Variations In Structural Style Research Articles

Influence of lateral variations in décollement strength on the structure of fold-and-thrust belts: Insights from viscous wedge models

Fold-and-thrust belts (FTBs) evolve over a mechanically weak basal décollement that separates the overlying intensely deformed rocks from the underlying less deformed ones. Although deformation structures in FTBs commonly show lateral continuity, a closer inspection reveals distinctive variations in structural style (e.g., fold style) along and across mountain belts. This study uses laboratory-scale viscous models to investigate the influence of lateral décollement strength variations on the spatio-temporal evolution of strain patterns in FTBs. These experiments, simulating crustal-scale deformation, show notable changes in the mode of tectonic wedge growth, including the topographic evolution and ductile strain pattern distribution. For example, the deformation front propagates faster over weakly coupled décollement than the laterally adjacent strongly coupled segment, leading to along-strike variations of the topographic slope and curved outline of the deformation front. Constrictional strain, characteristic of regions of weak coupling, is transient and replaced by flattening strain beyond ∼20 % bulk shortening. The latter prevails in regions over strong décollement, whereas complex strain histories mark the transition zone between weak and strong décollements. Based on our modelling results, we propose that variations in décollement strength may cause the segmentation of deformation processes and the development of transverse faults in FTBs.

Influence of rheologically weak layers on fault architecture: insights from analogue models in the context of the Northern Alpine Foreland Basin

We present a series of analogue models inspired by the geology of the Zürcher Weinland region in the Northern Alpine Foreland Basin of Switzerland to explore the influence of rheological weak, i.e. (partially) ductile layers on the 3D evolution of tectonic deformation. Our model series test the impact of varying weak layer thickness and rheology, as well as different kinematics of an underlying “basal fault”. Model analysis focuses on deformation in the weak layer overburden and, uniquely, within the weak layer itself. We find that for low to moderate basal fault displacements, the above-mentioned parameters strongly influence the degree of coupling between the basal fault and the weak layer overburden. Coupling between the basal fault and overburden decreases by reducing the strength of the weak layer, or by increasing the weak layer’s thickness. As a result, basal fault displacement is less readily transferred through the weak layer, leading to a different structural style in the overburden. By contrast, increasing the amount, or rate, of basal fault slip enhances coupling and leads to a more similar structural style between basal fault and overburden. Moreover, dip-slip displacement on the basal fault is more readily transferred to the overburden than strike-slip displacement of the same magnitude. Our model results compare fairly well to natural examples in the Northern Alpine Foreland Basin, explaining various structural features. These comparisons suggest that rheological weak layers such as the Jurassic Opalinus Clay have exerted a stronger control on fault zone architecture than is commonly inferred, potentially resulting in vertical fault segmentation and variations in structural style. Furthermore, the novel addition of internal marker intervals to the weak layer in our models reveals how complex viscous flow within these layers can accommodate basal fault slip. Our model results demonstrate the complex links between fault kinematics, mechanics and 3D geometries, and can be used for interpreting structures in the Alpine Foreland, as well as in other settings with similar weak layers and basal faults driving deformation in the system.

Structural development and tectonostratigraphic evolution of the Sylhet Trough (northeastern Bengal Basin) in the context of Cenozoic Himalayan Orogeny: Insights from geophysical data interpretation

The Sylhet Trough of the Bengal Basin in the northeastern Indian Plate had started to develop since the Late Mesozoic Gondwana break-up and subsequent onset of the Cenozoic Himalayan Orogeny. However, very little is known about evolution of the Cenozoic Sylhet Trough in response to the latter tectonic event. Therefore, we aim at providing insights into the nature of basement, variations in structural style, kinematics and possible timing of structural activation(s) and tectonostratigraphic evolution of the Sylhet Trough based on geophysical data interpretation. Results show that the Sylhet Trough is mostly underlain by an attenuated continental or oceanic crust, bounded by a NE-SW-trending geophysical feature in the northwest that corresponds to the boundary with the Precambrian continental crust. The overlying Cenozoic sediments show variations in style of deformations and can be divided into three structural domains. Domains 1 and 2 in the southeastern and northeastern parts of the Sylhet Trough, respectively, are characterized by anticlines that are highly deformed to the east and are mostly associated with west- or northwest-dipping reverse faults. However, the anticlinal trends are deviated from a generalized NS-orientation in the Domain 1 to a dominant NE-SW-orientation in the Domain 2. Many of these anticlines resemble positive flower structures, indicating possible impacts of regional transpression associated with the Dauki and Kaladan faults to the north and east; respectively. In contrast, the Domain 3 to the west shows NE-SW-trending extensional structures. A southward thickening of the Oligocene to Miocene pre-kinematic packages indicates an early episode of foreland basin subsidence, although there is no evidence of a major structural activation. Rather, the presence of the Pliocene syn-kinematic packages indicates a major structural reactivation in response to the E-W and N-S crustal shortening since the Pliocene. This younger episode of structural activation can be directly linked with the late Cenozoic subduction related processes of the Indian Plate to the north and to the east, which is also responsible for the transformation of the Sylhet Trough into a remnant ocean basin. This latter (i.e., Pliocene and onward) episode of crustal shortening played a significant role in structural trap formation and preservation of hydrocarbons in the Sylhet Trough.

STRUCTURE AND EVOLUTION OF THE TEREK‐CASPIAN FOLD‐AND‐THRUST BELT: NEW INSIGHTS FROM REGIONAL SEISMIC DATA

The Terek‐Caspian fold‐and‐thrust belt along the northern flank of the Greater Caucasus Mountain Range together with the adjacent foreland basin is one of the oldest oil‐producing regions in Russia. Despite the long history of exploration, recently acquired seismic data has provided new insights about the structural architecture and evolution of this area. Its structural development during the Neogene was constrained by a syn‐extensional tectonic fabric inherited from Jurassic rifting and extension of the Great Caucasus Basin. The structural framework of this basin controlled the distribution of syn‐extensional deposits, and the Cenozoic reactivation of lateral ramps resulted in along‐strike variations in structural style. Thus western, central and SE segments of the Terek‐Caspian foldbelt are recognised and are referred to here as the Terek‐Sunzha fold zone, the Dagestan Promontory, and the Maritime Zone in southern Dagestan.Three principal episodes of Cenozoic compression in the Terek‐Caspian fold‐and‐thrust belt took place. The first episode in the Oligocene resulted in the inversion of pre‐existing normal faults with the coeval development of a foreland basin to the north of the thrust belt. The dominance of sediments of northerly provenance in the foreland basin suggests there was only moderate uplift of the Greater Caucasus at this time. However, significant uplift of the orogenic belt took place during later phases of Sarmatian (Late Miocene) and Akchagylian (Late Pliocene) compression. Erosion of the uplifting Greater Caucasus gave rise to the development of large‐scale, northerly prograding clinoforms which are clearly observed on seismic profiles in the foreland basin. Shortening was largely accommodated by wedge‐shaped thrusting facilitated by the presence of mechanically weak stratigraphic units.Structural development of the Terek‐Sunzha fold zone in the west of the Terek‐Caspian fold‐and‐thrust belt was largely controlled by a Tithonian salt layer which provided an efficient basal detachment surface and which also supplied material to squeezed diapirs in front of the belt. To the east, the plan‐view shape and internal architecture of the Dagestan Promontory were influenced by the areal extent of the Lower‐Middle Jurassic depocentre of the palaeo‐Volga delta which is up to 10 km thick. In the Maritime Zone, the style of shortening was mostly controlled by the presence of a pre‐existing structural high composed of folded Palaeozoic‐Triassic strata in front of the fold‐thrust belt.

Contractional salt tectonics and role of pre-existing diapiric structures in the Southern Pyrenean foreland fold–thrust belt (Montsec and Serres Marginals)

Triassic Keuper evaporites have long been recognized as the main detachment level for thrusting in the Pyrenean fold–thrust belts. The deformed Late Cretaceous–Eocene foreland basin of the Southern Pyrenees has structures and stratal geometries that can be interpreted as related to salt tectonics (e.g. unconformities, rapid thickness variations, long-lived growth fans and overturned flaps), although they have been overprinted by shortening and thrusting. Based on field observations and published maps, we build new structural cross-sections reinterpreting two classic transects of the Southern Pyrenees (Noguera Ribagorçana and Noguera Pallaresa river transects). The sequential restoration of the sections explores the variations in structural style, addressing the role of halokinesis in the tectonic and sedimentary development. In the Serres Marginals area, we propose that salt pillows and diapirs started developing locally during the Mesozoic pre-orogenic episode, evolving into a system of salt ridges and intervening synclines filled with early synorogenic sediments. Rapid amplification of folds recorded by widespread latest Cretaceous–Paleocene growth strata is taken as marking the onset of contractional folding in the area. During Pyrenean compression, folding mechanisms transitioned from dominantly halokinetic to a combination of buckling and differential sedimentary loading. Squeezing of salt diapirs and thrust welding occurred as salt ridges were unroofed. We provide new field observations that lead to a reinterpretation of the regional structural development and contribute to the debate about the role of salt tectonics in the Pyrenees. Supplementary material: Table S1, giving the thickness of the main stratigraphic units, is available at https://doi.org/10.6084/m9.figshare.c.5287737

Secondary detachments within carbonates of the Saraburi Group, Triassic Khao Khwang fold and Thrust Belt, Thailand

Permian limestones of the Saraburi Group in Central Thailand, deformed during the Triassic Indosinian orogeny, exhibit detachments formed in response to the following variations in lithology: thin shale beds within a limestone sequence, massive limestone overlaying well-bedded limestone, massive limestone overlying interbedded limestones and shales, within well-bedded limestones, thin shales interbedded with limestones overlying limestones. Where shales are present the decoupling of layers is similar to examples previously described in the literature from outcrop and analogue models. However, more unusually, detachments are also present within limestone units. While detachment folds are typically developed above a basal weak unit (usually salt or shale), in the study area is an example developed entirely within well-bedded limestones. Instead of flow of the weak unit into the core of the fold, room problems in the core of the fold are accommodated by pressure solution parallel to bedding, thrusts, small fault propagation folds, and intense folding. Variations in structural style are largely influenced by the spacing/intensity of bedding, limestone clay content (and the efficiency of pressure solution during deformation), and the presence/absence and thickness of interbedded shales. A ductile detachment zone is developed between well-bedded limestones and overlying massive limestones, the detachment zone separates highly folded sequence dominated by steep dips, from a weakly folded sequence dominated by gentle dips. Pressure solution processes, and variability in pressure solution intensity (related to clay content) play a very strong role in accommodating the difference in deformation styles across the detachment.

Structure and kinematics of an extensional growth fold, Hadahid Fault System, Suez Rift, Egypt

Abstract. Normal faulting drives extensional growth folding of the Earth's upper crust during continental extension, yet we know little of how fold geometry relates to the structural segmentation of the underlying fault. We use field data from the Hadahid Fault System, Suez Rift, Egypt, to investigate the geometry and kinematics of a large (30 km long, up to 2.5 km displacement), exceptionally well-exposed normal fault system and to test and develop models for extensional growth folding. The Hadahid Fault System comprises eight up to 5 km long segments that are defined by unbreached or breached monoclines. These segments are soft-linked, hard-linked, or defined by a more subtle along-strike transition in overall structural style. High overlap : separation (O:S) ratios between its segments suggest the Hadahid Fault System comprises a single, now hard-linked structure at depth. We demonstrate that a progressive loss of at-surface displacement along-strike of the Hadahid Fault System results in surface-breaking faults and breached monoclines being replaced by unbreached monoclines developed above blind faults. However, shorter along-strike length-scale variations in structural style also occur, with unbreached monoclines developed between breached monoclines. The origin of this variability is unclear, but it might reflect local variations in host rock material properties that drive short length-scale variations in fault propagation-to-slip ratio, and thus the timing and location of fold breaching. We show that folding is a key expression of the strain that accumulates in areas of continental extension, arguing that tectono-sedimentary models for rift development should capture the related structural complexity.

Inhomogeneous thinning of a cratonic lithospheric keel by tectonic extension: The Early Cretaceous Jiaodong Peninsula–Liaodong Peninsula extensional provinces, eastern North China craton

Abstract The mechanisms of lithospheric thinning and craton destruction have been hotly debated in the last decades. The Early Cretaceous Jiaodong and Liaodong extensional provinces (JEP and LEP, respectively) of the eastern North China craton are typical areas where the cratonic Archean lithosphere has been intensely extended and thinned. Various extensional structures, e.g., metamorphic core complexes (MCCs), low-angle detachment faults, and extensional basins, characterize the Early Cretaceous crustal deformation of the two provinces. However, profound differences exist in structural development and related magmatic activities between the two provinces. Distributed small-scale extensional basins were formed in association with exhumation of the Liaonan and Wanfu MCCs in the LEP, whereas the major Jiaolai Basin was developed coevally with exhumation of the Wulian, Queshan, and Linglong MCCs in the JEP. Sr-Nd isotope compositions of volcanic rocks from the basins of the two provinces are compatible with syntectonic magmatic activities of evolving magma sources in the LEP, but multiple and hybrid magma sources in the JEP. It is shown, from variations in structural styles, plutonic and volcanic activities, and thermal evolution of the two extensional provinces, that two stages (ca. 135–120 Ma and 120–100 Ma) of tectonic extension affected the JEP and LEP in the Early Cretaceous. We demonstrate that regional tectonic extension (parallel extension tectonics, or PET) is responsible for the formation of major extensional structures and the occurrence of the magmatic associations. Progressive wide rifting by coupled crust-mantle detachment faulting of a hot LEP lithosphere was accompanied by evolving magma sources from dominant ancient crust and enriched mantle to juvenile crust. Two stages of narrow rifting of a cold JEP lithosphere led to early crustal detachment faulting transitioning to late crust-mantle faulting, which resulted in intense magmatic activity from hybrid to multiple magma sources. These processes contributed to destruction of the craton, with thinning of its lithospheric keel and local transformation of the nature of the lithospheric mantle. It is expected that such a model is also applicable to interpretation of tectonic extension of contiguous areas of the North China craton and the remobilization of other cratons.

Numerical modelling of inversion tectonics in fold-and-thrust belts

This work presents numerical experiments of inversion of rift basins and consequent sub-thrust imbrication in tectonic wedges. Half-graben basins initially develop and then are covered with a post-rift sequence bearing a décollement-prone horizon (i.e., the upper décollement). A total of twelve models of tectonic inversion have been conducted varying (i) the strength of inherited extensional fault arrays and (ii) applying different fluid pressure ratios (i.e., strength) within syn-rift strata. Combinations of those were simulated using different internal angles of friction for the inherited faults, different strengths for the syn-rift infill and for the upper décollement. Results show that changes in relative strength between inherited faults, syn-rift deposits and the upper crustal décollement leads to important variations in structural styles. Weak faults systematically favour the compressional reactivation of inherited extensional faults. Weak syn-rift sediments favour hanging wall by-pass structures instead of fault reactivation and less internal deformation of the syn-rift deposits. Weak upper décollements supports the accretion of basement in a hinterland antiformal stack, decoupling of basement and cover, and forward tectonic transport of rift basins. Strong upper crustal décollements favours basement and cover coupling, can lead to fault reactivation in the absence of weak faults and syn-rift sediments, however combinations of weak faults and strong upper décollement shows fault reactivation, weak syn-rift sediments and strong upper décollement form hanging wall by-pass structures. Modelling results are compared to natural case studies.

Wide-azimuth seismic and potential field data integration for salt recognition in the southern Gulf of Mexico

In the recently opened exploration frontier area of the deepwater Saline Basin in the southern Gulf of Mexico, we successfully applied a multiphysics workflow to integrate wide-azimuth seismic and potential field data. In areas of poor signal-to-noise ratio, subtlety and variation in structural style due to salt kinematics and deep-seated thrusting impose challenges for seismic imaging. High-velocity (impedance) contrasts at the interfaces between salt and carbonate rafts within the background sediments result in strong reflection and refraction at small angles of incidence, diffractions with complex wave patterns, and lack of illumination near and below these features. This creates high noise levels in the seismic image resulting in high uncertainty in the salt interpretation. To reduce the uncertainties of salt interpretation, a multiphysics workflow is devised to integrate gravity data with the seismic measurements. The multiphysics workflow employs a jointly collected suite of wide-azimuth, long-offset, broadband seismic data and potential field data. The process consists of conventional processing steps to enhance the salt-related signal and a full 3D seismogravity modeling. Velocity model building, petrophysical link estimation for domain conversion, and gravity forward modeling are hence used to interactively assess the scenario testing. The geophysical integration enables definition of a better constrained earth model of velocity and density distributions with improved geometries and reduced uncertainties.

Contemporary stress and neotectonics in the Otway Basin, southeastern Australia

AbstractGeomechanical and geological datasets from fold–thrust belts and passive margins that have been subject to neotectonic activity often provide contradictory evidence for the state of contemporary stress. Southeastern Australia has relatively high levels of neotectonic activity for a so-called stable continental region. In the eastern Otway Basin, this neotectonic activity consists of compressional deformation and uplift, indicating a reverse fault stress regime. However, this is inconsistent with the stress magnitudes estimated from petroleum exploration data, which indicate normal or strike-slip fault stress regimes. A new wellbore failure analysis of 12 wells indicates that the maximum horizontal stress azimuth in this basin is c. 135° N, consistent with neotectonic structural trends. Our results indicate that the lithology and variations in structural style with depth exert important controls on horizontal stress magnitudes. The observed partitioning of stress regimes and deformation styles with depth within the basin may reflect the contrasting mechanical properties of the basin-fill. There is an overall increase in the minimum horizontal stress gradient of c. 1–2 MPa km−1 from west to east, corresponding to a change in structural style across the basin. In the central Otway Basin, rift-related faults strike near-parallel to the maximum horizontal stress azimuth and there are comparatively low levels of neotectonic activity, whereas in the eastern Otway Basin, where rift-related faults strike near-orthogonal to the maximum horizontal stress azimuth, the level of neotectonic faulting and uplift is much higher. Our results show that the integration of structural geology with geomechanical datasets can lead to improved interpretations of contemporary stresses, consistent with neotectonic observations.

Basement composition and basin geometry controls on upper-crustal deformation in the Southern Central Andes (30–36°S)

AbstractDeformation and uplift in the Andes are a result of the subduction of the Nazca plate below South America. The deformation shows variations in structural style and shortening along and across the strike of the orogen, as a result of the dynamics of the subduction system and the features of the upper plate. In this work, we analyse the development of thin-skinned and thick-skinned fold and thrust belts in the Southern Central Andes (30–36°S). The pre-Andean history of the area determined the formation of different basement domains with distinct lithological compositions, as a result of terrane accretions during Palaeozoic time, the development of a widespread Permo-Triassic magmatic province and long-lasting arc activity. Basin development during Palaeozoic and Mesozoic times produced thick sedimentary successions in different parts of the study area. Based on estimations of strength for the different basement and sedimentary rocks, calculated using geophysical estimates of rock physical properties, we propose that the contrast in strength between basement and cover is the main control on structural style (thin- v. thick-skinned) and across-strike localization of shortening in the study area.

Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen, Australia

The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east–west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east–west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation during the Early Permian. This model differs to previously proposed reconstructions where curvature largely formed by orogen parallel strike-slip transportation during the Hunter Bowen Event.

Lateral variation in structural style along an evaporite-influenced rift fault system in the Halten Terrace, Norway: Influence of basement structure and evaporite facies

The Halten Terrace is underlain by a Triassic evaporitic package, resulting in vertically decoupled normal fault systems following subsequent extension. Four structural domains are identified along the eastern margin of the Halten Terrace, characterised by: i) thick-skinned normal faults affecting both sub-salt and supra-salt cover, ii) basement-restricted normal faults associated with fault-propagation folds, iii) thick-skinned, distributed normal faults, and iv) thick-skinned, localised normal faults. A fault domain boundary associated with an NE–SW striking basement fault corresponds to an abrupt change in style in the north of the Halten Terrace. Summed throw and estimated strain measurements show that throw and strain accommodated by the fault system increase southward, corresponding to a transition from distributed to localised faulting. The evaporite package is variable in thickness, but those variations do not correspond spatially to variations in structural style. Wells that penetrate the evaporite package, and volume attribute analysis of 3D seismic data, suggest variable evaporite facies. A change in seismic attributes from high-amplitude, low variance to low-amplitude, high variance corresponds to a change from decoupled to thick-skinned faulting. The sub-evaporite fault template, amount of strain accommodated across the fault system, and facies variations in the evaporite package are key influences on structural style.

Controls on structural styles along the deformation front of the Subandean zone of southern Bolivia

The Subandean zone of southern Bolivia is a typical thin-skinned fold and thrust belt with remarkable regularity in the geometry of the structures. However, when the structural geometry and evolution are analyzed in detail, it is verified that there are many deviations from such regularity. In this paper, special emphasis has been placed on analyzing those processes that could explain the along strike variations in structural styles of the deformation front.Particularly, the role that played the upper detachment level in the development of the different structural styles observed along the deformation front is analyzed herein. This analysis is focused on the development of overpressures, which may have been essential for the activation of the detachment level in the Devonian shales of the Los Monos Formation. To do this, we made a series of 1D petroleum system models at different locations along the deformation front. This analysis allowed to model primary gas generation and secondary cracking — processes that are related to overpressure occurrence, and therefore, to the efficiency of the upper detachment level. The models suggest a close relationship between thermal evolution of the foreland basin and generation of gas, with different structural styles observed at the deformation front. Thus, it was possible to divide the deformation front of the southern Subandean zone into two distinct segments. In the northern segment, where models suggest that gas generation and overpressures of the Los Monos Formation would be well developed, the existence of composite roof duplexes is verified. On the other hand, where the models suggest that generation of gas in the Los Monos Formation would be limited, the upper detachment level is not involved in the deformation, and the existence of fault bend fold structures is verified.

Structural analyses of Şaphane relay ramps and fault linkage evolution in active extensional regime, western Turkey

The Şaphane relay ramps (SAR-I and SAR-II) are well-developed structures formed by extensional tectonic settings in western Turkey. Their formation is controlled by the configuration of 2 different breaching faults located in between and the overlapping area of 3 normal faults, which are the Şaphane, Gürlek, and Yumrutaş faults. The relay ramps form within a ~3 km-wide and ~12 km-long interaction zone between 045° and 060° trending faults on the northern boundary of the Erdoğmuş-Yenigediz graben. Some variations in structural styles and its products (for example, overlapping slip-lines (slickenlines), fractures, and antithetic-synthetic faults) are observed along the breaching faults in the relay ramps that were probably created during the formation of the relay structures. In this research, field data from these segmented normal faults having displacement and an interaction area are presented. This normal faulting that resulted from the recent extensional tectonic regime is related to the whole crustal deformation in western Turkey, and the progressive stages have created such a characteristic structure, the relay ramp.

Lateral variation in structural style of mountain building: controls of rheological and rift inheritance

AbstractMany mountain belts exhibit significant along‐strike variation in structural style with changes in the width of the orogen, the geometry and kinematics of the crustal‐scale thrust system, and the degree of partitioning between pro‐ and retro‐wedge deformations. Although the main factors controlling first‐order structural style are understood, the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenees, characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part. Similarly, the prior Mesozoic rifting event was characterized by significant lateral variation in structure. We integrate available geological and geophysical data with forward lithospheric scale numerical models. We show that lateral variation in crustal strength attributed to inherited Variscan crustal composition accentuated during Mesozoic rifting explains the variation in structural style observed during Pyrenean mountain building.

The influence of Mesozoic Palaeogeography on the variations in structural style along the front of the Albanide thrust-and-fold belt

The front of the W-verging Albanide fold-and-thrust belt and its adjacent foreland have been investigated using a grid of seismic reflection profiles, purposely acquired in the Southern Adriatic Sea.This convergent margin is part of the continental collision that extends from former Yugoslavia to North-Western Greece. At the front of the Albanides, both the structural style of the external part of the fold-and-thrust belt, and the evolution of the related foredeep basin, are strongly controlled by the nature of the Mesozoic units that are progressively accreted to the belt, namely the Apulian Platform and its adjacent deep-water basins. Where a thick carbonate platform is accreted (souther Albania) the frontal mountain range presents a high topography, whereas the foredeep basin is relatively shallow. On the other hand, where the basinal domain is accreted (northern Albania), the topography of the fold-and-thrust belt is subdue, whereas the foredeep basin is very deep.

Structural geometries and magnitude of shortening in the eastern Kura fold‐thrust belt, Azerbaijan: Implications for the development of the Greater Caucasus Mountains

AbstractThe Greater Caucasus are the northernmost extent of the Arabia‐Eurasia collision and are thought to represent the main locus of shortening within the central portion of the collision zone between 40° and 48°E. Recent work suggests that in detail, since the Plio‐Pleistocene, much of the shortening in the eastern portion of the Caucasus system has been focused within the Kura fold‐thrust belt along the southeastern margin of the Greater Caucasus. Here we present new field mapping and stratigraphic investigations of the eastern termination of the Kura fold‐thrust belt in Azerbaijan to better constrain the structural geometries, magnitude of shortening, and initiation age for this portion of the fold‐thrust belt. Our work suggests that this area of the fold‐thrust belt exhibits significant along‐strike variations in structural style and evolution and can effectively be divided into two distinct domains at ~48°E. The western domain is characterized by a subcritical median surface slope and isolated folds and thrusts propagating out of sequence, whereas the eastern domain is dominated by a single duplex structure and a history of in‐sequence development in a critically tapered wedge. We hypothesize that these variations result from changes in relative rates of syn‐tectonic sedimentation, erosion, and convergence velocity along strike. We find that within the western domain, the fold‐thrust belt has accommodated ~12 km of total shortening. An unconformity within the western domain brackets the initiation age of this portion of the fold‐thrust belt to between 1.8 and 0.88 Ma yielding permissible average shortening rates of between 6.7 and 13.6 mm/yr. Comparison of these average shortening rates to the geodetically measured shortening rate of 8 mm/yr indicates that since initiation, the fold‐thrust belt has accommodated 83–100% of convergence between the Greater and Lesser Caucasus at this longitude.

Antarctic/Scotia plate convergence off southernmost Chile

The southern tip of South America off Chile has suffered a long phase of ocean-continent convergence which has shaped the continental margin through different phases of accretion and tectonic erosion. The present accretionary wedge is a discontinuous geological record of plate convergence and records only part of the accretionary processes resumed after Chile ridge consumption (14 Ma). The structural style of the subduction complex, such as rates of sediment accretion and tectonic erosion, structural vergence, width of the accretionary wedge, taper angle and deformation in the forearc basin, varies along the margin. Large taper values are related to narrow wedges and seaward vergent structures. Low tapers occur where deformation at the toe of the accretionary complex is spread over wide areas and is related both to landward and seaward vergent thrust faults. Seismic data interpretation contributes to define more accurately frontal wedge morphology and geometry of subduction and suggests that different modes of accretion together with tectonic erosion may be active concurrently along the trench at different locations. In areas of subduction driven accretionary processes the majority of trench sediments are involved in accretionary processes and sediments are uplifted and piled up in the form of imbricate thrust sheets. In areas where the wedge is non-accretionary the continental margin shows steeper continental slopes associated with narrow accretionary wedges, more intense sediment disruption and very shallow decollement levels. Variation in structural style and in the geometry of the forearc region setting off Southernmost Chile, has been interpreted as related to the existence of different structural domains: the nature of their boundaries is still unclear mainly for the lack of high resolution bathymetric data. They have been tentatively related to tectonic lineaments belonging to the Magellan Fault system and/or to the character and morphology of the converging plates (lateral heterogeneities, sea-mounts and fracture zones), which produce a segmentation of the margin.