Dynamics and structural evolution of fold-and-thrust belts: From nature to models across spatial and temporal scales

Fold-and-thrust belts are one of the most recognizable large-scale geological features occurring all around the globe. Fold-and-thrust belts mostly develop along convergent plate boundaries but they may also form along passive margins or other super-critical slopes driven by a gravitationally driven stress field. Fold-and-thrust belts may involve the basement of continental lithospheres to build entire mountain ranges or just the uppermost sedimentary sequence detaching along stratigraphic décollements. Although these different types of fold-and-thrust belts vary in spatial extent, longevity of their formation, and rock types involved, their dynamics and structural evolution strongly depends on the same internal and external effects, such as rheological and rock mechanical properties, temperature and surface processes, allowing to compare them with each other and to develop common mechanical predictions.
Fold-and-thrust belts have been intensely investigated to decipher their short- and long-term evolution. However, there are important questions that yet have to be fully understood: i) What is the effect of inherited structures within the basement, the cover sequence and potential décollement layers, and how can those inheritances be detected? ii) How are transient and long-term rheological/mechanical characteristics comparable during the formation of a fold-and-thrust belt? iii) Do present day fold-and-thrust belts reflect local, transient conditions, and how are large-scale, long-term tectonic processes affecting their evolution?
The here proposed session tackles to answer these questions by an interdisciplinary approach. We look forward to receive abstracts focusing on the short- and long-term dynamics and structural evolution of fold-and-thrust belts by means of structural fieldwork, seismics and seismology, analogue and numerical modelling, rock mechanics, geomorphology and thermochronology as well as quantification of uncertainties in order to improve our understanding of fold-and-thrust belts across spatial and temporal scales.

Convener: Jonas B. Ruh | Co-conveners: Sandra BorderieECSECS, Esther IzquierdoECSECS, Olivier Lacombe, Christoph von Hagke
vPICO presentations
| Mon, 26 Apr, 09:00–10:30 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Jonas B. Ruh, Sandra Borderie, Christoph von Hagke
Youseph Ibrahim and Patrice Rey

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.

Sreetama Roy, Santanu Bose, and Puspendu Saha

Fold and thrust belts (FTBs), formed by the collision of two continental plates, accommodate tectonic convergence through folding and faulting of crustal rocks. The effects of distributed deformation although ubiquitous in all fold-and-thrust belts, regionally occurring ductile structures are often interpreted as an outcome of localized deformation. Our study presents 3D laboratory-scale models using a viscous thin sheet as crustal layer to investigate the evolution of distributed ductile strain in FTBs. Here, we tested the role of mechanical coupling at the basal decollement (i.e., weak versus strong) on the nature of ductile strain variations within a deforming tectonic wedge. Convergence velocity has been kept constant in all experiments to avoid the influence of rate-dependence on viscous rheology. Our results reveal that the mode of wedge growth with changing basal coupling is crucial for varying strain pattern towards the hinterland. Weak decollement models yield a zone of constriction towards the central part of the hinterland, explaining the occurrence of isolated patches of L-tectonites and cross-folds in FTBs; while strong decollement condition allows the gravity-driven flow to be dominant over horizontal shortening, leading to rotation of earlier structures and formation of orogen-parallel recumbent folds, particularly towards the hinterland. The deformation towards the frontal part of the tectonic wedge, irrespective of coupling strength in both models is similar, forming a characteristic pattern of pervasive, hinterland dipping ductile fabrics. We correlate our findings to infer that spatio-temporal variations in basal coupling are responsible for the development of variably occurring ductile structures in FTBs.

How to cite: Roy, S., Bose, S., and Saha, P.: Spatially varying ductile structures in fold-and-thrust belts: insights from laboratory experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5075, https://doi.org/10.5194/egusphere-egu21-5075, 2021.

Nicolas Molnar and Susanne Buiter

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.

Ferdinando Musso Piantelli, David Mair, Marco Herwegh, Alfons Berger, Eva Kurmann, Michael Wiederkehr, Fritz Schlunegger, Roland Baumberger, and Andreas Möri

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.

Pablo Santolaria, Concepción Ayala, Emilio L. Pueyo, Félix M. Rubio, Ruth Soto, Pablo Calvín, Aranzazu Luzón, Adriana Rodríguez-Pintó, Carlota Oliván, and Antonio M. Casas-Sainz

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.

Pilar Clariana, Ruth Soto, Conxi Ayala, Aina Margalef, Antonio Casas-Sainz, Teresa Román-Berdiel, Emilio L. Pueyo, Carmen Rey-Moral, Belén Oliva-Urcia, Elisabet Beamud, and Felix M. Rubio

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.

Charlotte Peigney, Elisabet Beamud, Òscar Gratacós, Eduard Roca, Alberto Sáez, Luis Valero, and Josep Anton Muñoz

In foreland settings at the front of active orogens, the aggradation/progradation of fluvial fans and sedimentary changes in lacustrine systems depends greatly on the tectonic activity and the derived drainage pattern changes in the hinterland. As a result of the emplacement and erosion of the South-Pyrenean thrust sheets, a system of N-S fluvial fans prograded into the Ebro foreland basin from late Eocene to Oligocene times. After the synorogenic deposition of the Priabonian (late Eocene) marine evaporites of the Cardona Fm, the Ebro Basin was characterized by internal drainage, with the fluvial fans grading to lacustrine systems at the center of the basin, which developed and migrated in response to subsidence changes. All these deposits were deformed by variably oriented salt-detached folds, evidencing the basinwards propagation of the deformation. In this work, we study the Solsona-Sanaüja fluvial fan system by means of litostratigraphy and magnetostratigraphy aiming to determine the age of the transition from fluvial fan to lacustrine systems in the NE sector of the Ebro Basin. The precise dating of this succession reveals causal relationships between tectonic and climatic processes affecting the source-to-sink system, including changes in the depositional style linked to the evolution of the Pyrenean fold and thrust belt.

Our new magnetostratigraphic study consisted in the sampling and analysis of 195 samples along a ca. 1800m thick stratigraphic section of the late Eocene-Oligocene succession in the northern limb of the NW-SE oriented Sanaüja Anticline. Our results show overall Priabonian to Rupelian ages for the succession, considering an age of 36 Ma. (C16n) for the top of the Cardona Fm from previous magnetostratigraphic studies. This allows dating the end of the evaporitic sedimentation (top of the Barbastro Fm) as Priabonian and establishing a late Priabonian to early Rupelian (C13r) age for the transition from the younger lacustrine deposits (Torà Fm) to the continuous and most important fluvial fan episode of progradation in the study area. The final progradation of the fluvial fan system was coeval to a tectonically controlled reorganization of the drainage pattern of the basin responding to the emplacement of the South-Pyrenean thrust sheets. Meanwhile, smaller scale (hectometric-decametric) alternation between lacustrine and alluvial deposits was possibly driven by climatic changes related to orbital eccentricity cycles. The correlation and integration of these results with previous magnetostratigraphic studies in the area can help analyzing sedimentation patterns and architectural changes in the basin margins at a regional scale.

How to cite: Peigney, C., Beamud, E., Gratacós, Ò., Roca, E., Sáez, A., Valero, L., and Muñoz, J. A.: Magnetostratigraphic dating of the Paleogene synorogenic sediments of the NE sector of the Ebro Foreland Basin (Spanish Pyrenees), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14714, https://doi.org/10.5194/egusphere-egu21-14714, 2021.

Nadine McQuarrie and Mary Braza

One of the first order questions regarding a cross-section representation through a fold-thrust belt (FTB) is usually “how unique is this geometrical interpretation of the subsurface?”  The proposed geometry influences perceptions of inherited structures, decollement horizons, and both rheological and kinematic behavior.  Balanced cross sections were developed as a tool to produce more accurate and thus more predictive geological cross sections.  While balanced cross sections provide models of subsurface geometry that can reproduce the mapped surface geology, they are non-unique, opening the possibility that different geometries and kinematics may be able to satisfy the same set of observations. The most non-unique aspects of cross sections are: (1) the geometry of structures that is not seen at the surface, and (2) the sequence of thrust faulting.  We posit that integrating sequentially restored cross sections with thermokinematic models that calculate the resulting subsurface thermal field and predicted cooling ages of rocks at the surface provides a valuable means to assess the viability of proposed geometry and kinematics.  Mineral cooling ages in compressional settings are the outcome of surface uplift and the resulting focused erosion.  As such they are most sensitive to the vertical component of the kinematic field imparted by ramps and surface breaking faults in sequential reconstructions of FTB.  Because balanced cross sections require that the lengths and locations of hanging-wall and footwall ramps match, they provide a template of the ways in which the location and magnitude of ramps in the basal décollement have evolved with time.  Arunachal Pradesh in the eastern Himalayas is an ideal place to look at the sensitivity of cooling ages to different cross section geometries and kinematic models. Recent studies from this portion of the Himalayan FTB include both a suite of different cross section geometries and a robust bedrock thermochronology dataset. The multiple published cross-sections differ in the details of geometry, implied amounts of shortening, kinematic history, and thus exhumation pathways. Published cooling ages data show older ages (6-10 Ma AFT, 12-14 Ma ZFT) in the frontal portions of the FTB and significantly younger ages (2-5 Ma AFT, 6-8 Ma ZFT) in the hinterland. These ages are best reproduced with kinematic sequence that involves early forward propagation of the FTB from 14-10 Ma.  The early propagation combined with young hinterland cooling ages require several periods of out-of-sequence faulting. Out-of-sequence faults are concentrated in two windows of time (10-8 Ma and 7-5 Ma) that show systematic northward reactivation of faults.  Quantitative integration of cross section geometry, kinematics and cooling ages require notably more complicated kinematic and exhumation pathways than are typically assumed with a simple in-sequence model of cross section deformation.  While also non-unique, the updated cross section geometry and kinematics highlight components of geometry, deformation and exhumation that must be included in any valid cross section model for this portion of the eastern Himalaya.

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.

Chirantan Parui and Kathakali Bhattacharyya

Convergence-related shortening gets primarily accommodated in faults, fault-related folds and penetrative strain in fold thrust belts (FTB). For example, in the Himalayan FTB, ~477-919 km minimum orogen-scale shortening is accommodated by a series of folded, south vergent thrust systems that vary laterally in their geometry resulting in laterally varying shortening distribution. From hinterland to foreland, these major thrust faults are the Main Central thrust, the Pelling-Munsiari thrust, the Lesser Himalayan duplex, the Main Boundary thrust, and the Main Frontal thrust. In the Sikkim Himalayan FTB, the structural geometry of these thrust sheets laterally varies over ~15 km. Based on two regional, transport-parallel balanced cross-sections, ~542-589 km minimum wedge-scale shortening has been estimated. To quantify grain-scale shortening, we analyzed 201 thin-sections cut from 96 quartz-rich samples (sandstone, quartzite, phyllite, schist, and gneiss) and calculated penetrative strain from them. Penetrative strain results indicate that ~25-26% of total Himalayan shortening is recorded at the grain-scale in this section of the eastern Himalaya.

In the internal thrust sheets, the strain magnitude (RS) remains higher (~1.4-2.43 ), and it progressively decreases in the frontal thrust sheets (~1.08-1.51). The normalized Fry and the Rf-φ are the two most commonly used graphical methods to estimate best-fit strain ellipse parameters, i.e., RS and φ (long-axis orientation). However, in thrust sheets with less deformed sandstones, where initial grain shapes were not spherical, these graphical methods do not accurately estimate the best-fit strain ellipse parameters. The central vacancy in the Fry plot was objectively fitted using the enhanced normalized Fry (ENFRY), the point-count density (PCD), the continuous function method (CFM), and weighted least square (WLS) methods. From the Rf-φ data, we calculated the best-fit strain ellipse using the shape matrix eigenvector (SME), centroids of the hyperbolic plot (HP), Elliot’s polar graph (EPG), and Rf-φ graph, harmonic mean (HM) and vector mean (VM) methods. In this study, we calculate the accuracy of these strain methods as a function of the strain magnitude and structural position within the orogenic wedge. The SME and HP methods record the lowest bootstrap errors in the strain parameters in the internal thrust sheets. In contrast, RS and φ values estimated by the WLS method records the lowest bootstrap error in the frontal thrust sheets, followed by the SME, HP, and EPG methods. We also created six synthetic aggregates containing 150-170 random elliptical grains with random long-axis orientations. We deformed these aggregates under pure-shear, simple-shear, and general-shear conditions at various strain increments. We have generated 7560 strain data. To understand the accuracy of these strain methods in estimating penetrative strain, we calculated the Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) for every strain method and every type of deformation. Experimental results indicate that the SME and HP methods record the lowest errors in the RS and φ values. In low strain conditions (RS<1.5), the SME, HP, and EPG methods record lower errors in the strain parameters. Therefore, this study shows that the SME and HP methods overall yield a better penetrative strain estimate.

How to cite: Parui, C. and Bhattacharyya, K.: A comparison of different methods for estimating penetrative strain using natural and synthetic data: A study from the Sikkim Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3808, https://doi.org/10.5194/egusphere-egu21-3808, 2021.

Pritam Ghosh and Kathakali Bhattacharyya

We examine how the deformation profile and kinematic evolutionary paths of two major shear zones with prolonged deformation history and large translations differ with varying structural positions along its transport direction in an orogenic wedge. We conduct this analysis on multiple exposures of the internal thrusts from the Sikkim Himalayan fold thrust belt, the Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex (LHD), and the overlying Main Central thrust (MCT). These two thrusts are regionally folded due to growth of the LHD and are exposed at different structural positions. The hinterlandmost exposures of the MCT and PT zones lie in the trailing parts of the duplex, while the foreland-most exposures of the same studied shear zones lie in the leading part of the duplex, and thus have recorded a greater connectivity with the duplex. The thicknesses of the shear zones progressively decrease toward the leading edge indicating variation in deformation conditions. Thickness-displacement plot reveals strain-softening from all the five studied MCT and the PT mylonite zones. However, the strain-softening mechanisms varied along its transport direction with the hinterland exposures recording dominantly dislocation-creep, while dissolution-creep and reaction-softening are dominant in the forelandmost exposures. Based on overburden estimation, the loss of overburden on the MCT and the PT zones is more in the leading edge (~26km and ~15km, respectively) than in the trailing edge (~10km and ~17km, respectively), during progressive deformation. Based on recalibrated recrystallized quartz grain thermometer (Law, 2014), the estimated deformation temperatures in the trailing edge are higher (~450-650°C) than in the leading edge (350-550°C) of the shear zones. This variation in the deformation conditions is also reflected in the shallow-crustal deformation structures with higher fracture intensity and lower spacing in the leading edge exposures of the shear zones as compared to the trailing edge exposures.

The proportion of mylonitic domains and micaceous minerals within the exposed shear zones increase and grain-size of the constituent minerals decreases progressively along the transport direction. This is also consistent with progressive increase in mean Rs-values toward leading edge exposures of the same shear zones. Additionally, the α-value (stretch ratio) gradually increases toward the foreland-most exposures along with increasing angular shear strain. Vorticity estimates from multiple incremental strain markers indicate that the MCT and PT zones generally record a decelerating strain path. Therefore, the results from this study are counterintuitive to the general observation of a direct relationship between higher Rs-value and higher pure-shear component. We explain this observation in the context of the larger kinematics of the orogen, where the leading edge exposures have passed through the duplex structure, recording the greatest connectivity and most complete deformation history, resulting in the weakest shear zone that is also reflected in the deformation profiles and strain attributes. This study demonstrates that the same shear zone records varying deformation profile, strain and kinematic evolutionary paths due to varying deformation conditions and varying connectivity to the underlying footwall structures during progressive deformation of an orogenic wedge.

How to cite: Ghosh, P. and Bhattacharyya, K.: Effect of kinematics of orogenic wedge on kinematic evolutionary paths and deformation profiles of major shear zones: An example from the eastern Himalaya , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3824, https://doi.org/10.5194/egusphere-egu21-3824, 2021.

Jyoti Das and Kathakali Bhattacharyya

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 Rf-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.

Fabrizio Cocco and Antonio Funedda

The Variscan foreland of SW-Sardinia consists of a Cambrian to lower Carboniferous succession polydeformed under very low-grade metamorphism. It is characterized by the following superposed structures: 1) E-W-trending upright folds; 2) N-S-trending inclined folds, penecontemporaneous with 3) W-ward fore-thrusts, and 4) E-ward back-thrusts.

A peculiar feature of this sector of the Variscan foreland is the widespread occurrence of back-thrusts, apparently more common than fore-thrusts, unlike the majority of foreland fold-and-thrust belts.

Our research focuses on the role played by the folded basement in limiting extensive fore-thrusts development and how fold shape and orientation, along with litho-stratigraphic heterogeneity, influenced the back-thrust geometry.

Generally, back-thrusts occur when the shortening can no longer be accommodated by fore-thrusts, usually because of buttressing induced by fore-thrust-related thickening and duplication of the stratigraphic succession. However, in the segment of Variscan foreland outcropping in SW-Sardinia, back-thrusting seems to be activated by a different mechanism. The inherited structural setting is characterized by two perpendicular generation of superposed folds that gave rise to a type 1 interference pattern with pluri-km-scale domes and basins. In particular, in the western sector of the foreland (i.e., the farthest from the nappe zone thrusted over the foreland) domes are made up of about 500 m thick lower Cambrian sandstone and limestones formations that may have acted as a buttress, hindering fore-thrusting propagation and facilitating extensive E-ward back-thrusting. This is corroborated by the large number of back-thrusts that crop out between the buttress and the nappe front.

In this area, back-thrusts affect the folded sedimentary succession that is progressively younger and weaker E-ward. As commonly accepted in thrust faults, ramps developed in the competent stratigraphic sequence, here made up of sandstones and limestones, and flats in weak stratigraphic horizons, here consisting of marly limestones and shales. As a result, in the study area the dip of back-thrusts decreases towards the nappes front, where the weaker lithologies have been overthrusted.

The back-thrusts’ surface is characterized by discontinuous antiforms and synforms that do not affect the underlaying succession; so, a later deformation phase that folded the back-thrusts can be ruled out. Therefore, the fault plane should have been deformed throughout the back-thrusts growth and development.

Interestingly, strictly relationships can be noticed between the fault plane geometry and the inherited structures in the footwall of the back-thrusts. Where the back-thrusts cut across upright limbs perpendicular to the back-thrust strike, the fault plane shows an antiformal shape; where the back-thrusts take place above the pre-existing synforms with the axis plunging towards the back-thrust dip, the fault plane takes the form of the underlying synforms. Instead, back-thrusts are uninfluenced by pre-existing folds where they cut either synforms with the axis that plunges opposite to the dip direction of the fault plane or antiforms, regardless the plunging direction of their axis.

To conclude, this research highlights the relevant role of the inherited structural setting on fold-and-thrust belt style and suggests that the strata attitude and the axes plunging directions of pre-existing folds could have a control in the back-thrust geometry.

How to cite: Cocco, F. and Funedda, A.: Influence of inherited superposed folding on back-thrusting development: a case study from the Variscan foreland fold-and-thrust belt of Sardinia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9034, https://doi.org/10.5194/egusphere-egu21-9034, 2021.

Aurore Laurent, Olivier Averbuch, Laurent Beccaletto, Fabien Graveleau, Frédéric Lacquement, Laure Capar, and Stéphane Marc

In NW Europe, the Upper Carboniferous Variscan collision between Avalonia and the Armorica-Gondwana accretion complex led to the progressive tectonic inversion of the southern Avalonian margin and the development of a crustal-scale north-vergent thrust system propagating outward from the Late Mississippian to the Middle Pennsylvanian (330-305 Ma). The northern Variscan thrust front spreads over 2,000 km across NW Europe. In the Nord-Pas-de-Calais (NPC) coal district area (northern France), its 3D geometry and kinematics have been investigated through the reprocessing and interpretation of 532 km in length of industrial seismic reflection profiles acquired in the 1980s. The seismic interpretations point out the major compressional and extensional tectonic features affecting this fossil, deeply eroded, mountain front, highlighting its very atypical structure and kinematics.

The deformation front is characterized by a main frontal thrust zone localizing most of the northward displacement (i.e. several tens of kilometers) of the Ardennes Allochthonous Unit above the slightly-deformed part of the Avalonian margin, referred to as the Brabant Para-autochthonous Unit. This large displacement induced the underthrusting of the molassic foreland basin (NPC coal basin) over nearly 20 km and was associated to the out-of-sequence dislocation of the mountain front. The underthrust Brabant Para-autochthonous Unit, made of both the Namurian-Westphalian (330-305 Ma) molassic foreland basin and the underlying Mid-Upper Devonian (390-360 Ma) and Dinantian (360-330 Ma) carbonate platform, is deformed by a series of second-order north-vergent thrust faults, often associated with ramp-related folds. These thrust faults are rooted in décollement zones located either at the transition between the Namurian shales and the Dinantian carbonates or in the Famennian shales.

The 3D integration of the seismic interpretations led to the characterization of a major lateral ramp oriented NW-SE, affecting both the main frontal thrust zone and the basal thrust of some Overturned Thrust Sheets developed at its footwall. This lateral ramp represents a major zone of relay along the thrust front, in between two major segments, oriented respectively ENE-WSW to the east and WNW-ESE to the west. At the base of the underthrust Brabant Para-autochthonous Unit, the Mid-Upper Devonian platform is shown to be structured by synsedimentary normal faults responsible for the southward deepening and thickening of the southern Avalonian margin. These faults are oriented along two main directions i.e. N060-080° and N110-130°, that is the general orientation of the future Variscan structures. Overall, the results indicate that the Devonian pre-structuration of the southern Avalonian margin exerted a primary control on the dynamics and segmentation of the Northern Variscan Front in northern France by localizing both the frontal and lateral ramps within the thrust wedge.

How to cite: Laurent, A., Averbuch, O., Beccaletto, L., Graveleau, F., Lacquement, F., Capar, L., and Marc, S.: 3D geometry and kinematics of the Northern Variscan Thrust Front in Northern France: new insights based on reprocessing and interpretation of seismic reflection profiles., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8600, https://doi.org/10.5194/egusphere-egu21-8600, 2021.

Marios N. Miliorizos, Nicholas Reiss, Nikolaos S. Melis, and William A.J. Rutter

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.

Ismael Yarbuh, Juan Contreras, Antonio González-Fernández, Ronald M. Spelz, and Raquel Negrete-Aranda

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.

Willemijn S.M.T. van Kooten, Edward R. Sobel, Cecilia del Papa, Patricio Payrola, Alejandro Bande, and Daniel Starck

The Cretaceous period in NW Argentina is dominated by the formation of the Salta rift basin, an intracontinental rift basin with multiple branches extending from the central Salta-Jujuy High. One of these branches is the ENE-WSW striking Lomas de Olmedo sub-basin, which hosts up to 5 km of syn- and post-rift deposits of the Salta Group, accommodated by substantial throw along SW-NE striking normal faults and subsequent thermal subsidence during the Cretaceous-Paleogene. Early compressive movement in the Eastern Cordillera led to the formation of a foreland basin setting that was further dissected in the Neogene by the uplift of basement-cored ranges. As a consequence, the northwestern part of the Lomas de Olmedo sub-basin was disconnected from the Andean foreland and local depocenters such as the Cianzo basin were formed, whereas the eastern sub-basin area is still part of the Andean foreland. Thus, the majority of the Salta Group to the east is located in the subsurface and has been extensively explored for petroleum, while in northwestern part of the sub-basin, the Salta Group is increasingly deformed and is fully exposed in the km-scale Cianzo syncline of the Hornocal ranges. The SW-NE striking Hornocal fault delimits the Cianzo basin to the south and the Cianzo syncline to the north. During the Cretaceous, it formed the northern margin of the Lomas de Olmedo sub-basin, which is indicated by an increasing thickness of the syn-rift deposits towards the Hornocal fault, as well as a lack of syn-rift deposits on the footwall block. Structural mapping and unpublished apatite fission track (AFT) data show that the Hornocal normal fault was reactivated and inverted during the Miocene. Although structural and sedimentary features of the Cianzo basin infill provide information about the relative timing of fault activity, there is a lack of low-temperature thermochronology. Herein, we aim to constrain the exhumation of the Lomas de Olmedo sub-basin during the Cretaceous rifting phase, as well as the onset and magnitude of fault reactivation in the Miocene. We collected 74 samples for low-temperature thermochronology along two major NW-SE transects in the Cianzo basin and adjacent areas. Of these samples, 59 have been analyzed using apatite and/or zircon (U-Th-Sm)/He thermochronology (AHe, ZHe). Furthermore, 49 samples have been prepared for AFT analysis. The ages are incorporated in thermo-kinematic modelling using Pecube in order to test the robustness of uplift and exhumation scenarios. On the hanging wall block of the N-S striking east-vergent Cianzo thrust north of the Hornocal fault, Jurassic ZHe ages are attributed to pre-Salta Group exhumation. However, associated thrusts to the south show ZHe ages as young as Eocene-Oligocene, which might indicate early post-rift activity along those thrusts. AHe data from the Cianzo syncline show a direct age-elevation relationship with Late Miocene-Pliocene cooling ages, indicating the onset of rapid exhumation along the Hornocal fault in the Miocene. This is consistent with regional data and suggests that pre-existing extensional structures were reactivated during Late Miocene-Pliocene compressive movement within this part of the Central Andes.

How to cite: van Kooten, W. S. M. T., Sobel, E. R., del Papa, C., Payrola, P., Bande, A., and Starck, D.: Exhumation history of the Lomas de Olmedo basin: constraining multi-phase deformation using low-temperature thermochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12429, https://doi.org/10.5194/egusphere-egu21-12429, 2021.

Hardy Medry Dieu-Veill Nkodia, Florent Boudzoumou, Timothée Miyouna, Alex Ibarra-Gnianga, and Damien Delvaux

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.

Achraf Atouabat, Sveva Corrado, Dominique Frizon de Lamotte, Geoffroy Mohn, Faouziya Haissen, Remi Leprêtre, and Andrea Schito

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.

Ahmed Yaaqoub, Abderrahim Essaifi, Romano Clementucci, Paolo Ballato, and Claudio Faccenna

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.

Nadaya Cubas

Over the last decade, we have accumulated evidence that, along subduction zones, a significant part of the seismic cycle deformation is permanently acquired by the medium and reflects the variation of rupture properties along the megathrust. Assuming a persistence of the megathrust segmentation over several hundred thousand years, this permanent deformation and the forearc topography could thus reveal the mechanics of the megathrust. Numerous recent studies have also shown that the megathrust effective friction appears to differ significantly between aseismic or seismic areas. From mechanical modelling, I will first discuss how such differences in effective friction are significant enough to induce wedge segments with varying morphologies and deformation patterns. I will present examples from different subduction zones characterized by either erosive or accretionary wedges, and by different seismic behaviors. Secondly, I will present how this long-lived deformation can in turn control earthquake ruptures. I will show, that along the Chilean subduction zone, all recent mega-earthquakes are surrounded by basal erosion and underplating. Therefore, the deformation and morphology of forearcs would both be partly linked to the megathrust rupture properties and should be used in a more systematic manner to improve earthquake rupture prediction.

How to cite: Cubas, N.: Relationships between deformation and morphology of forearc wedges and earthquake ruptures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11594, https://doi.org/10.5194/egusphere-egu21-11594, 2021.

Simone Teloni, Chiara Invernizzi, Stefano Mazzoli, Pietro Paolo Pierantoni, and Vincenzo Spina

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 26th 2019 a 6.4 Mw 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.

sepideh pajang, Nadaya Cubas, Laetitia Le-pourhiet, Eloise Bessiere, Jean Letouzey, Seyedmohsen Seyedali, Marc Fournier, Philippe Agard, Mohammad Mahdi Khatib, Mahmoudreza Heyhat, and Mohammad Mokhtari

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.