TS7.1

The stacking of thrust sheets and mass transfer of sediment during fold and thrust belt accretion imposes a load on the basement and underlying mantle. This load induces an isostatic adjustment through a flexural response, which may also contribute to the overall architecture of the fold and thrust belt. Whereas plate kinematics imposes its tempo to evolving fold and thrust belts, the rheology of the mantle controls the tempo of the isostatic flexure. Using two-dimensional high-resolution numerical experiments, we explore how the interplay between the tectonic compressional rate and the isostatic flexural rate influences the structural evolution and final architecture of fold and thrust belts. 

We run a suite of numerical experiments using the well-tested code Underworld. Our geological model is mapped over a 42 km by 16 km numerical grid, with a cell resolution of 80 m. The geological model consists from top to bottom of  ‘sticky air’, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a basal layer which - in combination with a basal kinematic boundary condition - controls the amount of isostatic flexure. Materials have a mechanical behavior that results from elasto-visco-plastic rheology. The pressure at the base of the model is held constant, and the vertical velocity is updated at each timestep. The amount of material entering or exiting the model at each point along the base scales with the density of the basal layer, which is used to control the isostatic rate. Sedimentation and erosion are self-consistent through mechanical erosion and a hillslope diffusion law. Our models show that as the ratio between tectonic and flexural rates decreases (i.e. flexure gets faster), fold and thrust belts become narrower, lower in elevation, and structurally more complex. We compare these results with natural analogs including the Cordilleran and Jura fold and thrust belts.

How to cite: Ibrahim, Y. and Rey, P.: The Role of Isostasy in the Evolution of Thin-Skinned Fold and Thrust Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13767, https://doi.org/10.5194/egusphere-egu21-13767, 2021.

Shortening in fold-and-thrust belts can be accommodated with little or substantial basement involvement, with the former, thin-skinned, style arguably being the more common (Pfiffner, GSA Special Paper, 2006). Experimental studies on thin-skinned fold-and-thrust belts have confirmed critical taper theory and have highlighted the roles of bulk rheology, embedded weak layers, décollement strength, and surface processes in structural evolution. However, analogue models of thick-skinned fold-and-thrust belts are less common, which may be related to practical challenges involved in shortening thick layers of brittle materials. Here we focus on basement fault reactivation, which has been suggested for several fold-and-thrust belts, such as the Swiss Alps, the Laramide belt in North America and the Sierras Pampeanas in South America, which show evidence of deep-rooted thrust systems, pointing to a thick-skinned style of shortening.

Within an orogenic system, the shortening style may change between thin- and thick-skinned in space (foreland to hinterland) and time. This raises the question how inherited structures from one shortening phase may influence the next. We aim to use analogue experiments of multi-phase shortening to discuss the effects of deep-seated shortening-related inherited structures, such as thrusts and basement topography, on the structural evolution of fold-and-thrust belts.

We employ a push-type experimental apparatus that can impose shortening in both thick- and thin-skinned style. The device has two independently moving backstops, permitting to change between these shortening styles over time, allowing the simulation of multiple contractional scenarios. We start with an initial stage of thick-skinned shortening, followed by either thin- or thick-skinned reactivation. We use quartz sand to simulate crustal materials and microbeads for embedded weak (sedimentary) layers. Surface and lateral strain, as well as topography, is quantified using a high-resolution particle imaging velocimetry and digital photogrammetry monitoring system.

We will present preliminary results of this innovative experimental approach with the objective of discussing to what extent pre-existing conditions in the basement control the geometric, kinematic, and mechanical evolution of thick-skinned and basement-involved thin-skinned tectonics. In this presentation, we hope for a discussion of mechanisms of localisation of shortening in brittle analogue models, of sequences of thin- and thick-skinned deformation expected during multi-phase shortening, and comparisons to ongoing research and natural observations. Questions we aim to discuss are: Can weaknesses and anisotropies within the basement influence and control later structural evolution? Are pre-existing structures, such as thrusts or shear zones within the basement, responsible for subsequent fault nucleation, thin-skinned folding or basement uplift? What role does the rheology of the basement-cover interface play in the reactivation of basement thrusts? Can we model these reactivations with an analogue setup?

How to cite: Molnar, N. and Buiter, S.: A discussion of the (re)activation of basement structures during multi-phase shortening in thin- and thick-skinned styles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4162, https://doi.org/10.5194/egusphere-egu21-4162, 2021.

Inversion of passive margins and their transportation into fold-and-thrust belts is a critical stage of mountain building processes and their structural interpretation is fundamental for understanding collisional orogens. Due to the multitude of parameters that influence their formation (e.g. the interaction between sedimentary cover and basement, the mechanical stratigraphy or the rheology of different rock types) as well as along-strike internal variations, a single cross-sectional view is insufficient in exploring the 3D evolution of a fold-and-thrust belt. Hence, a 3D geological characterization is required to better comprehend such complex systems. Based on a detailed digital map, a 3D structural model of the current tectonic situation and sequential retrodeformation, we elaborate the 3D evolution of a part of the former European passive continental margin. In this setting, we focus on the Doldenhorn Nappe (DN) and the underlying western Aar massif (external Central Alps, Switzerland). The DN is part of the Helvetic nappe system and consists of a large-scale recumbent fold with a thin inverted limb of intensively deformed sediments (Herwegh and Pfiffner 2005). The sedimentary rocks of the DN were deposited in Mesozoic-Cenozoic times in a small-sized basin, which has been inverted during the compression of the Alpine orogeny (Burkhard 1988). Along NNW-SSE striking geological cross-sections, restoration techniques reveal the original asymmetric triangular shape of the DN basin and how the basin has been exhumed from ~ -12 km (Berger et al. 2020) to its present position at 4km elevation above sea level throughout several Alpine deformation stages. Moreover, the model allows to visualize the current structural position of the DN and the massif as well as the geometric and overprinting relationships of the articulated deformation sequence that shaped the investigated area throughout the Alpine evolution. Here we document that: (i) the DN is a strongly non-cylindrical recumbent fold that progressively pinches out toward the NE; (ii) significant along-strike (W-E) stratigraphy thickness variations are reflected in structural variations from a single basal thrust deformation (W) to an in-sequence thrust deformation (E); and (iii) the progressive exhumation of the basement units towards the E and thrusting towards the N. In this context, special emphasis is given to illustrate how three-dimensional geometry of inherited pre-orogenic structures (e.g., Variscan-Permian and rifting related basement cover structures) play a key role in the structural style of fold-and-thrust belts. In summary, today’s structural position of the DN is the result of the inversion of a small basin in an early stage of thrusting, which was followed by sub-vertical buoyancy driven exhumation of the Aar massif and subsequent thrust related shortening. All three stages are deeply coupled with an original non-cylindrical shape of the former European passive continental margin.

How to cite: Musso Piantelli, F., Mair, D., Herwegh, M., Berger, A., Kurmann, E., Wiederkehr, M., Schlunegger, F., Baumberger, R., and Möri, A.: The role of inherited structures and basin geometry during the 3D inversion of a passive continental margin: the case of the Doldenhorn-Aar Massif system (Central Swiss Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2609, https://doi.org/10.5194/egusphere-egu21-2609, 2021.

The presence of multiple evaporite levels strongly influence the structural style and kinematics of fold-and-thrust belts. Particularly (but not exclusively) in their fronts, it is common for these décollements to favor the formation of triangle zones. In the central portion of the Pyrenees, the South Pyrenean Triangle Zone represents the frontal part of this chain, that involves the Oligocene-Miocene Ebro Basin foreland deposits. We have focused on its western termination, characterized by a salt-cored anticline that laterally passes to a backthrust which dies out to the west. These structures are detached on the Upper Eocene-Lower Oligocene syntectonic evaporite Barbastro Formation (and lateral equivalents) that acted as a multidetachment unit. To the north, the south-directed Pyrenean thrust unit detached on Middle-Upper Triassic evaporites to finally glide along the Upper Eocene-Lower Oligocene décollement horizons.

In this contribution, we present a detailed structural and stratigraphic model of this triangle zone termination, constructed accordingly to two major approaches (1) constraining the geometry and structural architecture based on surface geology, interpretation of seismic lines (>900 km) and wells and, (2) obtaining the 3D density distribution of the detachment level (Barbastro Fm. and lateral equivalents as well as deeper, Triassic evaporites) using gravity stochastic inversion by means of more than 7000 gravity stations and 1500 actual density data from surface rocks. All in all, this multidisciplinary approach allows us to characterize the western termination of the South Pyrenean Triangle zone as the transition from a ramp-dominated and multiple triangle zone to a detachment-dominated one whose geometry, kinematics, and location were controlled by the distribution and heterogeneity of the Upper Eocene-Lower Oligocene syntectonic décollements and the southern pinch-out of the basal detachment of this unit.

How to cite: Santolaria, P., Ayala, C., Pueyo, E. L., Rubio, F. M., Soto, R., Calvín, P., Luzón, A., Rodríguez-Pintó, A., Oliván, C., and Casas-Sainz, A. M.: The western termination of the South Pyrenean Triangle Zone; a structural and geophysical characterization., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3417, https://doi.org/10.5194/egusphere-egu21-3417, 2021.

The characterization of the basement architecture of the Pyrenean Axial Zone, backbone of the chain, is crucial to understand its geodynamic evolution and the interplay between tectonism and magmatism. In this work, a new gravity-constrained cross section was built along the Central Pyrenees, between two of the largest Pyrenean Late Variscan granitic complexes, La Maladeta and Andorra-Mont Louis granites, to infer the geometry at depth of the basement host rocks. This cross section is ca. 65 km long and extends from the Mesozoic Bóixols basin in the South to the Late Variscan Bassiès granite to the North, close to the northern end of the Axial Zone. It is based on available geological maps, previous published works and new geological field data; together with newly acquired gravimetric stations (1141), to improve the existent spatial resolution of the gravity data from the databases of the Spanish and Catalan Geological Surveys, and density values from 65 rock samples covering all different lithologies in the cross section. Thus, its geometry at depth is constrained by means of an integrated 2.5D gravity/structural/petrophysical modelling.

The La Maladeta and Andorra-Mont Louis granites appear aligned in a WNW-ESE direction and both lie within the same Alpine basement unit, the Orri thrust sheet. They are separated about 40 km by the WNW-ESE-oriented Llavorsí syncline, formed by Devonian and Silurian rocks and limited to the north and south by south vergent thrusts. This syncline is located between two large Cambro-Ordovician anticlinorium structures, the La Pallaresa and Orri massifs to the north and south respectively, formed by a monotonous alternation of shales and sandstones with some intercalations of limestones and conglomerates affected by very low to medium grade of metamorphism. Most structures show southern vergence along the cross section, and its southern part is characterized by the occurrence of Triassic evaporites, a significant detachment level decoupling deformation between the Paleozoic basement and the Mesozoic-Cenozoic cover rocks.

The observed residual anomaly along the cross section shows a relative maximum, coinciding with the southern edge of the Axial Zone (Nogueras Zone) and southern half of the Orri massif, followed to the north by a relative large minimum. This gravity minimum in the core of the Axial Zone coincides with the northern half of the Orri massif, the Llavorsí syncline and southern half of the La Pallaresa massif and must be related at depth with rocks of lower density with respect to rocks located to the North and South. Two possible solutions have been postulated to explain the presence of lower density rocks: (i) the presence of Triassic evaporites at depth as a continuation to the North of the Triassic evaporites outcropping in the Rialp window located to the South and/or (ii) the presence of buried granitic bodies equivalent to the adjacent La Maladeta and Andorra-Mont Louis granites.

How to cite: Clariana, P., Soto, R., Ayala, C., Margalef, A., Casas-Sainz, A., Román-Berdiel, T., Pueyo, E. L., Rey-Moral, C., Oliva-Urcia, B., Beamud, E., and Rubio, F. M.: Characterization of a relative gravity minimum in the core of the Pyrenean Axial Zone (Central Pyrenees), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8249, https://doi.org/10.5194/egusphere-egu21-8249, 2021.

How to cite: McQuarrie, N. and Braza, M.: Using bedrock thermochronometer systems to constrain fold-thrust belt geometry and kinematics, insight from the eastern Himalayas, Arunachal Pradesh, India., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8977, https://doi.org/10.5194/egusphere-egu21-8977, 2021.

In a fold-thrust belt (FTB), penetrative strain within thrust sheets vary in its magnitude, orientation and type. Addressing variation in magnitude and orientation of strain from major thrust sheets in a FTB, both along the transport direction and along-strike, enable us to understand the complexity of strain partitioning during orogeny. Tectonic windows provide an opportunity to understand the impact of footwall structures on finite strain geometry and orientations of the overlying thrust sheets. In this study, we investigate how penetrative strain is partitioned from the internal to the external major thrust sheets in the Siang window in far-eastern Arunachal Himalayan FTB. We also compare these results with similar thrust sheets from well preserved tectonic windows in the eastern Himalaya, i.e., the Teesta window of the Sikkim and Kuru Chu window of the Bhutan Himalayan FTB.

We conduct finite strain analysis on quartz grains using R_f-φ, normalized Fry and Shape Matrix Eigenvector methods. The studied lithologies are gneiss for the internal Pelling-Munsiari-Bomdilla thrust (PT) sheet, while quartzite and sandstone dominantly comprise the external Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) sheets. The rocks north of the PT sheet are not accessible. Results from this study indicate that all the studied rocks record an overall flattening strain. Magnitude of the finite penetrative strain decreases from the internal PT sheet to the external MBT, MFT sheets in the Siang window. The long axes of the finite strain ellipsoids (X) generally have a low plunge and vary in bearing, irrespective of the structural positions of the different thrust sheets. Finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to folding of the thrust sheets. The results also indicate that the footwall structures affect the strain geometry in the interior part of the Himalayan wedge. The grain scale shortening percentage is highest for internal PT sheet and it progressively decreases towards the external MFT sheet. The results indicate greater contribution of thrust-parallel stretch than thrust-perpendicular component, in both internal and external thrust sheets in the Siang window. Preliminary results also suggest that the strain magnitude and grain-scale shortening percentage are the lowest, and orientations of X-axes are more variable with respect to the regional transport direction in the far-eastern Siang window as compared to the other westerly lying regional transects of the Himalayan FTB.

How to cite: Das, J. and Bhattacharyya, K.: Strain partitioning within thrust sheets of tectonic windows: Insights from eastern Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3854, https://doi.org/10.5194/egusphere-egu21-3854, 2021.

Decades of work has been completed on Variscan geology of the inner Bristol Channel and Severn Estuary, yet there are few structural models that correctly portray their regional framework. Many published charts loosely depict the positions, strikes and nature of the Variscan deformation front and its geometry across SE Wales. Thus, we correlate seismic data with coastal outcrop at appropriate scales and detail, to present a refined model for the front.

Coastal outcrops, in conjunction with known crustal-scale seismic data: BIRPS, SWAT and LISPB, are combined with archives of intermediate scale: wide-angle reflection, seismic refraction and reflection records. They justify a reinterpretation of the front and may explain the geometry and kinematics of its foreland. Using these data, we draw new sections from north Devon to South Wales showing the position of structural units, both Palaeozoic and Mesozoic, affected either directly by thrusts, folds and disturbances or indirectly through structural inheritance during reactivation.

We correlate extracts from SWAT lines 2 and 3, a reinterpretation of LISPB data and the new fine-scale sections, S-N across the inner channel and W-E across the estuary. They enable the synopsis of crustal-scale data and regional maps. We find from measurement of several hundred lineaments and planes along the borderlands that the predominant orientation is ENE-WSW, unlike the central Bristol Channel which is WNW-ESE. All these, plus outcrop scale geometries and striation analyses, support the new tectonic partition of SE Wales and west of England.

Much information on the partition boundaries can be gathered from the marine geography of the estuary using Admiralty charts that yield accurate soundings. Seabed profiles across the estuary illustrate the positions of bedrock. Many align with onshore structure both locally and on the grander scale and through 3D reconstruction, we find that a crucial confluence of three discrete trends of lineament converge near Flat Holm and Steep Holm and may represent the pristine Variscan WNW, the Caledonoid NE and pervasive NNW trends. These islands in the estuary are sentinels at a boundary to the hybrid terrane that underlies SE Wales.

Mesozoic strata of marginal to distal facies, preserved close to negatively inverted faults with partial growth, mark the reactivated stems of Variscan ramps and NE disturbances with significant thrust displacements. We note two phases of negative inversion require restoration in order to reconstruct the orientations within the Variscan basement. In addition, close examination of late (Tertiary) fault history of the estuary is required to adjust basement trends and displacements to get a better sense of rotation within the Palaeozoic foreland.

Through restoration the new hybrid sub terrane preserves characteristics of Variscan and Caledonoid trending faults and we deduce that a rotation in major thrust trajectory occurred contemporaneously with reactivation of deeper lineaments. This was followed by a structural decapitation as shallow-level thrusts encroached SE Wales, during late stages of the Variscan Orogeny. Finally, the detached stems were incorporated into an imbricate fan which was significantly affected by post-Carboniferous inversion.   

How to cite: Miliorizos, M. N., Reiss, N., Melis, N. S., and Rutter, W. A. J.: Structure of the Inner Bristol Channel and Severn Estuary borderlands, UK: regional mapping and seismic interpretation yield a refined model for mountain front deformation and inversion. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3225, https://doi.org/10.5194/egusphere-egu21-3225, 2021.

Examples of natural folds growing in a homogenous mechanical stratigraphy of alternating competent and incompetent thin layers of fine- and coarse-grained sediments are examined, and the fold growth process is quantified. Our analysis reveals that the overall response to loading of siliciclastic sequences corresponds to that of flexural flow and parallel-to-bedding heterogeneous pure shear. Folds start out as low-amplitude sinusoidal disturbances that rapidly become finite-amplitude folds of heterogeneous strain. We also derive the following scaling relations: (i) degree of amplification scales with both the height above the detachment and strain, (ii) wavelength selectivity broadens with increasing strain, and (iii) deposition of syn-sedimentary geometries is function of strain. These relations are a natural consequence of idealized area-preserving laws of fold growth. From these results we devise a method to estimate fold strain by means of an amplitude versus depth diagram. We are also able to define a progression of fold shape change as a function of the fundamental parameter strain. Initially, structures grow by limb rotation and the selective amplification of a single dominant wavelength giving rise to sinusoidal folds. When strain reaches ~8%, softening/plastic yielding around hinges results in the development of sharp fold profiles. Limbs lock their dips at 35°–45°, suggesting that growth in this stage is permitted by hinge mobility along ramps and blind faults. Moreover, hinge migration causes fold development to accelerate spontaneously. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.

How to cite: Yarbuh, I., Contreras, J., González-Fernández, A., Spelz, R. M., and Negrete-Aranda, R.: Development of Detachment Folds in the Mexican Ridges Foldbelt, Western Gulf of Mexico Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7009, https://doi.org/10.5194/egusphere-egu21-7009, 2021.

The West Congo belt is a Panafrican orogenic belt that evolved and resulted from the collision of the Sao Francisco craton and the Congo Craton during late Neoproterozoic (630 Ma)  to late Cambrian (490 Ma ?). It constitutes the counterpart of the most studied Araçuaì belt in Brasil. Over the past decades, most structural analysis focused in Araçuaì belt while few structural data were obtained from the West-Congo Belt. Understanding the West Congo belt and particularly in its foreland is relevant to establish a unified structural model for its evolution, as the late phases of deformation of both orogens are still debated. In the Comba basin at Mont Bélo, Loutété, Mfouati, most of the folds are gently plunging, upright to moderately inclined fold, with sometimes chevron shape, circular shape and box shape. Some of the folds show decollement within their limbs. Most of these fold display flexural slip displacements along the layers where slickensides are associated with calcite fibres. Most of the limbs developed boudinage in the carbonate layers. The folds are oriented WNW-ESE and they are cut by a system of conjugate NW-SE striking strike-slip dextral fault and NNE-SSW striking sinistral fault. A kinematic analysis from fault slip data using the Win-Tensor program reveal that faults originate from NNE-SSW shortening and ESE-WNW extension. This kinematic analysis is consistent with the orientation of the fold according the Riedel model. The brittle deformation occurred in continuity of the deformation after the folding as folds hinges are displaced in certain localities.This episode of progressive deformation probably ends with intense shearing of the belts, as several dominating regularly spaced NE-striking shear zones cut the orogen from the Republic of Congo to the democratic Republic of Congo. Further investigations will be conducted in the continuity of the west Congo Belt in the Democratic Republic of Congo in order to enlarge the regional perspective.

How to cite: Nkodia, H. M. D.-V., Boudzoumou, F., Miyouna, T., Ibarra-Gnianga, A., and Delvaux, D.: A progressive episode of deformation in the foreland of the West-Congo Belt: From folding to brittle shearing, in Republic of Congo, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10949, https://doi.org/10.5194/egusphere-egu21-10949, 2021.

Belonging to the Maghrebides system, the Rif belt (Northern Morocco) suffered an important Cenozoic Alpine compressional deformation as a consequence of the closure of the Maghrebian Tethys and the westward translation and docking of the Alboran Domain onto the African margin during the Late Burdigalian. The Mesozoic North African Margin is still partially preserved in the Eastern Rif (e.g., Senhadja Jurassic-Cretaceous unit) and inverted in its Central portion (North of the Nekor Fault Zone) due to the high shortening in this area. It is in agreement with sub-surface data suggesting that the thickest crust along the chain is located in the central Rif (Izzaren Area, External Rif), and can be interpreted as a deep-rooted crustal imbrication.

This contribution aims to characterize the role of the structural inheritance of the rifted North African margin in the development and the propagation of the Rif belt by the combination of paleothermal and structural data collected along a NE-SW regional transect (between Chefchaouen and Ouezzane provinces), focusing mainly on the external zones (namely, Intrarif, Mesorif and Prerif) sampling the deformed domains originally developed along the North African paleo-passive margin. A new paleo-thermal dataset of vitrinite reflectance (Ro%), micro-Raman spectroscopy on organic matter and XRD on clayey fraction of sediments displays levels of thermal maturity between early and deep diagenetic conditions (Ro% from 0.49% to 1.15%). The highest thermal maturity values along the section are concentrated in the Lower to middle Cretaceous Loukkos Intrarifain sub-unit that is structurally squeezed between Tangier Intrarif Upper Cretaceous sub-unit and the Mesorif “Izzaren Duplex”. It attests for an important amount of shortening leading to the development of an imbricate fan of thrusts.

The geometry of the “Izzaren Duplex”, limited at surface by two first-order thrust faults, is controlled by pre-existing tectonic structures, probably inherited by the former architecture of the North African paleomargin. Moreover, the Chattian-Middle Miocene siliciclastic succession filling the Zoumi basin is in a stratigraphic continuity with the Izzaren Upper Jurassic-Upper Cretaceous substratum, sheding new light on its geodynamic meaning. This observation is supported by the homogeneity of deformation and the absence of thermal jump between the Mesozoic and Cenozoic successions, attesting for an active compressive deformation in the area between the Late Serravalian and Late Tortonian.

In conclusion, the combination of paleo-thermal and structural analysis allowed to reconstruct robust tectono-thermal model in order to propose an accurate reconstruction of the structural evolution and a new geological restoration of the Rif belt with respect to the geometry of the rifted paleo-margin.

How to cite: Atouabat, A., Corrado, S., Frizon de Lamotte, D., Mohn, G., Haissen, F., Leprêtre, R., and Schito, A.: The structuration of the External Rif (Rif belt: Northern Morocco). An insights from paleo-thermal and structural analyses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12892, https://doi.org/10.5194/egusphere-egu21-12892, 2021.

The Middle Atlas mountain range represents the northeastern branch of the Atlas system, which spans approximately 2000 km from the Atlantic coast of Morocco to Tunisia. The Atlas system is a prominent example of active intracontinental mountain belts that developed in the African plate of the Cenozoic Alpine belt.

The Middle Atlas is an inverted Mesozoic rift that began to rise during the late Cretaceous with limited crustal thickening. It can be divided into two geomorphological provinces: 1) an elevated, low-relief area called Tabular Middle Atlas (TMA), which is located in the north-western orogenic sectors and consists of weakly deformed Mesozoic sediments in stratigraphic contact with the Paleozoic basement of the western Meseta, and 2) a deeply dissected, high-relief area known as Folded Middle Atlas to the southeast, where crustal deformation is dominated by transpressive tectonics induced by a NNW-SSE maximum shortening direction. Seismicity and geomorphic landforms suggest that tectonic deformation is still active, at least in some sectors of the orogen.

In order to investigate the tectonic evolution of the Middle Atlas, we combined structural and geomorphic analysis. Although the age control of the continental syn-orogenic deposits is limited, the eastern boundary of the orogen shows evidence of recent tectonic deformation and flexural subsidence with the development of a foreland basin. Conversely, the western boundary of the orogen does not include syn-orogenic foreland basins suggesting the lack of flexural subsidence. This boundary is also characterized by alkaline late Miocene-Quaternary lava flows over a wide surface of ca. 960 km². These lava flows cover part of the TMA where they fill valleys crossing the Meseta and draining towards the Atlantic Ocean. The degree of subsequent fluvial incision of the lavas is lower in the TMA than in the Meseta. While incision does not go beyond the stratigraphic contact lava-substratum in the TMA, it goes further down in the Meseta indicating a higher magnitude of uplift. The lack of contractional deformation, however, suggests that such an uplift is not controlled by tectonics.

Overall, our preliminary observations suggest the occurrence of an asymmetry between the two orogenic flanks. Uplift along the eastern orogenic boundary has been triggered by late Cenozoic contractional deformation, whereas deep-seated, most likely mantle-driven processes essentially control uplift of the western boundary.

How to cite: Yaaqoub, A., Essaifi, A., Clementucci, R., Ballato, P., and Faccenna, C.: Tectonic deformation and mantle-driven uplift of the Middle Atlas mountain belt (Morocco) during the late Cenozoic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13646, https://doi.org/10.5194/egusphere-egu21-13646, 2021.

Fault geometries are usually reconstructed through seismic data, which can provide a very good image of the subsurface. However, the recognition of deep structures is often difficult for the shallow depth of these data and their low resolution in depth. On the contrary, recent earthquakes and their parameters (e.g. hypocentre, focal mechanism, magnitude, etc.) may have an important role in better understanding deep features, outlining the active crustal structures.

November 26^th 2019 a 6.4 M_w Durres earthquake struck the Albanian coastal area, claiming 51 victims and hundreds of injured people. This seismic sequence sheds new light into the structural architecture and active tectonic setting of the northern outer Albanides. Stress field analysis performed through local mechanisms of the main seismic events of the sequence and those recorded since 1997 by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) confirm that the area is dominated by ENE trending, horizontal maximum compression, with a ENE dipping thrust faults roughly parallel to the coastline. Further analysis to investigate the structural architecture of this area was conducted plotting hypocentre distribution which show that shallower hypocentres tend to cluster around the deeper portion of projected fault segment proposed by the DISS ‘composite seismogenic source’ labelled ALCS002, whereas most of the seismic events including the Mw = 6.4 main shock are nucleated within the crystalline basement. This result unravels for the first time the fundamental role of deeply rooted, crustal ramp-dominated thrusting in seismogenesis, implying a profound reconsideration of the seismotectonic setting of the region.

The outcomes of this study show here that the recent earthquakes are pivotal in outlining the active crustal frontal structure of the thrust belt, providing new fundamental constraints, not only on the active tectonic setting of the region, but also on the crustal architecture of the outer Albanides. In this regard, the identification of such deep seismogenic sources and the definition of their dimensional parameters may have major implications on the correct assessment of the seismic hazard, especially for this large and densely populated area of Albania. Furthermore, the evidence provided in this study for a deep seismogenic thrust system in a foreland basin setting may be of general interest in similar tectonic contexts worldwide, where deep structures are possibly unidentified, and may represent a weakness in seismic hazard assessment.

How to cite: Teloni, S., Invernizzi, C., Mazzoli, S., Pierantoni, P. P., and Spina, V.: Seismogenic fault system of the Mw 6.4 November 2019 Albania earthquake: new insights into the structural architecture and active tectonic setting of the outer Albanides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2434, https://doi.org/10.5194/egusphere-egu21-2434, 2021.

Western Makran is one of the few subduction zones left with a largely unconstrained seismogenic potential. According to the sparse GPS stations, the subduction is accumulating some strain to be released during future earthquakes. Mechanical modelling is first used to retrieve the spatial variations of the frictional properties of the megathrust, and discuss its seismogenic potential. To do so, we first build a structural map along the Iranian part of the Oman Sea and investigate three N-S seismic profiles. The profiles are characterized by a long imbricated thrust zone that takes place at the front of the wedge. A diapiric zone of shallow origin lies in between the imbricated zone and the shore. Along the eastern and western shores, active listric normal faults root down to the megathrust. Eastern and western domains have developed similar deformation, with three zones of active faulting: the normal faults on shore, thrusts ahead of the mud diapirs, and the frontal thrusts. On the contrary, no normal faults are identified along the central domain, where a seamount is entering into subduction. From mechanical modelling, we show that along the eastern and western profiles, a transition from very low to extremely low friction is required to activate the large coastal normal fault. To propagate the deformation to the front, an increase of friction along the imbricated zone is necessary. These along-dip transitions could either be related to a transition from an aseismic to seismic behavior or the brittle-viscous transition.

To decipher, we run 2-D thermo-mechanical modelling incorporating temperature evolution, with a heat flow boundary condition. Our simulations are first calibrated to reproduce the heat flow estimates based on the BSR depth. Then the effects of the illite-smectite and brittle-viscous transitions on the deformation are investigated. The decrease in heat flow landward is due to the landward deepening of the oceanic plate and thickening of sediments of the accretionary wedge. Deformation starts at the rear of the model and migrates forming in-sequence, forward verging thrust sheets. The two brittle-viscous and illite-smectite transitions affect the topographic slope and friction. A reduction of friction due to the illite-smectite transition reduces the slope by normal faulting that does not appear in the brittle-viscous transition simulations. Therefore, the presence of normal faults could permit to distinguish viscously creeping segments from segments that deform seismically. As a consequence, the normal fault is most probably related to the presence of a seismic asperity, and the difference in deformation along strike would thus reveal the existence of two different patches, one along the eastern domain and a second along the western domain. Since no large earthquake has been historically reported and given the high convergence rate, a major earthquake will strike the Makran region. We suggest that the magnitude of this event will depend on the behavior of the Central region, and the ability of the earthquake to propagate from the eastern to the western asperity or the Pakistani Makran.

How to cite: pajang, S., Cubas, N., Le-pourhiet, L., Bessiere, E., Letouzey, J., Seyedali, S., Fournier, M., Agard, P., Khatib, M. M., Heyhat, M., and Mokhtari, M.: Seismic hazard of the Western Makran subduction zone: Effect of heat flow on frictional properties combining mechanical and thermo-mechanical modelling approaches, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13351, https://doi.org/10.5194/egusphere-egu21-13351, 2021.