Basement Normal Faults Research Articles

Potential field survey of subsurface structures of the NW segment of the Zagros Fold‐Thrust Belt, Kurdistan Region

AbstractThis study reports results of gravity and magnetic surveys conducted for the first time in the western segment of the Zagros Fold‐and‐Thrust Belt in the Kurdistan Region. This study attempts to delineate deep structures in an area, which has not been surveyed before. CG‐5 Autograv gravimeter and G‐857 portable proton‐precession magnetometer were used to acquire gravity and magnetic data from 750 stations along over eleven traverses across and parallel to the Zagros trend (NW–SE). Six of these traverses are parallel to the Zagros trend, whereas the others are perpendicular to the trend of the other traverses and can be tied where they intersect. The total length of the traverses is about 1000 km. Tilt Angle of horizontal gradient method is used to detect regional gravity and magnetic lineaments. The mapped lineaments from regional gravity and magnetic surveys are divided into two categories: the NE–SW lineaments, which represent transversal faults in the study area, and the NW–SE lineaments, which represent the Zagros Thrust Faults, some of which may be linked to the inverted basement normal faults of Arabian passive margin (the NW–SE Najd Fault system). The results show that there is a relationship between the regional gravity and magnetic lineaments outlining the same deep geological features. The data presented here confirm the presence of regional longitudinal and transversal lineaments documented in other studies (e.g. Anah‐Qalat Dizeh Fault, Surdash‐Tikrit Fault, Sirwan Fault, Khanaqin Fault, Zagros Mountain Front Fault, Baranan Back Thrust Fault and High Zagros Reverse Fault) and outlines new lineaments not mapped before. Most of the detected regional lineaments in the current study coincide with the previously confirmed lineaments, which have played a significant role in the tectonic evolution of the Zagros Fold‐and‐Thrust Belt. As such, this study contributes to a better understanding of the subsurface structure of the Kurdistan segment of the Zagros Fold‐and‐Thrust Belt and probably the rest of the belt.

Reconstructing the Iberian Salt‐Bearing Rifted Margin of the Southern Pyrenees: Insights From the Organyà Basin

AbstractReinterpretation of the Organyà Basin, based on new detailed field observations and subsurface data, emphasizes the key contribution of Upper Triassic evaporites in the tectono‐sedimentary evolution of the South‐Central Pyrenees. Results are integrated in a 65‐km long restored cross‐section through the Serres Marginals, Montsec and eastern Organyà salt‐related depocenters. The reconstructed part of the Jurassic–Cretaceous northern Iberian salt‐rich rifted margin shows a template characterized by inherited Permo‐Triassic basement normal faults and an initial salt thickness of 0.7 km to the south and 1.5 km to the north. The Organyà Basin is part of the South Pyrenean Diapiric Province, a large system of salt related depocenters and minibasins, that is limited to the north by the more than 120‐km long Senterada salt wall complex separating the supra‐salt and sub‐salt domains in the Southern Pyrenees. Three main stages of diapiric activity are recognized along the northern Iberian margin from Asturias to the Eastern Pyrenees: a Jurassic early salt mobilization; a latest Jurassic–middle Albian main diapiric evolution associated with rifting; and a Campanian–Miocene diapiric reactivation during basin inversion that produced salt welds and thrust welds and translated the salt province some 60 km to the south.

Neogene-Quaternary evolution of the La Baja Guajira basin, Colombia: Burial and thermal history with implications on petroleum systems

The La Baja Guajira Basin is South America's northernmost part that extends from onshore Colombia into the Caribbean Sea and represents an important gas-producing region in Colombia. The dataset used in this study consists of twenty-four (24) regional 2D seismic-reflection lines integrated with information from nine (9) boreholes across the basin. We present seismic interpretations of key stratigraphic and structural features constrained by 1D basin modeling. This allows us to describe the Neogene-Quaternary evolution of the basin, and understand the possible implications for the hydrocarbon potential.We have interpreted eight (8) seismic units within the La Baja Guajira Basin (acoustic basement to U7) affected by three fault groups: 1) NW-SE-striking basement normal faults, 2) inverted normal faults, and 3) strike-slip faults. Considering the evidence, we propose four tectonic phases along the basin's evolution. The basement was affected by normal faulting as the result of extensional tectonics prior to the deposition of the sedimentary infill (Paleogene?). Another extensional interval occurred during the Early Miocene, triggering a moderate accommodation within the proximal area. Subsequently, extension migrated westwards resulting in the formation of the main La Baja Guajira Basin depocenter. In the Middle Miocene (post-extension), sedimentation was controlled by exhumation pulses in adjacent areas to the basin, which resulted in significant sediment input contemporaneous with rapid tectonic subsidence. Yet, local inversion of NW-SE-oriented basement-rooted faults affected today's offshore domain until the Late Miocene. This inversion can be associated with regional compressive deformation linked to the activity of the Oca and Cuiza faults. A second episode of rapid tectonic subsidence happened in the Early Pliocene. In the south of the basin, rocks were subjected to maximum burial and highest temperatures in recent times, conditions that might lead to hydrocarbon generation. The high Neogene to recent sedimentation rates calculated for the La Baja Guajira Basin support microbial gas generation; structural reactivation in this time supported trap formation, particularly in the offshore parts of the basin.

Crustal configuration of the Naama and El Bayadh region of northwest Algeria: Inferences from gravity and magnetic analysis

Northwestern Algeria is located where the Saharan Atlas borders the Saharan Platform along the South Atlas Front. The origin of the South Atlas Front is controversial, thus a detailed gravity and magnetic analysis constrained by seismic reflection profiles and well data were conducted to determine the structural configuration of this region. A residual gravity anomaly map created using upward continuation and a reduced to the pole magnetic anomaly map indicated a series of northwest-trending maxima anomalies parallel to the Atlassic orogeny folds and faults and east-trending maxima within the Benoud foreland basin. These maxima mostly coincide with Paleozoic basement uplifts based on seismic reflection profiles. Depth analyses based on upward continuation, and two-dimensional forward modeling of the gravity and magnetic data indicated that the source of the maxima are mainly 5 km in depth with the magnetic sources being approximately 0.5 km deeper than the gravity sources. The gravity and magnetic models indicate that the source bodies are steep-sided and coincide with interpreted faults from the seismic reflection profiles. The maxima anomalies are mainly caused by basement uplifts instead of variations in density and/or magnetic susceptibility in the Paleozoic or older basement lithologies. The South Atlas Front and the Saharan Atlas on the central and eastern portions of Algeria is governed by a thin-skinned tectonics style involving the Mesozoic-Cenozoic sedimentary cover. While the western part was probably controlled by a mixed thin- and thick-skinned tectonic style with the reactivation of deeply rooted Triassic and Jurassic faults during Tertiary compressive events in the interpreted Paleozoic basement.Our gravity and magnetic models illustrate a crustal architecture model of the Paleozoic basement which is consistent with a north-dipping basement normal fault between less deformed Saharan Platform than the more deformed Atlassic domain. Such a crustal model aids in determining the nature of the Atlassic orogeny as geological and geophysical studies have been determining the variations of structural styles along its entire length.

Evaluating the role of stratigraphy and inherited basement structures on the evolution of thick and thin-skinned related folds: Insights from the Salar de Atacama Basin in the Central Andes

We investigated the role of pre-existing basement normal faults, and ductile layers present within post-rift sequences on the final geometry and kinematics of hybrid (thick and thin-skinned) fold and thrust belts that result of tectonic inversion, using as an example the northern region of the Salar de Atacama Basin in the Central Andes. The study was developed integrating field data consisting of regional and local observations and geological mapping, and the seismic and structural interpretation of 2-D industrial seismic profiles to understand the current structure of the basin. A regional-scale balanced cross-section was constructed and restored to validate our interpretations and determine the pre-shortening tectonic configuration of the basin. Our results suggest that low angle normal faults result in favorable structures for the development of thick-skinned thrust ramps from their full tectonic inversion during the growth of fold and thrust belts. In the Salar de Atacama Basin, these structures allowed for the uplift and exhumation of large Paleozoic stratified and crystalline rock on its western section. The existence of ductile strata within post-rift sequences commonly facilitates the transition from thick to thin-skinned structural style during the progressive tectonic inversion of pre-existing half-graben structures. This mechanism allowed for the creation of prominent east-verging basement cored anticlines and doubly verging and narrow, shallow thin-skinned folds and thrusts at the northernmost part of the Salar de Atacama Basin. Pieces of evidence from the basin suggest that the initial areal extension (length) of the post-rift ductile strata exert a fundamental role in the lateral extension of thin-skinned thrusts. Finally, the existence or not of important ductile strata in the overburden stratigraphy conditions the progression of hybrid fold and thrust belts after pre-existing normal faults are reactivated.

Seismo‐Tectonic Model for the Southern Pre‐Rif Border (Northern Morocco): Insights From Morphochronology

AbstractLocated at the southern boundary of the Alpine chain in Morocco, the deformation front of the Southern Rif Mountains is a region of moderate tectonic activity, which makes it a good natural laboratory to understand whether, and how, low compressional strains are located on specific structures. Along the ≈80 km‐long left‐lateral, transpressive and reverse fault zone that runs at the toe of the Pre‐Rif Ridges, an analysis of high‐resolution digital topography provides new geomorphic lines of evidence supporting Quaternary activity along, 20 km‐long fault segments. The fault zone can be divided into the Meknès and the Fès segments, which are constrained at depth by reactivated, NE‐trending basement faults, delimitating paleo‐grabens associated with the Late Triassic‐Jurassic opening of the Atlantic Ocean. For selected sites, we used in situ‐produced36Cl,10Be, and26Al and high‐resolution topography to infer the timing of abandonment of fluvial markers, which suggest incision rates on the order of 0.6–2 mm/yr. Given their lengths, scaling laws suggest that the identified fault segments should root at about 7–12 km‐depth, possibly reactivating former basement normal faults and making them potential seismogenic sources capable of generating Mw6+ earthquakes, with return times of the order of several hundreds of years. Our new morphochronological data set confirms that the Southern Rif deformation front is a key structure that may have accommodated most of the lateral extrusion of the Rif between the Nubia and Iberia tectonic plates.

Four-dimensional analog and geometrical modeling of the Hides anticline, Papua New Guinea: Structure of a giant gas field

The Hides anticline in the Papua New Guinea fold belt is the core part of the Papua New Guinea liquified natural gas project that commenced production in 2014, yet little is known about its internal structure, particularly at depth. The structure of the upper 3 km (9843 ft) is relatively well constrained from surface data, recent development wells, and poor-quality two-dimensional seismic data, but the deep structure has been obscure. Seismic data and gravity modeling suggest an underlying large normal fault in basement. Forward geometrical modeling using MoveTM and four-dimensional analog sandbox modeling under an x-ray tomography scanner have greatly constrained the structural interpretation. The modeling confirms the presence of the underlying basement normal fault and suggests that it was mildly inverted prior to the onset of pure compression deformation in the sedimentary section. The main detachment level was just above basement approximately 8–10 km (∼5–6 mi) beneath the surface, but this ramped up at the basement fault creating an array of triangle-zone faults through the overlying sedimentary section, building the Hides anticline. The early inversion was instrumental in initiating a back thrust and hinterland-verging tight fold in the Mesozoic section, including the source, reservoir, and seal. Deformation in the thick Miocene carbonates at the surface was detached from that in the underlying reservoir along an Upper Cretaceous mudstone horizon that varies strongly in thickness such that the 2–4-km (6562–13,123-ft)-high Mesozoic structure verges northeast and the overlying Miocene structure verges southwest. Analog modeling shows that a ductile stratigraphy is required with slow strain rates of approximately 1–3 km/m.y. (∼3281–9843 ft/m.y.).

Monocline formation during growth of segmented faults in the Taranaki Basin, offshore New Zealand

Precursor monoclines or fault propagation folds are commonly associated with normal faults. Many previous studies have analysed these structures at outcrop or in analogue models but there are few natural examples which have been studied in detail in 3D. Here we present the results of analysis of two monoclines imaged in a 3D seismic reflection survey in the Taranaki Basin, offshore New Zealand. The monoclines formed in response to Miocene-Recent extensional reactivation of underlying basement normal faults of Cretaceous age. The Miocene structures have associated growth strata that allows their geometrical evolution to be established. The two monoclines initiated at the same time (~3.4Ma). The smaller of the two monoclines has only limited associated faulting while the larger structure was cross-cut by upward propagation of the basement fault as a segmented array of normal faults. Variations in fold amplitudes and fault displacements within the growth strata associated with the larger structure record the progressive transition from folding to faulting, with folding limited to the relay zones between fault segments in the later stages of growth. This structure demonstrates spatial and temporal complementary relationships between folding and faulting so that together they define a single kinematically coherent structure. Although the monoclines widen upwards with geometries consistent with the trishear model of forced folding, there is also a positive correlation between monocline amplitude and wavelength measured along the length of the structure which suggests that the wavelengths of these structures increased as they grew.

Analysis of gravity-driven normal faults using a 3D seismic reflection dataset from the present-day shelf-edge break of the Otway Basin, Australia

The growth, interaction and controls of gravity-driven normal faults is somewhat understudied. Using three-dimensional (3D) and two-dimensional (2D) seismic reflection data, located at the present-day shelf-edge break and into the deepwater province of the Otway Basin, southern Australia, we aim to temporally and spatially constrain the development of a normal fault system and determine the controls on growth. The Otway Basin is a Late Jurassic to Cenozoic age, rift-to-passive margin basin. The seismic reflection data images a gravity-driven fault array, consisting of ten fault segments, striking NW-SE (128-308), located within Upper Cretaceous clastic sedimentary rock. We analyse the growth of a gravity-driven hard-linked fault assemblage interacting with basement normal faults. Our analysis shows that the fault assemblage is linked to major basement faults and displays Turonian-Santonian nucleation, continued growth until the latest-Maastrichtian and a maximum throw of 1.74 km. High variability of throw along-strike and down-dip of the fault assemblage indicates growth via lateral and vertical segment linkage. We interpret that the spatial and temporal evolution of the fault assemblage is the result of rifting basement fault control during Upper Cretaceous resumed crustal extension in the Otway Basin. The control of the rifting basement faults on these gravity-driven normal faults has implications towards the growth and petroleum prospectivity of gravity-driven normal faults on passive margins such as the Niger Delta and Gulf of Mexico, but also towards gravity-driven normal faults developed in supra-salt sedimentary rock in rift basins, such as the North Sea and Suez Rift.

Intersection patterns of normal faults in the Lufeng Sag of Pearl River Mouth Basin, China: Insights from 4D physical simulations

Interpretation of seismic data from the Lufeng Sag of the Pearl River Mouth Basin (PRMB) in the northern part of South China Sea shows that different intersection patterns developed in the cover units above basement normal faults. A series of analogue models are used to investigate the intersection patterns and deformation in the sedimentary cover sequences above a basement horst bounded by two non-parallel faults. Modelling results show that during their upward propagation, the basement faults may intersect within the cover sequences and form a graben above the basement horst. Length and width of the graben increase with cover thickness. The strike and dip intersection points are controlled directly by the thickness of the cover sequences, dip and strike of the basement faults, and width of the basement horst. The intersection point migrates along the axis of the graben toward the wide end of the basement horst, when the cover sequence thickens. In contrast, it migrates toward the narrow end of the basement horst, where both fault dip and angle of strike difference increase. The intersection point moves upward with increasing width of the basement horst crest. Model profiles also indicate that in the presence of a ductile layer between the cover and basement such intersection patterns do not form. Interpretation of seismic data and model results show that the intersection pattern developed in the Lufeng Sag is a result of propagation of basement faults into cover units during different extension stages of the basin. Results of this study can be applied to many other sedimentary basins where such fault intersection patterns are likely to form when non-parallel conjugate basement faults are active during sedimentation.

Seismicity on Basement Faults Induced by Simultaneous Fluid Injection–Extraction

Large-scale carbon dioxide (CO2) injection into geological formations increases pore pressure, potentially inducing seismicity on critically stressed faults by reducing the effective normal stress. In addition, poroelastic expansion of the reservoir alters stresses, both within and around the formation, which may trigger earthquakes without direct pore-pressure diffusion. One possible solution to mitigate injection-induced earthquakes is to simultaneously extract pre-existing pore fluids from the target reservoir. To examine the feasibility of the injection–extraction strategy, we compute the spatiotemporal change in Coulomb stress on basement normal faults, including: (1) the change in poroelastic stresses $$\Delta \tau _s+f\Delta \sigma _n$$ , where $$\Delta \tau _s$$ and $$\Delta \sigma _n$$ are changes in shear and normal stress. respectively, and (2) the change in pore-pressure $$f\Delta p$$ . Using the model of (J. Geophys. Res. Solid Earth 99(B2):2601–2618, 1994), we estimate the seismicity rate on basement fault zones. Fluid extraction reduces direct pore-pressure diffusion into conductive faults, generally reducing the risk of induced seismicity. Limited diffusion into/from sealing faults results in negligible pore pressure changes within them. However, fluid extraction can cause enhanced seismicity rates on deep normal faults near the injector as well as shallow normal faults near the producer by poroelastic stressing. Changes in seismicity rate driven by poroelastic response to fluid injection–extraction depends on fault geometry, well operations, and the background stressing rate.

Reconstruction of the Provence Chain evolution, southeastern France

The Provence fold-and-thrust belt forms the eastern limit of the Pyrenean orogenic system in southeastern France. This belt developed during the Late Cretaceous-Eocene Pyrenean-Provence compression and was then deformed by Oligocene-Miocene Ligurian rifting events and Neogene to present-day Alpine compression. In this study, surface structural data, seismic profiles, and crustal-to-lithospheric-scale sequentially balanced cross sections contribute to the understanding of the dynamics of the Provence Chain and its long-term history of deformation. Balanced cross sections show that the thrust system is characterized by various structural styles, including deep-seated basement faults that affect the entire crust, tectonic inversions of Paleozoic-Mesozoic basins, shallower decollements within the sedimentary cover, accommodation zones, and salt tectonics. This study shows the prime control of the structural inheritance over a long period of time on the tectonic evolution of a geological system. This includes mechanical heterogeneities, such as Variscan shear zones, reactivated during Middle Cretaceous Pyrenean rifting between Eurasia and Sardinia. In domains where Mesozoic rifting is well marked, inherited basement normal faults and the thermally weak crust favored the formation of an inner thick-skinned thrust belt during Late Cretaceous-Eocene contraction. Here 155 km (similar to 35%) of shortening was accommodated by inversion of north verging crustal faults, north directed subduction of the Sardinia mantle lithosphere, and ductile thickening of the Provence mantle lithosphere. During the Oligocene, these domains were still predisposed for the localized faulting of the Ligurian basin rifting and the seafloor spreading.

Structure of an inverted basin from subsurface and field data: the Late Jurassic-Early Cretaceous Maestrat Basin (Iberian Chain)

The Maestrat Basin experienced two main rifting events: Late Permian-Late Triassic and Late Jurassic-Early Cretaceous, and was inverted during the Cenozoic Alpine orogeny. During the inversion, an E-W-trending, N-verging fold-and-thrust belt developed along its northern margin, detached in the Triassic evaporites, while southwards it also involved the Variscan basement. A structural study of the transition between these two areas is presented, using 2D seismic profiles, exploration wells and field data, to characterize its evolution during the Mesozoic extension and the Cenozoic contraction. The S-dipping Maestrat basement thrust traverses the Maestrat Basin from E to W; it is the result of the Cenozoic inversion of the lower segment–within the acoustic basement–of the Mesozoic extensional fault system that generated the Salzedella sub-basin. The syn-rift Lower Cretaceous rocks filling the Salzedella sub-basin thicken progressively northwards, from 350m to 1100m. During the inversion, a wide uplifted area –40km wide in the N-S direction– developed in the hanging wall of the Maestrat basement thrust. This uplifted area is limited to the North by the E-W-trending Calders monocline, whose limb is about 13km wide in its central part, dips about 5oN, and generates a vertical tectonic step of 800-1200m. We interpreted the Calders monocline as a fault-bend fold; therefore, a flat-ramp-flat geometry is assumed in depth for the Maestrat basement thrust. The northern synformal hinge of the Calders monocline coincides with the transition from thick-skinned to thin-skinned areas. The vast uplifted area and the low-dip of the monocline suggest a very low-dip for the basement ramp, rooted in the upper crust. The Calders monocline narrows and disappears laterally, in coincidence with the outcrop of the Maestrat basement thrust. The evaporitic Middle Muschelkalk detachment conditioned the structural style. Salt structures are also related to it; they developed during the Late Triassic extension, as deduced from the Keuper seismic reflectors that onlap the folded Upper Muschelkalk and form growth strata above some basement normal faults.

Structural model of the Balkassar area, Potwar Plateau, Pakistan

Balkassar is an important hydrocarbon producing area of the Potwar Plateau, Pakistan. Two-dimensional seismic reflection data of the area revealed tectonically controlled, distinct episodes of (1) normal faulting in the basement followed by (2) reverse faulting in the cover sequence. Himalayan orogeny and associated diapirism of the Precambrian Salt Range Formation have produced many salt-cored anticlines in the Potwar Plateau, and one such salt-cored anticline is present in the Balkassar. This anticline has NE–SW-oriented axis, and both the SE and NW limbs are bounded by reverse faults. The basement normal faults indicate Jurassic rifting and splitting of Pangaea. We interpret reverse faults with dip angles of about 60°–75° in the cover sequence, having both hinterland and foreland vergence. Both NW- and SE-dipping faults are present in contrast to the only southward-directed thrusts of previous models. Duplexes and triangle zones, which are common in the northern part of the Potwar Plateau, are not developed in the Balkassar area due to comparatively less crustal shortening in the area. The present interpretation can help in understanding the complex structures in other parts of the Potwar Plateau for hydrocarbons exploration and also in deformed foreland basins worldwide that display similar characteristics but are considered to be dominated by low-angle thrust tectonics.

Physical analog (centrifuge) model investigation of contrasting structural styles in the Salt Range and Potwar Plateau, northern Pakistan

We use scaled physical analog (centrifuge) modeling to investigate along- and across-strike structural variations in the Salt Range and Potwar Plateau of the Himalayan foreland fold-thrust belt of Pakistan. The models, composed of interlayered plasticine and silicone putty laminae, comprise four mechanical units representing the Neoproterozoic Salt Range Formation (basal detachment), Cambrian–Eocene carapace sequence, and Rawalpindi and Siwalik Groups (Neogene molasse), on a rigid base representing the Indian craton. Pre-cut ramps simulate basement faults with various structural geometries.A pre-existing north-dipping basement normal fault under the model foreland induces a frontal ramp and a prominent fault-bend-fold culmination, simulating the Salt Range. The ramp localizes displacement on a frontal thrust that occurs out-of-sequence with respect to other foreland folds and thrusts. With a frontal basement fault terminating to the east against a right-stepping, east-dipping lateral ramp, deformation propagates further south in the east; strata to the east of the lateral ramp are telescoped in ENE-trending detachment folds, fault-propagation folds and pop-up structures above a thick basal detachment (Salt Range Formation), in contrast to translated but less-deformed strata with E–W-trending Salt-Range structures to the west. The models are consistent with Salt Range–Potwar Plateau structural style contrasts being due to basement fault geometry and variation in detachment thickness.

Frontal compression along the Apennines thrust system: The Emilia 2012 example from seismicity to crustal structure

The evolution of the Apennines thrust-and-fold belt is related to heterogeneous process of subduction and continental delamination that generates extension within the mountain range and compression on the outer front of the Adria lithosphere. While normal faulting earthquakes diffusely occur along the mountain chain, the sparse and poor seismicity in the compressional front does not permit to resolve the ambiguity that still exists about which structure accommodates the few mm/yr of convergence observed by geodetic data. In this study, we illustrate the 2012 Emilia seismic sequence that is the most significant series of moderate-to-large earthquakes developed during the past decades on the compressional front of the Apennines. Accurately located aftershocks, along with P-wave and Vp/Vs tomographic models, clearly reveal the geometry of the thrust system, buried beneath the Quaternary sediments of the Po Valley. The seismic sequence ruptured two distinct adjacent thrust faults, whose different dip, steep or flat, accounts for the development of the arc-like shape of the compressional front. The first shock of May 20 (Mw 6.0) developed on the middle Ferrara thrust that has a southward dip of about 30°. The second shock of May 29 (Mw 5.8) ruptured the Mirandola thrust that we define as a steep dipping (50–60°) pre-existing (Permo-Triassic) basement normal fault inverted during compression. The overall geometry of the fault system is controlled by heterogeneity of the basement inherited from the older extension. We also observe that the rupture directivity during the two main-shocks and the aftershocks concentration correlate with low Poisson ratio volumes, probably indicating that portions of the fault have experienced intense micro-damage.

Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland, across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30km length N–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour External Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dominant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10km (21%) whereas on the easternmost one shows ~5km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback.

Thermal‐mechanical modeling of salt‐based mountain belts with pre‐existing basement faults: Application to the Zagros fold and thrust belt, southwest Iran

Two‐dimensional thermal‐mechanical models of thick‐skinned, salt‐based fold and thrust belts, such as the Zagros, SW Iran, are used to address: (1) the degree of deformation and decoupling between cover and basement rocks due to the presence of a weak salt detachment, (2) the reactivation potential of pre‐existing basement normal faults due to brittle or ductile behavior of the lower crust (as related to cold or hot geothermal gradients), and (3) variations in deformation style and strain distribution. The geometry and kinematics of the orogenic wedge and the activity of pre‐existing basement faults are strongly influenced by the geothermal gradient (defined by the Moho temperature, MT) and basement rheology. We infer that the MT plays a major role in how the lower and upper crust transfer deformation toward the foreland. In relatively hot geotherm models (MT = 600°C at 36 km depth), the lowermost basement deforms in a ductile fashion while the uppermost basement underlying the sedimentary cover deforms by folding, thrusting, and displacements along pre‐existing basement faults. In these models, cover units above the salt detachment occur within a less deformed, wide plateau in the hinterland. In relatively cold geotherm models (MT = 400°C at 36 km depth), deformation is mainly restricted to basement imbricate thrusts that form within the orogenic hinterland. Detachment folding, thrusting, and gravity gliding occur within cover sediments above uplifted basement blocks. Gravity gliding contributes to a larger amount of shortening in the cover compared to the basement.

Cover deformation above steep, basement normal faults: Insights from 2D discrete element modeling

A discrete element model is used to investigate progressive cover deformation above a steep (70°), basement normal fault. The cover materials are homogenous with frictional material behavior. In the model shown here both normal and reverse faults in the cover accommodate displacement on the underlying basement fault. The earliest faults are curved, reverse faults which propagate upwards from the basement fault tip into the proto hanging wall. These are replaced, progressively towards the footwall, by subvertical to steep normal faults and finally by a normal fault which dips at an angle predicted by Mohr-Coulomb theory. Thus, most early, secondary structures are located in the hanging-wall of the final, through-going, fault. This structural evolution produces an asymmetric, triangular zone of deformation above the basement fault tip which superficially resembles that associated with trishear; however, its progressive development is quite different. Results also emphasize that the occurrence of reverse faults in extensional settings is not diagnostic of inversion.

Reelfoot rift and its impact on Quaternary deformation in the central Mississippi River valley

Geophysical and drill-hole data within the Reelfoot rift of Arkansas, Tennessee, Missouri, and Kentucky, USA, were integrated to create a structure contour map and threedimensional computer model of the top of the Precambrian crystalline basement. The basement map and model clearly defi ne the northeast-trending Cambrian Reelfoot rift, which is crosscut by southeast-trending basement faults. The Reelfoot rift consists of two major basins, separated by an intrarift uplift, that are further subdivided into eight subbasins bound by northeast- and southeast-striking rift faults. The rift is bound to the south by the White River fault zone and to the north by the Reelfoot normal fault. The modern Reelfoot thrust fault, responsible for most of the New Madrid seismic zone earthquakes, is interpreted as an inverted basement normal fault. Geologic interpretation of 5077 shallow borings in the central Mississippi River valley enabled the construction of a structure contour map of the Pliocene‐Pleistocene unconformity (top of the Eocene‐base of Mississippi River alluvium) that overlies most of the Reelfoot rift. This map reveals both river erosion and tectonic deformation. Deformation of the Pliocene‐Pleistocene unconformity appears to be controlled by the northeast- and southeast-trending basement faults. The northeast-trending rift faults have undergone and continue to undergo Quaternary dextral transpression. This has resulted in displacement of two major rift blocks and formation of the Lake County uplift, Joiner ridge, and the southern half of Crowley’s Ridge as compressional stepover zones that appear to have originated above basement fault intersections. The Lake County uplift has been tectonically active over the past ~2400 yr and corresponds with a major segment of the New Madrid seismic zone. The aseismic Joiner ridge and the southern portion of Crowley’s Ridge may refl ect earlier uplift, thus indicating Quaternary strain migration within the Reelfoot rift.