<p>Although the origin of outer-arc extension fractures in folded sequences is well-understood and documented in many natural examples, geometric and geomechanical factors controlling their spacing are hitherto unexplored. This study investigates the formation of bending-induced tensile fractures during constant-curvature forced folding using two-dimensional Distinct Element Method (DEM) numerical modelling. The DEM model comprises a central brittle layer embedded within elastic layers; the layer interfaces are cohesionless. Folding of this three-layer system is enforced by a velocity boundary condition at the model base, while a constant overburden pressure is maintained at the model top.</p> <p>The models illustrate several key stages of fracture array development: (i) Prior to the onset of fracture, the neutral surface is located midway between the layer boundaries, consistent with pure bending; (ii) Once the outer-fibre stress equals the tensile strength of the layer, fractures nucleate and propagate through the brittle layer; (iii) The rate of fracture formation as a function of curvature decreases nonlinearly, with new fractures developing approximately midway between two existing fractures; (iv) Eventually no new fractures form, irrespective of any further increase in fold curvature, a state referred to as fracture saturation.</p> <p>On the basis of these numerical model results, an approximate analytical solution for fracture spacing based on classic beam theory is developed. The predicted range of fracture spacing as a function of fold curvature is in good agreement with the numerical model results. Importantly, the analytical solution reveals which geometric and geomechanical factors control fracture spacing, namely layer thickness, radius of curvature, Young’s modulus, tensile strength and confining pressure. The fracture spacing to layer thickness ratio at saturation however depends only (nonlinearly) on the ratio of tensile strength to overburden pressure.</p> <p>The numerical model results are qualitatively compared with field observations at outcrops located in the Montpellier Fold region. The folded lithologies are of Jurassic age and comprise (brittle) limestones, with (ductile) marl intercalations. Fracture-bound limestone blocks located within the fold hinges and observed on the fold-profile plane are laterally bound by V-shaped veins that thin towards the fold core. In the inner-arc of each vein-bound limestone block, the marl interbed thickens towards the veins, whereas in the outer-arc it thins towards the veins. Clearly, the thickness distribution of the marl interbeds reflects non-uniform loading, which is consistent with the loading conditions hypothesised on the basis of the theoretical models.</p>

The spatio-temporal evolution for the frontal part of the Kirthar fold and thrust belt in Pakistan is constrained for the first-time using apatite (U-Th-Sm)/He (AHe) and apatite fission track (AFT) dating of samples from the Oligocene Nari Formation in combination with a published balanced cross-section. The AHe ages appear to be fully reset, allowing us to date the timing of exhumation above ramps. Comparison of partially reset AFT ages (similar to 22-24 Ma) with previously published zircon fission track (ZFT) ages of the Nari Formation from nearby areas suggest that rocks in the frontal zone were not buried deeper than-4 km after deposition. The close range of AFT ages have implications for the timing of hinterland exhumation and Oligocene age of the Nari Formation. The AHe ages suggest that deformation along the major basement ramp was active since similar to 7 Ma, forming a major topographic step (similar to 1.5 km) in the frontal part of the Kirthar fold and thrust belt. Combined analysis of the structural cross-section and thermal modeling of the samples suggest that major cooling occurred between similar to 7 Ma and similar to 5 Ma. This cooling was due to the erosion of Oligocene to Miocene strata from above the samples when Precambrian to Miocene strata was thrust above the pre-existing normal fault that acted as a basement ramp. The temporal structural evolution suggests that deformation in the frontal part of the Kirthar fold and thrust belt was characterized by faster rates (similar to 2.5 km/Ma) of orogenic growth and exhumation between similar to 7 and 5 Ma, followed by slower shortening rates (<1 km/Ma) since-5 Ma.

The frontal Zagros fold belt in the Fars area of Iran is generally considered to be detached on the Hormuz salt. Shortening rates and the proportion of thick-skinned deformation are debated and published balanced sections show large differences in style and amount of deformation. We believe that the main uncertainties are related to stratigraphic thickness variations, the degree of thick-skinned deformation and/or the flexural response of the crust. We analysed the stratigraphic thickness ranges interpreted from seismic data and conclude that the interpretational uncertainties, combined with the uncalibrated interval velocities of the largely undrilled Paleozoic strata, yield a wide range of outcomes for the total Phanerozoic thickness (9.5-16.5 km). With an area balancing approach, we show that geological surface cross sections could be matched by various structural subsurface scenarios, including pure thin-skinned deformation with flexure and/or thick-skinned deformation. Given the input uncertainties, we discuss the merits and shortcomings of our balanced sections and propose a geological model including a subtle control on the deformation style by the inherited pre-salt basin morphology. Given the uncertainties, we conclude that the contribution of thick-skinned deformation to the structural evolution remains indefinite and published shortening values may be much more unconstrained than typically specified.

The petroleum province in Lower Austria resulted from the Alpine collision and the subsequent formation of the Vienna Basin. OMV is active in this area since its foundation in 1956. Several plays have been successfully tested and produced in this complex geological region. The main exploration focus is currently on the deep plays. However, this paper proposes a so far unrecognized and therefore undrilled play in a shallower level to broaden OMV's portfolio in Austria. Seismic re-interpretations of reprocessed 3D seismic data and structural reconstructions were used to review some of the existing plays and get novel ideas from improved understanding of processes. In the frontal accretion zone of the Alpine wedge, the Waschberg-Ždánice zone discoveries are limited to the frontal thrust unit and associated structures. The more internal parts of the thrust belt have only sparsely been drilled and are perceived not to have high-quality reservoir rocks. The detailed structural interpretations indicated that the foredeep axis during the Early Miocene was positioned in the thrust sheet located directly in front of the advancing Alpine wedge (comprising the eroding Rhenodanubian Flysch in its frontal part). Seismic amplitude anomalies can be interpreted to represent Lower Miocene basin floor and slope fans. Nearby wells did not penetrate these fans but drilled instead shale-dominated lithologies. Thus, the presence of potential sand-rich fans in front of the advancing alpine wedge is considered a potential new play in Lower Austria. Analogues are found in Upper Austria some 250 km to the West, where several large gas fields in Lower Miocene deposits located in front of the advancing Alpine wedge have been discovered by another operator. In that area the fans are only partly involved in the fold-thrust belt. In Lower Austria, these fans are located within the rear thrust sheet(s), providing a structural component to a mixed structural-stratigraphic trap. Two potential charge mechanism can be considered: a) biogenic gas charge from the organic matter of surrounding shales (like the Upper Austria analogues) or b) oil charge via the thrust fault planes from the Jurassic Mikulov Formation (the proven main source rock in the broader area). Our results add to the understanding of the Miocene structural-stratigraphic evolution of the Alpine collision zone. The definition of a potential new play may add significant value to OMV's upstream efforts in a very mature hydrocarbon province.

This study focuses on structure and kinematics of the 1979 Mw 7.1 Montenegro earthquake. Although this event represents the strongest instrumentally recorded event in the entire Dinarides-Hellenides fold-and-thrust belt, no tectonic model relating this event to any particular fault existed so far. We combined onshore geological information with well logs, seismic lines, bathymetric data, seismotectonic and seismological data as well as cross-section balancing techniques into a new structural model for the area. Our results suggest that main shock and strongest aftershock (Mw 6.2) occurred on the NE-dipping basal thrust of a largely Palaeogene-age nappe system involving Cretaceous neritic carbonates. Ongoing propagation of this thrust system is documented by the existence of elongated ridges located 15 km offshore. Reflection seismic and bathymetry data reveal that the ridges form crests of actively growing fault-related anticlines. Slip distribution models of the strongest events imply that the basal thrust below the ridges accommodated up to 2.7 m of coseismic displacement. Ongoing shortening along the basal thrust also induced surface uplift of structurally higher thrust imbricates, evidenced by dry valleys incising onshore anticline crests. Combining all evidence, we speculate that the observed structural and geomorphic features resulted from repeated seismogenic faulting events as in 1979.

The southern Fars region of Iran is a classical and very well-studied area of salt tectonics for more than a century. Our study area is located in the “Simply Folded Belt” of the Zagros Mountains, including the nearby offshore of the Persian Gulf, and has a large number of well-known salt diapirs. These diapirs, composed of the infra-Cambrian Hormuz evaporites, have a surface diameter between 2-12 km and may extend vertically beneath the surface down to anywhere between 6-12 km.In outcrop, the most striking aspect of these diapirs is the very large proportion of non-evaporitic rocks embedded within the evaporites. Also, these extraclasts (or megaclasts) are sometimes very large, reaching even the kilometer scale. We interpret their present-day dominance and ubiquitous „crowding“ in the outcropping apex of any given diapir as quite misleading as to their overall compositional contribution to these salt bodies. In our view, their seemingly large proportion in the internal make-up of the diapirs should be attributed to the preferential preservation of non-evaporitic rocks exposed on the surface. We argue that the real proportion of the overall non-evaporitic rocks within a typical Hormuz diapir could be as low as 1-2%, but certainly not more than 10%. Nevertheless, given their typical lithologies composed of crystalline basement, Eocambrian carbonates and sandstones with very high seismic velocities on the order of 5,000-5,500 m/s, the megaclasts may make the „dirty“ salt faster than the typical 4,500 m/s velocity of a typical “clean” rock salt sequence. These distinct crystalline and poorly dated Lower Paleozoic carbonate and clastic rocks found in the diapirs appear to have analogue formations outcropping only very far from the study area, like in Central Iran.Importantly, as reported by others earlier, we have not found any evidence for the presence of post-Hormuz (i.e. post-Cambrian) host-rock lithologies incorporated into the diapiric material. Therefore, the strikingly selective nature of the extraclast lithologies within the diapiric bodies points to their original intra-Hormuz stratigraphic position. During Cenozoic diapirism, these infra-Cambrian Hormuz “stringers”, also including some pre-rift basement lithologies, were selectively incorporated into the ascending evaporite material as megaclasts and were carried to the surface from large depth. Therefore, one of the important conclusions of our study is that the various Hormuz intra-salt lithologic units must have deposited in a broad, wide-rift extensional setting.

The Faroe-Shetland Basin (FSB) belongs to the sedimentary basins that encompass outer parts of the NE Atlantic continental margin. The FSB contains several kilometres thick Cenozoic and Cretaceous strata, which overlie poorly understood pre-Cretaceous strata. Regional high-resolution 3D seismic reflection data used in this study covers the eastern half of the basin, generally devoid of thick basaltic successions. The pre-Cretaceous successions are preserved as erosional remnants of originally more extensive deposits throughout the FSB. Devono-Carboniferous and Permo-Triassic rifting probably took place in the FSB, but the magnitude and regional extent of each phase is less constrained than in the neighbouring inner Hebrides-Shetland rift basins. Major rifting took place in Jurassic times and generated a fault-controlled marine basin. During the Apto-Albian and Cenomanian, distributed extension occurred throughout the FSB, suggesting wide rift mode of extension. The subsequent Campanian - Maastrichtian extensional phase, on the contrary, was localized in the Flett Sub-basin controlled by the Westray and Corona faults along its western margin and oppositely dipping faults along the eastern basin margin. The fault-controlled Cretaceous deposition in the FSB is a distinct feature that is documented in the basins along the mid-Norwegian margin but rarely recorded in other large Cretaceous basins along the NE Atlantic margin. In the FSB, Late Cretaceous faults frequently reactivated earlier structures, suggesting that a structural inheritance played an important role in basin development. The existence of Permo-Triassic evaporites is postulated to explain geometries of Jurassic and Lower Cretaceous strata and fault patterns in the Foula Sub-basin. A partial mantle serpentinization during Early Cretaceous times proposed for other outer basins along the NE Atlantic margin is unlikely to have occurred in the FSB.

The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

Integration of detrital zircon geochronology and three‐dimensional (3D) seismic‐reflection data from the Molasse basin of Austria yields new insight into Oligocene‐early Miocene palaeogeography and patterns of sediment routing within the Alpine foreland of central Europe. Three‐dimensional seismic‐reflection data show a network of deep‐water tributaries and a long‐lived (>8 Ma) foredeep‐axial channel belt that transported Alpine detritus greater than 100 km from west to east. We present 793 new detrital zircon ages from 10 sandstone samples collected from subsurface cores located within the seismically mapped network of deep‐water tributaries and the axial channel belt. Grain age populations correspond with major pre‐Alpine orogenic cycles: the Cadomian (750–530 Ma), the Caledonian (490–380 Ma) and the Variscan (350–250 Ma). Additional age populations correspond with Eocene‐Oligocene Periadriatic magmatism (40–30 Ma) and pre‐Alpine, Precambrian sources (>750 Ma). Although many samples share the same age populations, the abundances of these populations vary significantly. Sediment that entered the deep‐water axial channel belt from the west (Freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable age distributions that include populations of Variscan, Caledonian and Cadomian zircon at modest abundances (15–32% each). Sandstone from a shallow marine unit proximal to the northern basin margin consists of >75% Variscan (350–300 Ma) zircon, which originated from the adjacent Bohemian Massif. Mixing calculations based on the Kolmogorov–Smirnoff statistic suggest that the Alpine fold‐thrust belt south of the foreland was also an important source of detritus to the deep‐water Molasse basin. We interpret evolving detrital zircon age distributions within the axial foredeep to reflect a progressive increase in longitudinal sediment input from the west (Freshwater Molasse) and/or southwest (Inntal fault zone) relative to transverse sediment input from the fold‐thrust belt to the south. We infer that these changes reflect a major reorganization of catchment boundaries and denudation rates in the Alpine Orogen that resulted in the Alpine foreland evolving to dominantly longitudinal sediment dispersal. This change was most notably marked by the development of a submarine canyon during deposition of the Upper Puchkirchen Formation that promoted sediment bypass eastward from Freshwater Molasse depozones to the Molasse basin deep‐water axial channel belt. The integration of 3D seismic‐reflection data with detrital zircon geochronology illustrates sediment dispersal patterns within a continental‐scale orogen, with implications for the relative role of longitudinal vs. transverse sediment delivery in peripheral foreland basins.

The Kirthar Fold Belt is part of the lateral mountain belts in Pakistan linking the Himalayan orogeny with the Makran accretionary wedge. This region is deforming very obliquely, nearly parallel to the regional plate motion vector (Figure 1, Mohadjer et al., 2010). The Chaman fault (Figure 1A), a largescale strike-slip fault, is considered to represent the lithospheric plate boundary (transform fault) in this lateral collision zone (Bannert et al., 1992). East of the plate boundary, a 150-200-km-wide deformation zone is present (Bannert et al., 1992; Szeliga et al., 2009). Due to the highly oblique orientation to the plate vector, strain partitioning is ongoing in this lateral deformation zone (i.e., dividing overall displacement into components of shortening and strike-slip deformation, cf. Szeliga et al., 2009, and references therein). For the frontal part of this deformation belt neither the deep structural architecture nor many aspects of the complex deformation are well understood.

The Vøring and the Faroe-Shetland basins are offshore deep sedimentary basins which are situated on the outer continental margin of the northeast Atlantic Ocean. Both basins are underlain by thinned continental crust whose structure is still debated. In particular the nature of the lower continental crust and the origin of high velocity bodies located at the base of the lower crust are a subject of discussion in recent literature. Regional interpretation of 2D and 3D seismic reflection data, combined with well data, suggest that both basins share several common features: (i) Pre-Cretaceous faults that are distributed across the entire basin width. (ii) Geometries of pre-Jurassic strata reflecting at least two extensional phases. (iii) Three common rift phases, Late Jurassic, Campanian-Maastrichtian and Palaeocene. (iv) Large pre-Cretaceous fault blocks that are buried by several kilometres of Cretaceous and Cenozoic strata. (iii). (v) Latest Cretaceous/Palaeocene inversion. (vi) Occurrence of partial mantle serpentinization during Early Cretaceous times, as proposed by other studies, seems improbable.

The Mountain Front Flexure or Fault (MFF) of the Zagros Mountains separates the foreland or foothills area from the morphological apparent mountain belt. Across this feature the regional elevations of Mesozoic to Neogene stratigraphic horizons substantially rise towards the mountain belt. Thin-skinned and thick-skinned structural styles have been proposed for this rise in other parts of the Zagros region. In our study area, in the Kurdistan Region of Iraq (KRI), we integrated surface and subsurface data and constructed a (semi-) balanced cross-section across the MFF. The section features duplex structures in the deeper subsurface, related to a deeper Palaeozoic and a shallower Triassic decollement horizon. On a smaller scale, layer-parallel shortening and intense deformation is observed in the incompetent lithologies, leading to an incipient disharmonic folding. Restoration of the section reveals a distinct imbalance between shortening in the upper part of the stratigraphic section (approximately 4 km or 16% on top Jurassic level) to the lower part (approximately 20 km or 49% on top Permian level). The imbalance can only be equalised on a regional section if the shortening is transferred from the lower to the higher decollement levels, which is connected to folds and thrusts in the foothills area. Based on observations from the mechanical stratigraphy, geometric relationships in map and cross-section, as well as morphological considerations, we argue that the origin of the MFF in the area of the considered section is related to active roof duplexes rather than basement-involved thrusting.

The Molasse Basin represents the northern foreland basin of the Alps. After decades of exploration, it is considered to be mature in terms of hydrocarbon exploration. However, geological evolution and hydrocarbon potential of its imbricated southernmost part (Molasse fold and thrust belt) are still poorly understood. In this study, structural and petroleum systems models are integrated to explore the hydrocarbon potential of the Perwang imbricates in the western part of the Austrian Molasse Basin.The structural model shows that total tectonic shortening in the modeled north south section is at least 32.3 km (20.1 mi) and provides a realistic input for the petroleum systems model. Formation temperatures show present-day heat flows decreasing toward the south from 60 to 41 mW/m2. Maturity data indicate very low paleoheat flows decreasing southward from 43 to 28 mW/m2. The higher present-day heat flow probably indicates an increase in heat flow during the Pliocene and Pleistocene.Apart from oil generated below the imbricated zone and captured in autochthonous Molasse rocks in the foreland area, oil stains in the Perwang imbricates and oil-source rock correlations argue for a second migration system based on hydrocarbon generation inside the imbricates. This assumption is supported by the models presented in this study. However, the model-derived low transformation ratios (<20%) indicate a charge risk. In addition, the success for future exploration strongly depends on the existence of migration conduits along the thrust planes during charge and on potential traps retaining their integrity during recent basin uplift.