretaceous, Miocene and Pliocene detachment levels affected by recent thick-skinned deformation. The section is characterized by multiple fault-propagation folds detached at various level within the Mesozoic and Cenozoic. In the central Aure FTB, two main structural steps show major uplifts that correspond to the wide Dude Anticlinorium and the Kapau Margin interpreted as crustal scale thrusting rooted at the brittle/ductile transition and connected with the Cretaceous décollement level. Crustal scale deformation seems to be transmitted into Mesozoic and Cenozoic decollements and disharmonic levels forming the frontal folded zone. In the frontal Aure FTB, Miocene carbonate may be involved in the deformation forming potentially Pleistocene structural traps. Based on flexural slip restoration technique, 28 km of shortening have been calculated within the sedimentary cover along 250 km that correspond to a ratio of 11,2 %.

How to cite: kergaravat, C., ringenbach, J., and Verges, J.: Evolution of the Aure-Moresby Foreland FTB (Papua New Guinea): Constraints from balanced crustal scale cross-section and forward modeling., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13310, https://doi.org/10.5194/egusphere-egu2020-13310, 2020.

D1264 |
EGU2020-7670
Antonio M. Casas, Pablo Calvín, Pablo Santolaria, Tania Mochales, Hmidou El-Ouardi, ESther Izquierdo, Teresa Román-Berdiel, Sara Torres, Andrés Pocoví, Belén Oliva-Urcia, Bennacer Moussaid, Marcos Marcén, Andrés Gil-Imaz, Vicente Carlos Ruiz, María Felicidad Bógalo, Elisa Sánchez, Ángela Herrejón, Ángela Jiménez, Juan José Villalaín, and Irene Falcón

Multiple constraints, including poorly known parameters, determine along-strike changes of frontal thrust structures in fold-and-thrust belts. Along the 400 km long, continuous Central Moroccan Atlas belt, structural style shows significant changes, preserving similar figures of shortening. This implies the absence of large-scale vertical-axes rotations, as demonstrated by paleomagnetic studies accomplished during the development of this project. The main factors controlling thrust geometry are:

- the geometry of Triassic-Jurassic extensional basins subsequently inverted during Cenozoic compression, with especial mention to changes of cover thickness and orientation of structures

- transfer of displacement between the northern and southern thrust systems

- transfer of displacement between the basement (Paleozoic) units and the Mesozoic cover through the Upper Triassic detachment. This factor strongly determines the width of the belt in each transect, as it occurs in other basement-and-cover fold-and-thrust belts

- cover/detachment thickness ratio.

- localization and partitioning of deformation between different structures in the inner part and the borders of the massif

- amount of superposition between different cover thrust sheets, including folded thrusts

- structural style, changing from thin-skinned style to large recumbent folds along strike, probably depending on P-T conditions and cover thickness

- backthrusts related to low cover thickness/detachment thickness ratio, especially frequent in the northern Atlas thrusts

- differential shortening between sections related to layer-parallel shortening and folds associated with cleavage development in the central part of the chain

- influence of previous structures, such as individual diapirs, salt walls or igneous intrusions that modify the pre-compressional geometry of the detachment level, nucleate structures and favor buttressing. This feature can also be a source of errors in the calculation of shortening.

All these factors result in strong along-strike changes such as branching of thrust surfaces, progression of deformation towards the foreland and differential cleavage development. Influence of structures developed during the basinal/diapiric/igneous stage results in a variability of trends that varies between from less than 10° to more than 30°, what allows in some cases to distinguish between structures controlled by basinal features and newly formed thrusts.

In spite of the different techniques for cross-sections reconstruction, and in some cases, the different interpretations for the origin of structures, the shortening figures obtained along the chain are remarkably constant, on the range of 35 km, thus implying a 18 to 30% of shortening for most of the transects what attests for the reliability of the results.

Recognition and quantification of factors controlling the development of structures is the fundamental step to determine the main thrust surfaces, and the secondary backthrusts in a region where basin inversion is one of the main constraints. Structural criteria point to a dominant southward vergence and secondary northwards-directed thrusts. Minor strike-slip components were probably localized in the core of the chain. Present-day 3-D reconstruction of the Atlas is currently being done considering all these inputs as well as those obtained from merging the vast dataset obtained.

How to cite: Casas, A. M., Calvín, P., Santolaria, P., Mochales, T., El-Ouardi, H., Izquierdo, E., Román-Berdiel, T., Torres, S., Pocoví, A., Oliva-Urcia, B., Moussaid, B., Marcén, M., Gil-Imaz, A., Ruiz, V. C., Bógalo, M. F., Sánchez, E., Herrejón, Á., Jiménez, Á., Villalaín, J. J., and Falcón, I.: Varying thrust geometry along the Central Atlas fronts: structural criteria for 3-D reconstruction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7670, https://doi.org/10.5194/egusphere-egu2020-7670, 2020.

D1265 |
EGU2020-13016
Mortaza Pirouz, Jean-Philippe Avouac, Adriano Gualandi, Muhammad Hassan Quddusi, and Weitong Huang

We constrain foreland basins geometry to assess the equivalent elastic thickness of the lithosphere and the loads that have originated due to the collisional process. Geometry of the foreland relates to the topography loading and hidden subsurface load based on simple 2D flexural models have been done for many foreland basins during 80 and 90s (for example for Zagros, Taiwan, and Colville foreland in Alaska) and most of them highlighted that present topographic and basin loads are not enough to provide such a deflection and extra buried loads are expected. Recent 3D flexure models using conventional approaches are used to estimate the elastic thickness of the lithosphere and may highlight a need for the buried loads. In the conventional approach, we apply topographic and basin loads and use the common assumption that the space created due to the deflection is filled with material of crustal density. This takes place in the deflection function by Δρ=ρmantlecruct. The conventional approach thus includes the static subsurface load associated with the buoyancy of the crustal root. In the new approach, we assume that the deflection is filled with air Δρ=ρmantleair and the sub-surface load is proportional to the topographic load. The load from topography and sub-surface loads is then simply λ times the topographic load. This approach allows accounting for quantifying all sub-surface loads correlated with topography, including the effect of the crustal root. In the new approach the Moho depth, representing the ratio between the root and topographic height, can be considered and the results give more clue about recognizing and quantifying buried load or mantle dynamics loads. With the new approach, we investigated Zagros, Taiwan, and Colville basin-Alaska foreland basins and obtained very precise models with less than 5% misfit between observations and predictions. Previous studies highlight that the buried loads are needed to obtain comparable results to the observations. However, our results show that the best models do not need extra buried loads with a reasonable ratio between topographic relief and crustal root using the new modeling approach.

Abbreviations: Tc, thickness of undeformed lithosphere; wt, deflection due to topographic load; Pt, topographic load; ρm, density of Mantle; ρc, density of crust; ρa, density of air; g, gravity acceleration; β, flexural parameter; Hr, root thickness; Ht, topographic height, and λ, ratio between root thickness and topographic height.

How to cite: Pirouz, M., Avouac, J.-P., Gualandi, A., Quddusi, M. H., and Huang, W.: New Versus Conventional Approach for Modeling Flexure of Foreland Basins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13016, https://doi.org/10.5194/egusphere-egu2020-13016, 2020.

D1266 |
EGU2020-11043
Frédéric Mouthereau, Paul Angrand, Juliette Rat, and Maxime Daudet

Abstract : The heterogeneous continental lithosphere of Western Europe inherits billion of years of tectonic evolution, mineral transformation and magmatic addition. Its deformation over broad regions form collisional orogens and large forelands basins, which tectonic evolution is controlled by the interactions between its inherited properties, large-scale plate convection and smaller-scale plate subduction. How these first-order interactions are connected through time and space to control collision evolution is however largely unknown. Here we explore the evolution of the Alpine collision along a transect stretching between northern Africa and Europe. We show that the complex patterns of Alpine deformation from the Rif-Betic, Pyrenean collision and Europe primarily reflects continental fragmentation and drastic weakening of the lithosphere that occurred during the Late Permian-Triassic. Subsequent rifting episodes from Jurassic to Early Cretaceous left imprints on the thermal evolution of sedimentary basins, together with significant increase of Iberia topography, asthenospheric flow, and plate-scale dispersion of terrigenous sediments. The lack of large oceanic domain, at the transition between Atlantic and western Tethys, resulted in the distributed of shortening over a broad region from north Africa, Iberia and Europe, in the upper Cretaceous (~70 Ma). Detailed contraints on the sequence of shortening throughout West Europe from Late Cretaceous to the Tortonian reveals that the overall evolution of the west-alpine orogenic domain is primarily controlled by the nature and architecture of the continental lithosphere but became progressively controlled by sub-lithospheric processes associated with late/post-orogenic tectonic evolution.  

How to cite: Mouthereau, F., Angrand, P., Rat, J., and Daudet, M.: Western Europe tectonic evolution : probing the relative role of inheritance and sub-lithospheric processes , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11043, https://doi.org/10.5194/egusphere-egu2020-11043, 2020.

D1267 |
EGU2020-2601
Saskia Köhler, Florian Duschl, Hamed Fazlikhani, and Daniel Köhn

The Franconian Basin in SE Germany has seen a complex stress history indicative of several extensional and compressional phases e.g. the Iberia-Europe collision acting on a pre-faulted Variscan basement. Early Cretaceous extension is followed by Late Cretaceous inversion with syntectonic sedimentation and deformation increasing progressively from SW to NE culminating in the Franconian Line where basement rocks are thrusted over the Mesozoic cover. The development of this intracontinental fold-and-thrust belt is followed by Paleogene extension associated with the formation of the Eger Graben, which is then succeeded by a new compressional event as a consequence of the Alpine orogeny.

We use existing data from literature and geological maps and new field data to construct balanced cross-sections in order to reveal the architecture of the Cretaceous fold-and-thrust belt. In addition, we undertake paleostress analysis using a combination of fault slip information, veins and tectonic and sedimentary stylolites to identify stress events in the study area, as well as their nature and timing. Furthermore, we try to understand how basement faults influence younger faults in the cover sequence.

Our paleostress data indicates that at least five different stress events existed in Mesozoic to Cenozoic times (from old to young): (1) an N-S directed extensional stress field with E-W striking normal faults, (2) a NNE-SSW directed compressional stress field causing thrusting and folding of the cover sequence, (3) a strike slip regime with NE-SW compression and NW-SE extension, (4) an extensional event with NW-SE extension and the formation of ENE-WSW striking faults according to the formation of the Eger Graben in the E, and finally (5) a strike slip regime with NW-SE compression and NE-SW extension related to Alpine stresses. The geometry of faulting and deformation varies significantly over the regions with respect to the influence of and distance to inherited Variscan structures.

We argue that the extensional event of stress field (1) provides spacing for Early Cretaceous sedimentation in the Franconian Basin. This is followed by the creation of an intracontinental fold-and-thrust belt during stress fields (2) and (3) with a slight rotation of the main compressive stress during these events in Late Cretaceous. We associate the following extension to the development of the Eger Graben in Miocene time. Finally, a NW-SE directed compression related to Alpine stresses in an intracontinental strike-slip regime is following. Reconstruction of the Cretaceous fold-and-thrust belt reveals mainly fault propagation folding with deep detachments sitting below the cover sequence indicating thick-skinned tectonics. We argue that the Franconian Line is a thrust with a steeply dipping root that belongs to the same fold-and-thrust belt.

How to cite: Köhler, S., Duschl, F., Fazlikhani, H., and Köhn, D.: Inversion tectonics, intracontinental fold-and-thrust belts and complex stress fields in central Europe: the history of the Franconian Basin revealed by paleostress analysis, geological maps and field observations. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2601, https://doi.org/10.5194/egusphere-egu2020-2601, 2020.

D1268 |
EGU2020-6742
Marco Herwegh, Alfons Berger, Edi Kissling, Nicolas Bellahsen, and Yann Rolland

Late stage Alpine collision is the result of collision of the European and Adriatic continental plates. For this stage, particularly the External Crystalline Massifs (ECM) and their forelands provide information on deformation style/kinematics, exhumation history and geodynamic driving forces. Using the ECMs as marker for the non-thinned European passive continental margin, the margin’s paleogeography with its curved geometries controls in parts later compressional tectonics. During closure of the Valaisan in Eocene times, a major NW-SE trending sinistral transfer zone must have acted as lateral ramp between the NNW migrating Penninic front and the westerly situated European margin units (Argentera and Maritime Alps, stage 1 of Schmid et al. 2017). Hence, first transpressive movements were documented by transpressional strike-slip faults in the case of Argentera and southern Belledonne s.l. Massifs. The SW-NE trending ECMs became affected during a first stage of horizontal tectonics in Oligocene, when first the margin sediments were scrapped off their substratum (Helvetics, Chaînes Subalpines, Dauphinois) followed by thick-skinned thrusting. Transport directions gradually changed from W to N from the Western towards the Eastern ECMs. This spread in transport direction is the consequence of an Alp-internal vertical uplift (Internal Crystalline Massifs (ICM), Lepontine dome) induced by indentation of Adriatic mantle into European lithosphere (stage 2 of Schmid et al. 2017). With further down-bending of the European crust, a major change to a vertical tectonic deformation style occurred in Mid to Late Miocene. Steep reverse faults in the ECMs, in parts with oblique slip components (Mont Blanc), witness this stage with its enhanced vertical rock uplift component. The latter is most pronounced in the Aar Massif and gradually decreases towards the West (Belledonne s.l.). With continuous ICM exhumation in Late Miocene, deformation style switches again to horizontal tectonics, leading to ‘en bloc’ exhumation above basement thrusts of all massifs (Belledonne to Aar Massifs). Progressive shortening induced thrust propagation into the foreland sediments as well as the Jura mountains. In the case of the Argentera Massif, oroclinal bending probably led to a substantial anticlockwise rotation, which goes in hand with the rotation of the entire SW Alpine arc (stage 3 of Schmid et al. 2017). In a geodynamic context, the ‘Adriatic push model’ could explain aforementioned stages of classical horizontal tectonics. Not so, however, the observed severe components of vertical tectonics in the case of ECMs. Here, lower crustal delamination, with a loss of lithospheric negatively buoyant forcing and consequent strong increase in positive buoyancy explains generation and activity of steep reverse faulting in the ECMs. In this context, the ‘orogeny slab rollback’ model provides a physically more consistent framework to explain the observed deformation sequences of late-stage continent-continent collision.

Schmid, S.M., Kissling, E., Diehl, T., van Hinsbergen, D.J., Molli, G., 2017. Ivrea mantle wedge, arc of the Western Alps, and kinematic evolution of the Alps–Apennines orogenic system. Swiss Journal of Geosciences 110, 581-612.

How to cite: Herwegh, M., Berger, A., Kissling, E., Bellahsen, N., and Rolland, Y.: Late-Stage Alpine Continent-Continent Collision: Implications from Exhumation of the External Crystalline Massifs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6742, https://doi.org/10.5194/egusphere-egu2020-6742, 2020.

D1269 |
EGU2020-19638
Sébastien Ternois, Julien Léger, Raphaël Pik, and Mary Ford

A bivergent orogenic belt and its foreland basins form an intimately coupled system for which cause-effect relationships have generated much debate. This is the case in the Pyrenees, where the contemporaneity of crustal thermal events (cooling) with foreland tectonic events (subsidence) during convergence has, until recent years, remained unclear due to little quantification of processes during early orogenesis. Over the last 5 years, the development of new tools for generating, manipulating and modelling data and processes in geo-thermochronology has been stimulating new research that challenged established paradigms in both Pyrenean pro- and retro-foreland systems. Results obtained for these systems must now be compared and integrated into a reconstruction of the Pyrenean orogen history.

In this study we first review published detrital geo-thermochronology data in the eastern to central Pyrenees, harmonizing the way by which they are inverted for the inference of sediment provenance with use of current statistical modelling methods. Second, we re-interpret these detrital data in terms of provenance for both Pyrenean pro- and retro-foreland systems. Last, we compare the geo-thermochronological signals in both foreland basins to reconstruct orogen dynamics in the Pyrenees. We critically assess the implications of this newly proposed reconstruction for the validity of conceptual models for orogenic evolution and of plate kinematic models in the northwestern Mediterranean region.

How to cite: Ternois, S., Léger, J., Pik, R., and Ford, M.: Reconstructing orogen dynamics from detrital geo-thermochronological signals in pro- and retro-foreland basins: A case sudy of the Pyrenees, France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19638, https://doi.org/10.5194/egusphere-egu2020-19638, 2020.

D1270 |
EGU2020-18993
Mjahid Zebari, Christoph Grützner, Philipp Balling, and Kamil Ustaszewski

Within the NW segment of the Zagros belt in the Kurdistan Region of Iraq, the Zagros Mountain Front Flexure separates the High Folded Zone from the Foothill Zone and forms a pronounced topographic and structural step. Due to the lack of outcrops and subsurface data, balanced and kinematic valid geometrical interpretations for the subsurface deformation associated with this step are not well constrained yet. To solve this, we estimated the structural relief across seven regional transects crossing the Mountain Front Flexure and we constrained the geometry of deformation from deformed-state and forward-modeled balanced cross-sections. The calculated structural relief for six out of seven transects ranges from 2 to 3 km. By using forward modeling, we show that predominantly thick-skinned deformation is needed to explain this amount of relief across the Mountain Front Flexure. Our best-fitting result suggests c. 6.5 km of displacement along a basement thrust fault that dips c. 25° at the top of the basement and that is shallowing downwards. About 4.2 km of this displacement on the basement fault were accommodated up-section by thrust-related and detachment folding of the Triassic and younger units within two prominent anticlines. About 2.3 km of displacement was transferred to the Foothill Zone, forming detachment folds above the Triassic detachment level. Inclined river terraces on the flank of anticlines within the Foothill Zone indicate ongoing displacement on this basement fault. The amount of shortening within the low topographic part of the belt from the deformation front to the limit of seismogenic thrusting within the Imbricated Zone, implies that the Late Miocene to Quaternary shortening rates there were much lower than the present-day geodetically derived convergence rates for this part of belt. These results shed new light on the geometry of the Zagros and its structural evolution.

How to cite: Zebari, M., Grützner, C., Balling, P., and Ustaszewski, K.: Structural relief across the NW segment of the Zagros Mountain Front Flexure in the Kurdistan Region of Iraq: implications for basement thrusting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18993, https://doi.org/10.5194/egusphere-egu2020-18993, 2020.

D1271 |
EGU2020-11758
Cédric Bulois, François Michaud, Marianne Saillard, Nicolas Espurt, Marc Regnier, María-José Hernandez Salazar, Yvonne Font, Pedro Reyes Benítez, Jean-Yves Collot, Jean-Noël Proust, Elia D'Acremont, Laure Schenini, and Diego Barba

Over the last 23 Myr, the roughly east-directed subduction of the Nazca Plate beneath South America led to the formation of several mountain ranges associated with the overall northern Andes evolution. Along the active southwestern Ecuadorian margin, the compressional setting involves the Cretaceous-Miocene Chongón-Colonche / Santa Elena terranes, overlain by recent sedimentary basins. This geological setting, generally interpreted as an onshore-offshore forearc system, evolves in close relation with the active tectonic escape of the North Andean Sliver and the opening of the Gulf of Guayaquil. This region is characterised by a widespread extensional deformation in the upper plate that overprints moderate subduction and crustal earthquakes.

To better document such extensional processes, we specifically explore the offshore shelf and the littoral area of the Santa Elena Peninsula using academic and industrial 2D seismic profiles calibrated with local wells and field observations. We document a trench-parallel fault network, composed of >20km-long normal faults that take place on top of the former Chongón-Colonche accretionary wedge. These faults are linearly-steep along the trench, and are listric toward the continent where they clearly control fault-block rotation. They separate flexural basins developing on the platform ahead the Chongón-Colonche Cordillera, and are associated with immerged terraces most likely formed during the Last Glacial Maximum. They also may link to further onshore marine terraces developing since the Pleistocene across the coastline.

These observations suggest a peculiar dismantlement of the margin, mainly affected by tectonic erosion involving reactivation of former compressional features. Normal faults are specifically interpreted as a regional syn-orogenic collapse of the Chongón-Colonche Cordillera, which may result from transecting subducting ridges, fracture zones and seamounts controlling, at least partially, the geometry and the nature of the deformation along the southwestern Ecuadorian margin. This deformation pattern is likely linked to a weak interseismic coupling along the subduction interface to which the active opening of the Gulf of Guayaquil overlaps. This project is funded by the project ANR MARACAS ANR-18-CE31-0022 (MARine terraces along the northern Andean Coast as a proxy for seismic hazard ASsessment).

How to cite: Bulois, C., Michaud, F., Saillard, M., Espurt, N., Regnier, M., Hernandez Salazar, M.-J., Font, Y., Reyes Benítez, P., Collot, J.-Y., Proust, J.-N., D'Acremont, E., Schenini, L., and Barba, D.: Inheritance processes of active extensional deformation in ocean-continent subduction zones: the example of the northern Andes compressional margin in Ecuador, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11758, https://doi.org/10.5194/egusphere-egu2020-11758, 2020.

D1272 |
EGU2020-9494
Hugo Ortner and Sinah Kilian

Tectonic subdivisions of larger geologic units reflect the geologic knowledge at the time of creation. In many thrust belts the original subdivisions had been created during the first comprehensive mapping campaigns at the end of the 19th to early 20th century and reflect the geologic knowledge at that time. Even if many thrusts were identified correctly, no formal framework existed to give guidelines of how to distinguish tectonic units. Nevertheless, these subdivisions are still in use.

We analyze the thrust sheets of the Northern Calcareous Alps of western Austria and southern Germany and test the implicit assumptions underlying most tectonic subdivisions against field observations:

Assumption 1: Thrust transport is large and thrusts do not end laterally. However, several major thrusts do loose stratgraphic offset and end laterally.

Assumption 2: Allochthons are surrounded by thrusts on all sides. Unfortunately, any fault has been used to delimit allochthons.

Assumption 3: Thrusting should bring old on young rocks. In some cases, allochthons have been delimited by out-of-sequence thrusts, that stack young on old rocks. In other cases, the allochthon is a mountain-size glide block that was buried by younger sediments, and the trace of the thrust is an unconformity in the field.

As a consequence we propose a revised tectonic subdivision of the western part of the NCA, that avoids some of the problems discussed here, and is entirely based on the emplacement of old-on-young rocks across thrusts.

How to cite: Ortner, H. and Kilian, S.: Tectonic subdivisions in thrust belts – cleaning up the western Northern Calcareous Alps (NCA), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9494, https://doi.org/10.5194/egusphere-egu2020-9494, 2020.

D1273 |
EGU2020-9969
Klaus Pelz, Pablo Granado, Michael König, Elizabeth P. Wilson, Philipp Strauss, Eduard Roca, Wolfgang Thöny, and Josep Anton Muñoz

As shown for fold and thrust belts worldwide and for the Northern Calcareous Alps (NCA) in particular, the initial thickness and spatial distribution of autochthonous salt exerts fundamental control on deformation localization and structural style. The initial sedimentary geometries of mini-basins formed by downbuilding into or rafting on salt do influence the geometries of thrust sheets during subsequent shortening. The lateral extent and spacing of individual thrust sheets and the overall cylindricity of structures is governed by initial facies changes and thickness variations within and across mini-basins and salt ridges between them. During convergence, remaining inflated salt localizes shortening whereas mini-basins may react as rather rigid blocks. As deformation culminates at these secondary welds that eventually become thrusted and squeezed, apparent structural closures might become exploration targets but potentially yield more complex internal geometries and less predictable facies distribution.

In this contribution we show several cross sections constrained by surface and subsurface data in the eastern NCA and below the Vienna Basin. We compare areas with abrupt changes in stratigraphic thickness, limited lateral extent of individual thrust sheets and highly non-cylindrical structural style along strike to areas where thrust sheets extend over several tens of kilometers along strike with remarkably cylindrical structures, little thickness variations and less abrupt facies changes. Predictive capabilities in underconstrained areas (i.e., insufficient seismic imaging and/or resolution, lack of well control, bad outcrop conditions) are analyzed and compared to closures with well control and pre-drill expectations. Evidently, culminations can be predicted with more confidence in areas with little variation in facies and sedimentary thicknesses. Reliability of predictions generally degrades with decreasing thrust sheet size, observable non-cylindricity within and in between thrust sheets, and with increased complexities at the edges of mini-basins (e.g., squeezed and thrusted flaps). Internal geometries of mini-basins need to be imaged and analyzed properly to narrow down these uncertainties at potential culminations along the edges.

How to cite: Pelz, K., Granado, P., König, M., Wilson, E. P., Strauss, P., Roca, E., Thöny, W., and Muñoz, J. A.: Prediction uncertainties in salt detached fold and thrust belts – examples from the surface and subsurface of the Northern Calcareous Alps (Austria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9969, https://doi.org/10.5194/egusphere-egu2020-9969, 2020.

D1274 |
EGU2020-2583
Pablo Granado and Jonas B. Ruh

Contractional rejuvenation of syn-rift salt-bearing minibasins by numerical simulations

Pablo Granado1, Jonas B. Ruh2

1 Institut de Recerca Geomodels, Departament de Dinàmica de la Terra i de l'Oceà, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain

2 Structural Geology and Tectonics Group, Geological Institute, Department of Earth Sciences, ETH Zurich, Switzerland

This work presents numerical experiments of contractional rejuvenation of passive margin minibasins and related diapiric structures and the involvement in inverted rift and fold-and-thrust belt systems. We use 2D finite difference numerical experiments with a temperature-dependent Maxwell-type visco-elasto-plastic rheology. Our experiments consist of a first phase of extension controlled by basement faults overlaid by a thick salt-bearing unit covered by a pre-kinematic layer. Extension led to forced folding and stretching of the pre-kinematic layer triggering diapirism, fixing the lateral dimensions of minibasins, whereas syn-rift accommodation space was controlled by extension of the basement faults plus salt evacuation provided by sediment load. Rate of extension controlled: i) internal growth geometries of minibasins; ii) the amount of downbuilding, and iii) the timing and extent of primary welds. Contractional reactivation was then carried out as end member modes of thin-skinned shortening over the basement steps, full inversion of extensional faults (i.e. thick-skinned), and combinations of both, always including erosion and syn-contractional sedimentation. Results provide an extensive template of structural styles and related kinematic evolutions including minibasin rotation and imbrication, squeezing of salt structures and surface flaring, and development of deep contractional growth synclines. Modelling results will be compared to natural case studies.

How to cite: Granado, P. and B. Ruh, J.: Contractional rejuvenation of syn-rift salt-bearing minibasins by numerical simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2583, https://doi.org/10.5194/egusphere-egu2020-2583, 2020.

Chat time: Wednesday, 6 May 2020, 16:15–18:00

D1275 |
EGU2020-14942
Marc Schori, Anna Sommaruga, and Jon Mosar

The Jura Mountains are a thin-skinned fold-and-thrust belt (FTB) in the northern foreland of the European Alps, extending over northern and western Switzerland and eastern France. The Jura FTB was detached in Triassic evaporites during Late Miocene and Pliocene compression. Prior to this, the pre-Mesozoic basement was intensely pre-structured by inherited faults that had been reactivated under changing stress fields during the Mesozoic and Cenozoic structural evolution of continental Europe. In order to understand the connection between thin-skinned FTB formation and pre-existing basement structures, we compiled boreholes and geological cross-sections across the Northern Alpine Foreland and derived elevation, thickness and erosion models of defined Mesozoic units and the top of the pre-Mesozoic basement.

Our models confirm the presence of basement faults concealed underneath the detached cover of the Jura Mountains. The pre-Mesozoic basement shows differences in structural altitudes resulting from partially overlapping lithospheric processes. They include graben formation during evolution of the European Cenozoic Rift System (ECRIS), flexural subsidence during Alpine forebulge development and lithospheric long-wavelength buckle folding. Faults in connection with these processes follow structural trends that suggest the reactivation of inherited Variscan and post-Variscan fault systems. We discuss the spatio-temporal imprint of lithospheric signatures on the pre-Mesozoic basement and their consequence on the formation of the Jura Mountains FTB. Untangling structures within the pre-Mesozoic basement leads us to a modern understanding of the long-term evolution of the detached Mesozoic cover. Furthermore, it allows us to improve the prediction of ages that are potentially preserved within the Mesozoic cover of the Jura FTB.

How to cite: Schori, M., Sommaruga, A., and Mosar, J.: The pre-Mesozoic basement underneath the Jura Mountains fold-and-thrust belt: an overview from models and maps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14942, https://doi.org/10.5194/egusphere-egu2020-14942, 2020.

D1276 |
EGU2020-18669
| Highlight
Sarah Louis, Elco Luijendijk, Christoph von Hagke, István Dunkl, Ralf Littke, and Jonas Kley

Foreland basin sediments mirror the history of an orogeny. Deformation and geodynamic processes in low spatial extend (e.g. dozens of km) can be quantified using kinematic restoration. Processes happening deep underneath an orogen show a large spatial manifestation that is difficult to quantify in time and space. Marine units at surface outcrops show 900 m of net uplift since deposition in undeformed parts of the alpine foreland basin. Existing low-temperature thermochronology data from the Swiss part of the Molasse Basin show a thermal overprint that indicates exhumation of more than 1.5 km. We quantify the wavelength of deep seated processes of the Alpine orogen by generating and analyzing a holistic dataset of the entire alpine foreland basin. In addition to compiling existing data from the western part of the basin we have generated a new apatite (U-Th)/He and vitrinite reflectance data set from the central and eastern part of the basin. The new apatite (U-Th)/He ages in the German part of the basin show exhumation below or close to the detection limit (~1.5 km). Within the folded and thrusted Molasse, exhumation is localized along thrusts and the thermochronological data indicates thrusting between 10 to 20 Ma. Vitrinite reflectance data reveals a trend of exhumation increasing from East to West. Parts of the central German Molasse basin have been exhumed as well. Thus, on the large scale we can see longwave exhumation patterns in the western part of the basin that affect both the deformed and undeformed parts of the basin which cannot only be related to Jura thrusting.

How to cite: Louis, S., Luijendijk, E., von Hagke, C., Dunkl, I., Littke, R., and Kley, J.: Low-temperature thermochronology and vitrinite reflectance data reveal longwavelength uplift in the Alpine foreland basin , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18669, https://doi.org/10.5194/egusphere-egu2020-18669, 2020.

D1277 |
EGU2020-9889
Chiara D'Ambrogi, Maurizio Marino, Fabio Carlo Molinari, Michele Morelli, Andrea Irace, Luca Barale, Fabrizio Piana, Gianfranco Fioraso, Pio Di Manna, and Pietro Mosca

The Po Plain is a one-of-a-kind place where to study the evolution of the orogen-foreland pair. It is a complex geological system consisting mainly of Triassic to Quaternary sedimentary successions that have recorded the tectonic evolution of the paleoAdriatic continental margin during the Middle-Late Triassic and Jurassic time interval, as well as the development and mutual interaction of the Western Alps, Southern Alps and Northern Apennines orogenic belts, and related synorogenic basins, during the Cenozoic. 

These peculiarities allow for achieving analyses on several topics (e.g., the relations between opposite-verging thrust systems, the role of the inherited paleogeography, the geometry and evolution of the foredeep and thrust-top basins, the presence and the activity of seismogenic faults), individually treated by previous studies, with focus on limited time-intervals, or detailed 3D models of limited areas.

Nevertheless, a comprehensive and accessible 3D model of the general framework of the entire Po Plain subsurface is still unavailable.

In this respect, the HotLime Project (GeoERA Programme) will fill the gap providing a publicly accessible 3D framework model of the geometry of some stratigraphic horizons, focusing on crucial stratigraphic intervals, extended from Piemonte to Emilia-Romagna Region - Adriatic coastline, covering an area larger than 21,000 km2. In the HotLime Project, the model will be used as input for the geothermal assessment of carbonate reservoirs.

The 3D model, built as a whole, will include five regional-wide stratigraphic horizons  (e.g. top or unconformity surface), from Triassic to Pleistocene, plus additional less extended horizons, and the 3D geometry of more than 150 faults (i.e., Mesozoic extensional faults and Paleogene to Neogene thrusts).

This comprehensive 3D geological model of the Po Plain subsurface is based on an integrated analysis of surface and subsurface geological/geophysical data (the latter provided by ENI SpA), that allows for better interpreting and correlating the key horizons. The input dataset includes: 305 well data; 799 2D seismic profiles, with a mean spacing of 5 km; detailed surface data from geological maps at different scales. The final 3D model benefits from the comprehensive and coherent interpretation of the overall input dataset, and the time-depth conversion of the 3D model as a whole through a 3D velocity model.

The objective of this work is to build a general-purpose 3D geological model that will serve a multiplicity of specific topics, and provide a powerful 3D image of this complex foreland basin. It highlights the position and geometry of inherited structures and allows for analyzing their relations with stratigraphic variations of the sedimentary infill (e.g., unit thickness); besides, the comparison of the mutual relations of the compressional faults with the pre-existing discontinuities. The fault distribution and clustering will be also compared with the deformations observed on the highly-detail modeled Pliocene and Pleistocene horizons, giving a fundamental input for the calculation of slip/uplift rates and definition of the activity of the faults.

 

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 731166

How to cite: D'Ambrogi, C., Marino, M., Molinari, F. C., Morelli, M., Irace, A., Barale, L., Piana, F., Fioraso, G., Di Manna, P., and Mosca, P.: 3D geological model of the Po Plain subsurface: an example of open geological base data for basin analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9889, https://doi.org/10.5194/egusphere-egu2020-9889, 2020.

D1278 |
EGU2020-7685
Charlotte Peigney, Elisabet Beamud, Óscar Gratacós, Luis Valero, Ruth Soto, Eduard Roca, and Josep Anton Muñoz

The South-Pyrenean fold-and-thrust belt consists of three major thin-skinned thrust sheets (Bóixols, Montsec and Serres Marginals) made up of uppermost Triassic to Oligocene cover rocks emplaced during Late Cretaceous-Oligocene times. In its central part, it forms a major salient (the Pyrenean South-Central Unit) whose geometry is controlled by the areal distribution of the pre-orogenic Upper Triassic and synorogenic Eocene salt décollement layers. Both westwards and eastwards, the salient is fringed by Paleogene synorogenic deposits that are deformed by detachment folds with orientations ranging from N-S to E-W. In the western edge of the salient, the varying trend of the folds is a result of synorogenic vertical axis rotations (VAR) which caused the clockwise rotation of the folds from an initial predominant E-W trend to the current NW-SE to NNW-SSE trend. The salient, at least on its western part, developed from a progressive curve originated from divergent thrust transport directions and distributed shortening.

The aim of our study is to get a better understanding of the whole salient, by studying the kinematics of the deformation on the most frontal part of its eastern edge. Here, some sparse anticlockwise rotations have been reported but their origin and their possible relationship with the distribution of the salt décollements has not yet been addressed. For this purpose, 78 paleomagnetic sites have been sampled on the synorogenic upper Eocene-Oligocene materials of the NE Ebro foreland Basin, in the Artesa de Segre area, focusing on the limbs of oblique salt-cored anticlines (Ponts, Vilanova de l’Aguda, Cardona) which are detached above the synorogenic Eocene-Oligocene evaporites of the Cardona and the Barbastro formations. VAR analyses principally show anticlockwise rotations similar to those previously identified to the North in the Oliana Anticline, although a small number of clockwise rotations were also detected.

In addition to the VAR analysis, a magnetostratigraphic study of the Eocene-Oligocene continental materials of the northern limb of the Sanaüja Anticline has been conducted in order to constrain the age of these rotations from stratigraphic correlations. The demagnetization of 104 samples from a ca. 1100 m thick magnetostratigraphic section shows Priabonian to Rupelian ages for this succession. The integration of our results on timing, direction and magnitude of foreland VAR with previous paleomagnetic and structural data from both the western and eastern boundaries of the frontal thrust of the Pyrenean South-Central Unit will allow the understanding of the kinematics of the thrust salient as a whole.

How to cite: Peigney, C., Beamud, E., Gratacós, Ó., Valero, L., Soto, R., Roca, E., and Muñoz, J. A.: Timing and magnitude of vertical-axis rotations in the eastwards-flanking synorogenic sediments of the South-Pyrenean fold-and-thrust belt. Kinematics and origin of the salient curvature., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7685, https://doi.org/10.5194/egusphere-egu2020-7685, 2020.

D1279 |
EGU2020-5585
Inmaculada Expósito, Alejandro Jiménez-Bonilla, José Luís Yanes, Juan Carlos Balanyá, and Francisco Moral

How to cite: Expósito, I., Jiménez-Bonilla, A., Yanes, J. L., Balanyá, J. C., and Moral, F.: Relief rejuvenation in response to intraplate neotectonics in the Betics foreland (southern Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5585, https://doi.org/10.5194/egusphere-egu2020-5585, 2020.

D1280 |
EGU2020-438
Dan Mircea Tamas, Alexandra Tamas, Zsolt Schleder, and Csaba Krezsek

Fold and thrust belts are a notorious challenging environment when it comes to providing structural models for the subsurface, and the Eastern Carpathians Bend Zone (ECBZ) is no exception.

Hosting the largest onshore oil fields in Romania, this is a highly mature hydrocarbon area, with most of the fields producing since the late nineteenth century. Characterise by superimposed tectonic events, most notably the mid-Miocene compression (when most of the shortening occurred) the area is also well known for multiple detachment levels and salt tectonics. As a consequence, the reservoirs, especially the Oligocene - lower Miocene (sub-salt), thought very prolific are structurally complex, heterogeneous and compartmentalised. It is a constant struggle for the geologist to create structural maps of these reservoirs due to complex deformation, inconsistent data and poor seismic resolution. Some of the most significant issues are related to scattering of dip data and the overall difficulties in correlating well logs. In some cases, even the logs of the side-track well do not correlate with the initial log.

In order to get a better understanding of these complex structures, we used the nearby Oligocene - lower Miocene surface exposures. First, detailed fieldwork coupled with drone photogrammetry and interpretation of 3D virtual outcrop models revealed that upright, gently plunging folds as well as overturned and recumbent folding occurs at these stratigraphic levels. Fold limbs are occasionally cross-cut by forethrust or backthrust. Also, parasitic folding and fold-accommodation faults have been identified. Apart from this rather complex but typical tectonic structures, a network of sand intrusions is also present in the Oligocene - lower Miocene sequence. The injectites, dykes, sills or composite intrusions are sourced from the quartz-rich sandstones and injected into the adjacent rocks. The dyke networks are intersecting the adjacent rocks at high angles and appear to follow fold-related fractures. Also, some preserved fluidized layers respond to fold tightening by thickness redistribution and intrusion. Injection is therefore considered to be syn-kinematic with the mid-Miocene tectonic stage when most of the shortening in the area occurred. Intrusion was most likely driven by the fluid overpressures built up due to active contractional tectonics, with intrusion events potentially triggered by associated seismicity.

Their presence can explain some of the reservoir heterogeneities and challenges in well correlation. For example, one well through a misinterpreted dyke will provide misleading information regarding reservoir architecture, including dip data and highlighting that not every change in dip is due to folding or faulting.  Finally, as the dykes commonly follow fold-related fractures, it is highly possible to intrude fault planes as well, thus potentially influencing the shale gouge ratio and fault seal capacity.

The outcrops in the ECBZ are good surface analogues for global examples of hydrocarbon reservoirs affected by remobilized sand intrusions. A better understanding of these complex structures, especially in a compressional setting, can improve both subsurface structural interpretation and reservoir characterisation.

How to cite: Tamas, D. M., Tamas, A., Schleder, Z., and Krezsek, C.: Syn-tectonic sand intrusions - an added complexity to a highly deformed fold and thrust belt and implication for subsurface structural interpretation: Eastern Carpathians Bend Zone, Romania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-438, https://doi.org/10.5194/egusphere-egu2020-438, 2020.

D1281 |
EGU2020-2564
Alexander Razmadze

Gare Kakheti foothills are located between Lesser Caucasus and Kakheti Ridge and are mainly represented by the series of NEN dipping thrust faults, most of which are associated with fault‐related folds. Gare Kakheti foothills as a part of the Kura foreland fold-and-thrust belt developed formerly as a foreland basin (Oligocene-Lower Miocene) (e.g. Alania et al., 2017). Neogene shallow marine and continental sediments in the Gare Kakheti foothills keep the record on the stratigraphy and structural evolution of the study area during the compressive deformation. Interpreted seismic profiles and structural cross-sections across the Udabno, Tsitsmatiani, and Berebisseri synclines show that they are thrust-top basins. Seismic reflection data reveal the presence of growth fault-propagation folds and some structural wedges (or duplex). The evolution of the Udabno, Tsitsmatiani, and Berebisseri basins is compared with simple models of thrust-top basins whose development is controlled by the kinematics of competing for growth anticlines. Growth anticlines are mainly represented by fault-propagation folds. The geometry of growth strata in associated footwall synclines and the sedimentary infill of thrust-top basins provide information on the thrusting activity in terms of location, geometry, and age.
This work was supported by Shota Rustaveli National Science Foundation (SRNSF - #PHDF-19-268).

 

How to cite: Razmadze, A.: Structure of the Gare Kakheti foothills using seismic reflection profiles: implications for kinematic evolution of the Georgian part of Kura foreland fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2564, https://doi.org/10.5194/egusphere-egu2020-2564, 2020.

D1282 |
EGU2020-2325
| solicited
| Highlight
Dennis Brown, Joaquina Alvarez-Marron, Hao Kuo-Chen, Yih-Min Wu, Giovanni Camanni, and Cristina Biete

Studies of mountain belts worldwide have shown that the structural, mechanical, and kinematic evolution of their foreland fold-and-thrust belts are strongly influenced by the structure of the continental margins that are involved in the deformation. The area on and around the island of Taiwan provides an unparalleled opportunity to investigate this because the entire profile of the SE margin of the Eurasian plate, from the shelf in the north to the slope and continent-ocean transition in the south and the offshore, is currently involved in a collision with the Luzon arc on the Philippine Sea plate. Taiwan can, then, provide key insights into how such features as rift basins on the shelf, the extensional faults that form the shelf-slope break in the basement, or the structure of the extended crust and morphology of the sedimentary carapace of the slope can be directly reflected in the structural architecture, the location and pattern of seismicity, topography, and the contemporaneous stress and strain fields of a fold-and-thrust belt. For example, east-northeast striking faults that have been mapped on the necking zone of the Eurasian margin can be traced into the island of Taiwan where they are causing important along-strike changes in various aspects of the structural, mechanical, kinematic, and morphological behavior of the fold-and-thrust belt. In particular, across the upper part of the necking zone there is an abrupt north-south change in structure, an increase in the amount of seismicity, an increase in topography, a rotation of the direction of maximum compressive horizontal stress, of the GPS displacement vectors, the compressional strain rate, and the maximum shear strain rate. These changes are interpreted to be caused by east-northeast striking, dextral strike-slip faulting in the basement that is taking place as a result of the reactivation of pre-existing faults along the upper part of the necking zone. The abrupt southeastward increase in topography across the upper part of the necking zone is the surface expression of the basal thrust of the fold-and-thrust belt ramping down into the basement, with maximum elevations reached in the basement-involved thrust sheets, suggesting a causal link between basement involvement in the thrusting and high topography. On the shelf, the roughly northeast-oriented Hsuehshan Trough is inverting along almost north-south striking basin bounding faults that penetrate into the middle crust and have well-clustered, deep seismicity. There are no substantial differences in the contemporaneous stress and strain field. There is, however, a clear relationship between basement involvement in the thrusting and the development of high topography in the Hsuehshan Range. Only the upper part of the slope is involved in the fold-and-thrust belt in southernmost Taiwan. In this area, there is a reduction of the amount of seismicity and lower topography. The largest part of the corresponding thrust wedge developed in the lower slope is offshore. This work is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades grant PGC2018-094227-B-I00.

How to cite: Brown, D., Alvarez-Marron, J., Kuo-Chen, H., Wu, Y.-M., Camanni, G., and Biete, C.: Structure of the south-central Taiwan fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2325, https://doi.org/10.5194/egusphere-egu2020-2325, 2020.

D1283 |
EGU2020-3800
| solicited
Yani Najman

Interrogation of sediment archives allows for documentation of both hinterland and foreland deformation. Examples of their use as an archive of Himalayan foreland deformation include the work of Govin et al. (Geology, 2018) in which determination of the timing of drainage rerouting of the palaeo-Brahmaputra has allowed us to date the timing of surface uplift of the Shillong Plateau, and the work of Najman et al (Tectonics, 2018) in which the presence of the major Paleogene unconformity previously recognised in the Himalayan foreland basin, was shown to extend much further south into the foreland, allowing for a broader range of possible causal mechanisms to be discussed. There are numerous examples of the use of the Himalayan foreland basin sediment record to determine orogenic tectonics, this being a complementary approach to bedrock studies of the orogen. For example, Govin et al. (in review) and Lang et al (GSAB 2016), used detrital mineral lag time studies targeted to the Siwalik Himalayan foreland sediment archive, to demonstrate when the rapid exhumation of the eastern Himalayan syntaxis commenced. Comparison with a similar dataset derived from a more distal sediment archive of the Bengal Fan (Najman et al. GSAB 2019), shows the advantages (as well as disadvantages) in the use of proximal sediment archives.

How to cite: Najman, Y.: Foreland basin sediment archives: highlighting their use in documenting deformation of the orogenic hinterland and foreland., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3800, https://doi.org/10.5194/egusphere-egu2020-3800, 2020.

D1284 |
EGU2020-18768
Maryline Le Béon, John Suppe, Yu-Huan Hsieh, Mong-Han Huang, Hsin-Hua Huang, and Chih-Tung Chen

            The fold-thrust belt of southern Taiwan currently accommodates rapid westward shortening in the order of 4.5 mm/a and is estimated to have developed since the Late Pliocene. It is the locus of the 2016 Mw6.4 Meinong earthquake, which involved fault slip at multiple levels in the mid-to-upper crust. It nucleated at ~15 km depth and also triggered shallow structures that reach the surface or nearly. To characterize the structure of the piedmont and investigate the broader tectonic setting of the event, we build two east-west regional balanced cross-sections based on surface geology, subsurface data, coseismic and interseismic geodetic data, and published nanno- and magneto-stratigraphy. We also document the first-order evolution of the piedmont and how the piedmont structures relate to the inner part of the mountain belt based on the cross-section restoration and the analysis of the seismic velocity structure of the plate boundary.

From the Coastal Plain towards the east, we propose a series of three active west-dipping backthrusts, rooted on a ~4.0-km-deep detachment, the Tainan detachment. The detachment lies within the base of the 3-km-thick Plio-Pleistocene Gutingkeng mudstone, which represents the initial foreland basin sediments. Syntectonic sediments and rapid shortening and uplift observed from geodetic measurements attest for the activity of these structures since the Late Pleistocene. Further east, the Tainan detachment ramps down to ~7 km depth, allowing the east-dipping Lungchuan and Pingchi thrusts to bring Upper Miocene continental-shelf formations to the surface. The cross-section restoration indicates less than 10 km shortening since ~450 ka or less on the Tainan detachment and the frontal backthrusts, while the east-dipping Lungchuan and Pingchi thrusts each consumed ~10 km shortening. Another ramp from ~7 to ~11 km depth is expected further east based on older sediments and slates exposed on the hanging wall of faults in the inner part of the mountain belt. This ~11-km-depth detachment seems to correspond with an inversion in seismic velocities at ~12 km depth beneath the slates belt, interpreted as slates over-riding lower-velocity passive margin sediments. Therefore, the detachment and thrusts system proposed from our cross-section appears to correspond to the main plate interface, where significant shortening was consumed in a thin-skinned deformation style, involving only foreland basin sediments near the deformation front.

The Meinong earthquake coseismic surface deformation suggests that, in addition to the deepest (15-20 km) main fault plane, the ~4-7-km-depth ramp, the Tainan detachment and the backthrusts slipped aseismically during after the earthquake. In contrast, the earthquake nucleated below the main detachment and, based on tomographic models, there is no clear structural connection between deep and shallower structures. The Meinong event locates near the interface between Cenozoic basement rocks and post-rifting sediments, similarly to the 2010 Mw6.3 Jiashian event. We propose that this interface is the locus of moderate-magnitude events, which seismic waves triggered limited slip on shallower faults, rooted within the weak, fluid-rich Gutingkeng mudstone. This interface may have developed as a secondary detachment level with limited total shortening.

How to cite: Le Béon, M., Suppe, J., Hsieh, Y.-H., Huang, M.-H., Huang, H.-H., and Chen, C.-T.: Geological structure and active deformation in the fold-thrust belt of southern Taiwan in relation to crustal-scale structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18768, https://doi.org/10.5194/egusphere-egu2020-18768, 2020.

D1285 |
EGU2020-4172
Lining Wang

The northwestern Sichuan region experienced the evolutionary process of a marine Craton basin in the Sinian-Middle Triassic and a continental basin in the Mesozoic-Cenozoic. Several regional tectonic activities cause the complicated stratigraphic distribution and structural deformations in deep layers. During key tectonic periods, the characteristic sedimentary and deformation structures were formed, including the platform margin of Dengying formation, the western palaeohigh at the end of Silurian, and the passive continental margin of late Paleozoic-middle Triassic. The Meso-Cenozoic intra-continental compressional tectonic processes since the late Triassic controlled the formation of complex thrusting structures surrounding and inside the basin. The northern Longmenshan fold-thrust belt has footwall in-situ thrust structures, controlled by two sets of detachments in the Lower Triassic and the Lower Cambrian, presenting a multi-level deformation structure with shallow folds, the middle thin-skin thrusts and the deeper basement-involved folds. From the perspective of structural geology, the Dengying formation of the Upper Sinian is mainly distributed in the eastern and northern areas of the northwest Sichuan basin where the Jiulongshan fold is the favorable exploration belts. Using the three-dimensional seismic reflection data, we recognize the structural characteristics of the platform margin of Dengying formation. Meanwhile, we apply new methods of two-dimensional and structural restoration based on mechanical constrains to gain insights into the development of the Jiulongshan anticline which forms the trap for the Jiulongshan field. The result of structural restoration indicates that, the formation of the Jiulongshan anticline is controlled by two-stage contractional thrusts. In the early days, there was no significant relief in Jiulongshan area, and the southwestern top of the Sinian Dengying formation was the paleo-high. The anticline was gradually formed in the Late Jurassic-the Early Cretaceous, presenting an approximately E-W strike structure. This structure was transformed by the N-E contractional stress to become an anticline in NE-SW direction.

 

How to cite: Wang, L.: Mechanics and paleo geomorphy of the platform margin of Dengying formation in the Jiulongshan field, Sichuan Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4172, https://doi.org/10.5194/egusphere-egu2020-4172, 2020.

D1286 |
EGU2020-7817
Jiawei Cui

The North China (NCC) is one of the oldest cratons in the world. The tectonic evolution processes of the NCC have been debated for decades (Zhao and Zhai, 2013; Zhao, 2007; Zhao et al., 2002, 2003, 2005, 2009; Zhai et al., 2005; Zhai and Santosh, 2011; Wilde et al., 2002, 2005; Kroner et al., 2005; Kusky et al., 2001, 2007; Kusky and Li, 2003; Faure et al., 2007; Trap et al., 2012; Hu et al., 2013; Zhao et al.,2019). The controversy focuses on the time of the formation of the NCC is in the late Paleoproterozoic or the late Archean. The key point of the controversy is that there are serious disagreement about the nature and implications of the late Paleoproterozoic orogen in the NCC. Some researchers thought the NCC underwent compression in 1.85 Ga according to previous researchers (Zhai et al., 2005; Zhai and Santosh, 2011; Zhao et al., 2019). Some researchers even thought that the NCC was finally formed resulted from the collision of the east block and the west block (Kusky et al., 2001, 2007; Kusky and Li, 2003; Trap et al., 2012; Zhao et al., 2002a, 2003a, 2005, 2009;). Recently, we found that NE-NEE trending extensional ductile shear zones developed in the Paleoproterozoic granitic gneiss (2.4Ga) in the northern margin of the Zhongtiaoshan, the middle part of the NCC. The ductile shear zone was unconformity covered by the Changcheng System and the deformation ages according to the 40Ar/39Ar dating results is 1.92 Ga, which indicate that the deformation time was in the late Paleoproterozoic. Therefore, We propose that that the NCC was in the post-collision extension environment or lateral and vertical extrusion of blocks might have happened after the orogeny in late Paleoproterozioc.

How to cite: Cui, J.: Indentation tectonics of the Zhongtiaoshan Block in the Trans-North China Orogen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7817, https://doi.org/10.5194/egusphere-egu2020-7817, 2020.

D1287 |
EGU2020-20903
Zhidong Gu

The eastern Sichuan Basin, South China, is characterized by approximately parallel thin-skinned fold-and-thrust belts with exposed narrow anticlines and wide synclines. The structural deformation, however, has remained controversial due to the previous poor seismic data. In this study, the new collected pre-stack long-offset 2D- and 3D seismic data have been applied, and a 200-km long cross section perpendicular to the fold-and-thrust belts has been constructed to analyze the structural style and geometric and kinematic evolution. The stratigraphic succession is composed of competent layers separated by three main incompetent layers being multiple detachments, which are the Cambrian evaporites, the Lower Silurian shales, and the Middle-Lower Triassic evaporites, respectively. The basal detachment, the Cambrian evaporites, played a dominant role in the structural deformation, above which the fold-and-thrust belts were generated, and the middle and top detachments accommodated the displacement during the deformation. The main structural styles are detachment folds, fault propagation folds, back thrusts and basement-involved folds. The evolution succession of the fold-and-thrust belts should be kink band, detachment folds, and sequential thrusts of the forelimb and backlimb of the folds. The style of deformation is dependent on the mechanical characters of stratigraphic succession, i.e., the thickness variation of competent and incompetent layers in the stratigraphic units.

How to cite: Gu, Z.: Multiple detachments of thin-skinned fold-and-thrust belts in the eastern Sichuan Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20903, https://doi.org/10.5194/egusphere-egu2020-20903, 2020.

D1288 |
EGU2020-7785
Tania Mochales, Ahmed Manar, Antonio María Casas-Sainz, Pablo Calvin, Pablo Santolaria, Juan José Villalaín, Vicente Carlos Ruiz, Andrés Gil-Imaz, Sara Torres, Andrés Pocoví, Bennacer Moussaid, Teresa Román-Berdiel, Hmidou El Ouardi, Esther Izquierdo-Llavall, Belén Oliva-Urcía, Marcos Marcén, María Felicidad Bógalo, Elisa Sánchez-Moreno, Ángela Herrejón, and Ángela Jiménez and the CHA Team

New residual magnetic map is presented to help decipher the magnetic imprints in the Central High Atlas (CHA) fold-and-thrust belt. The total intensity map shows a main direction mimicking the N070 trend which features the Atlas range. Detailed structural and paleomagnetic studies performed in the selected area demonstrate that similar shortening figures are observed in western and eastern portions. Differences in structural style are the consequence of (i) the inherited structure from the Triassic-Jurassic rifting stage, to Cenozoic inversion, (ii) the differential displacement through the Upper Triassic detachment level and (iii) superposition of cover thrust sheets.

 

Remarkable magnetic anomalies are recorded, from negative values (<-400 nT) in the southern foreland, to positive (>500 nT) in the core of the range. The western sector of the chain is defined by intermediate to high anomalies, probably related to NE-SW basement structures, which favored the emplacement of Triassic CAMP basalts and/or Jurassic gabbro bodies, within the syn-rift sequence. The central part of the range is characterized by high and very high positive anomalies with an irregular distribution, probably linked to Middle-Late Jurassic diapirism produced during extension and intrusion of gabbroid bodies at the core of diapirs, whose structure nucleated NE-SW anticlinal ridges. The eastern sector is characterized by intermediate to low intensity anomalies, likely associated to thick series of basinal Jurassic limestones, whose sequences were stacked (during the Cenozoic compressional stage) by means of kilometer-scale thrusts. Very high positive, linear anomalies seem to be related to Jurassic gabbro intruding directly into the carbonate facies in the eastern sector. Widespread negative anomalies are detected in the foreland southern basin. In this case, the remanent signature could be related to the Paleozoic magmatic provinces.

How to cite: Mochales, T., Manar, A., Casas-Sainz, A. M., Calvin, P., Santolaria, P., Villalaín, J. J., Ruiz, V. C., Gil-Imaz, A., Torres, S., Pocoví, A., Moussaid, B., Román-Berdiel, T., El Ouardi, H., Izquierdo-Llavall, E., Oliva-Urcía, B., Marcén, M., Bógalo, M. F., Sánchez-Moreno, E., Herrejón, Á., and Jiménez, Á. and the CHA Team: Geometrical and chronological constraints for magnetic signatures in the Central High Atlas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7785, https://doi.org/10.5194/egusphere-egu2020-7785, 2020.

D1289 |
EGU2020-8036
Pablo Santolaria, Antonio M. Casas, Pablo Calvín, Tania Mochales, Hmidou El-Ouardi, Esther Izquierdo, Teresa Román-Berdiel, Sara Torres, Andrés Pocoví, Belén Oliva-Urcia, Bennacer Moussaid, Marcos Marcén, Andrés Gil-Imaz, Vicente Carlos Ruiz, María Felicidad Bógalo, Elisa M. Sánchez-Moreno, Ángela Herrejón, Ángela Jiménez, Juan José Villalaín, and Irene Falcón

The Atlas system, an ENE-WSW intracontinental chain in the NW of Africa, grew because of the inversion of Mesozoic extensional basins during the Cenozoic convergence between the African and European plates. The Central High Atlas (CHA) is located in the mid-western sector of the chain and is characterized by (i) the presence of an Upper Triassic décollement, (ii) thick Lower-Middle Jurassic sedimentary sequences, and (iii) the occurrence of diapirs and igneous bodies which are especially common in the central part of the chain. The northern and southern borders are characterized by fold-and-thrust-systems involving the Paleozoic basement and the Mesozoic cover and showing significant displacements, especially towards the South.

Framed on a multidisciplinary structural project aiming to reconstruct the 4-D structure of the CHA, the purpose of this work is to gather a vast constraining dataset into a present-day, regional, 3-D geological model of the CHA fold-and-thrust belt. This 3-D reconstruction gives special weight to along- and across-strike variations of the geometry of the basement and cover structures and the distribution of salt and igneous bodies. The 3-D model is founded by 23 serial cross-sections, constrained by surface geology and more than 1900 structural data and complemented by geophysical modelling. The model considers regional structures having enough lateral continuity and so we ruled out minor, local features. Stratigraphically, we considered 5 horizons: (1) the top of the Triassic located below the detachment level, and partially equivalent to the top of the basement, (2) the base of the Jurassic succession (i.e. the top of the detachment level), (3) the Lower-Middle Jurassic limit and, towards the southern and northern fronts and foreland basins, (4) the bases of the Cretaceous and (5) the Cenozoic succession.

The reconstruction of the 3-D model entailed a strong feedback between the model and the cross-sections. The incipient 3-D model helped to refine the lateral consistency between cross-sections regarding branch and tip lines, cut-offs, fault angles, etc., and so to improve and further constrain them.

Thick to thin skinned deformation dominates the eastern, central and northwestern areas of the chain while thick skinned deformation occurs in the westernmost transects. The chain is defined as an asymmetric, doubly verging fold-and-thrust belt. A north-dipping, basement regional fault represents the main rooting structure of the CHA. Its geometry varies from and almost horizontal (West), to a 10°-12° (Centre) and 15° (East) thrust ramp surface. As this fault intersects the cover, it splits into a regional thrust front with several thrust branches. To the north, antithetic basement faults change to a thrust relay system as they intersect the Mesozoic sequence. Along the core of the chain, the structural style is characterized by open salt bodies, welded diapirs and steep thrusts having relatively limited lateral continuation. The Toundoute Unit, located in the central-western sector represents a basement-and-cover thrust stack where the basement is exhumed and crops out.

This 3-D structural model provides the bases for further 4-D reconstruction of the CHA and, at the same time, served as a constraining approach for cross-section construction.

How to cite: Santolaria, P., Casas, A. M., Calvín, P., Mochales, T., El-Ouardi, H., Izquierdo, E., Román-Berdiel, T., Torres, S., Pocoví, A., Oliva-Urcia, B., Moussaid, B., Marcén, M., Gil-Imaz, A., Carlos Ruiz, V., Felicidad Bógalo, M., Sánchez-Moreno, E. M., Herrejón, Á., Jiménez, Á., Villalaín, J. J., and Falcón, I.: 3-D geological model of the Central High Atlas fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8036, https://doi.org/10.5194/egusphere-egu2020-8036, 2020.

D1290 |
EGU2020-8342
Betti Hegyi, Zoltan Erdos, Ritske S. Huismans, and Christoph von Hagke

Triangle zones in fold and thrust belts are enigmatic structures bound by foreland verging thrust zones and back-thrusts verging towards the hinterland. The geometry as well as kinematic evolution of these structures has been the subject of a wide range of studies over the last few decades. The understanding of triangle zone mechanics is incomplete although different driving mechanisms for their formation have been proposed. So far few – mostly analogue – modeling studies have focused on understanding the primary factors controlling their formation. Factors suggested to have a first order control on the formation of triangle zones include the rheological properties of the detachment and overburden rocks, the thickness of the overburden rocks, syn-tectonic erosion and sedimentation rate, fluid over-pressure conditions, and the angle of the detachment. Here we use the arbitrary Lagrangian-Eularian finite element code FANTOM to examine the development of triangle zones. We focus especially on the effect of the angle and rheology of the detachment, the rheology of the overburden strata, and syn-tectonic deposition. 

How to cite: Hegyi, B., Erdos, Z., Huismans, R. S., and von Hagke, C.: Why do triangle zones exist? Insights from numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8342, https://doi.org/10.5194/egusphere-egu2020-8342, 2020.

D1291 |
EGU2020-18590
Stefano Tavani, Giovanni Camanni, Michele Nappo, Marco Snidero, Alessandra Ascione, Ettore Valente, Gholamreza Gharabeigli, Davoud Morsalnejad, and Stefano Mazzoli

The Mountain Front Flexure is a major structure of the Zagros orogenic system, and is underlain by the deeply rooted and seismically active Mountain Front Fault system. These coupled structural features divide the belt from its foreland and their trace is sinuous, forming salients and recesses. The origin and tectonic significance of the Mountain Front Fault system and its sinuosity are still unclear, with most of hypotheses pointing to a strong structural control exerted by geological inheritances. In this work we combine interpretation of seismic reflection profiles, earthquake data, geomorphic analysis, and geological observations, to build a balanced cross section across the Mountain Front Flexure in the Lurestan region. Our data are suggestive of a hybrid tectonic style for the Lurestan region, characterised by a major and newly developed crustal ramp in the frontal portion of the belt (i.e the Mountain Front Fault) and by the reactivation of steeply dipping pre-existing basin-bounding faults, along with a minor amount of shortening, in the inner area. Specifically, the integration of our results with previous knowledge indicates that the Mountain Front Fault system developed in the necking domain of the Jurassic rift system, ahead of an array of inverted Jurassic extensional faults, in a structural fashion which resembles that of a crustal-scale footwall shortcut. Within this structural context, the sinusoidal shape of the Mountain Front Flexure in the Lurestan area arises from the re-use of the original segmentation of the inverted Jurassic rift system.

How to cite: Tavani, S., Camanni, G., Nappo, M., Snidero, M., Ascione, A., Valente, E., Gharabeigli, G., Morsalnejad, D., and Mazzoli, S.: Role of structural inheritances in the development of the Mountain Front Flexure in the Lurestan region of the Zagros belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18590, https://doi.org/10.5194/egusphere-egu2020-18590, 2020.