Reactivation Of Normal Faults Research Articles

Normal fault reactivation induced by hydraulic fracturing: Poroelastic effects

Numerous surface-felt earthquakes have been spatiotemporally correlated with hydraulic-fracturing operations. Because large deformations occur close to hydraulic fractures (HFs), any associated fault reactivation and resulting seismicity must be evaluated within the length scale of the fracture stages and based on the precise fault location relative to the simulated rock volumes. To evaluate the changes in Coulomb failure stress (CFS) with injection, we conduct fully coupled poroelastic finite-element simulations using a pore-pressure cohesive zone model for the fracture and fault core in combination with a fault-fracture intersection model. The simulations quantify the dependence of CFS and the fault reactivation potential on the host rock and fault properties, spacing between the fault and the HF, and the fracturing sequence. We find that fracturing in an anisotropic in-situ stress state does not lead to fault tensile opening but rather dominant shear reactivation through a poroelastic stress disturbance over the fault core ahead of the compressed central stabilized zone. In our simulations, poroelastic stress changes significantly affect fault reactivation in all the simulated scenarios of fracturing 50–200 m away from an optimally oriented normal fault. Asymmetric HF growth due to the stress shadowing effect of the adjacent HFs leads to (1) a larger reactivated fault zone following the simultaneous and sequential fracturing of multiple clusters compared with single-cluster fracturing and (2) a larger unstable area ([Formula: see text].1) over the fault core or a higher potential of the fault slip following sequential fracturing compared with simultaneous fracturing. The fault reactivation area is further increased for a fault with lower conductivity and a higher opening-mode fracture toughness of the overlying layer. To reduce the risk of fault reactivation by hydraulic fracturing under the reservoir characteristics of the Barnett Shale, Fort Worth Basin, it is recommended to (1) conduct simultaneous fracturing instead of sequential and (2) maintain a minimum distance of approximately 200 m for HF operations from known faults.

The fault network in the southern foot of Rila Mountains, SW BulgariaThe fault network in the southern foot of Rila Mountains, SW Bulgaria

Southwestern slopes of Rila Mountain and northern parts of the Mesta graben are one of the least studied areas in terms of brittle tectonics. We carried out studies in the area around Bachevo and Dobarsko villages aiming to observe the fault zone network. The Bachevo fault zone is defined as Paleogene–Eocene Low-Angle Normal Fault that later underwent strike-slip and normal fault reactivation. The zone is an important tectonic boundary, separating the ca. 56 Ma Kapatnik granite from the Ograzhden Unit. Another important and possibly younger extensional zone is the Semkovo Fault. The relationships between the both zones are unclear. In terms of the Quaternary tectonics, it is important to differentiate between the Semkovo fault and the active Southeastern Rila Normal Fault. There are no data about Quaternary activity of the Bachevo fault zone.

Pumping-induced reactivation of a pre-existing normal fault: insights from a centrifuge model test

Pumping-induced normal fault reactivation and the resultant ground fracture have been observed in faulted basins worldwide, but the process and mechanism are poorly understood thus far. In this study, we conducted centrifuge model tests to investigate and analyze these issues. Two simplified faulted models, Models 1 and 2, were developed based on an actual event that occurred in the Beijing Plain, China. Our model tests reproduced the pumping-induced normal fault reactivation, characterized by localized hanging wall subsidence with new fault scarp formation in the models. The monitoring results showed that water table decline drastically accelerated uneven subsidence. Although the deformation pattern did not respond to variations in the material properties of the hanging wall and footwall, the magnitude of the reactivated normal faulting was influenced. The maximum vertical offset in Model 2 was much larger than that in Model 1 due to a larger compression modulus in the hanging wall and footwall in Model 2. Furthermore, the reactivation mechanism was revealed based on Anderson’s faulting theory. Normal faulting occurs once the maximum principal stress becomes vertical and the intermediate and minimum principal stresses become horizontal. Groundwater pumping increases the effective stress, leading to the addition of vertical stress and the resultant normal fault reactivation. Our findings provide a better understanding of human interactions with the Earth’s surface and are helpful for mitigating faulting-caused disasters.

Inversion of extensional basins parallel and oblique to their boundaries: inferences from analogue models and field observations from the Dolomites Indenter, European eastern Southern Alps

Abstract. Polyphase deformation of continental crust is analysed through physical analogue models for settings wherein platform–basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle–ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength difference between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel and oblique (10 to 20∘) to the basin axes has been applied, leading to the inversion of basins formed earlier. The experiments show that strength contrasts across platform–basin transitions control the localisation and overall style of compressional deformation, irrespective of the nature of the basal décollement (frictional versus viscous), the rheology of the basin fill, or changing platform–basin thickness ratios. Orientations of thrust faults change laterally across inherited platform–basin transitions throughout all experiments; higher obliquity of basin inversion leads to stronger alignment of thrust curvature with the orientation of pre-existing rift axes. At individual thrust faults, variations in the strike of thrust fronts are accompanied by changes in the shortening direction during incremental phases of deformation. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform–basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Our experiments are relevant for natural cases such as the Dolomites Indenter of the eastern Southern Alps, underlining the importance of inherited geologic features for the subsequent shortening geometries. Field structural data from the western segment of the Belluno thrust of the Valsugana fault system support predicted variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along-strike) along one single fault. Based on our modelling results, we suggest that this variability of thrust fault orientation and shortening directions, controlled by inherited structures, is consistent with strain partitioning during a single phase of deformation and does not necessarily reflect different deformation phases.

Constraining the tectonic evolution of rifted continental margins by U–Pb calcite dating

We employ U–Pb calcite dating of structurally-controlled fracture fills within crystalline Caledonian basement in western Norway to reveal subtle large-scale tectonic events that affected this rifted continental margin. The ages (15 in total) fall into four distinct groups with ages mainly ranging from latest Cretaceous to Pleistocene. (1) The three oldest (Triassic-Jurassic) ages refine the complex faulting history of a reactivated fault strand originated from the Caledonian collapse and broadly correlate with known rifting events offshore. (2) Two ages of ca. 90–80 Ma relate to lithospheric stretching and normal fault reactivation of a major ENE-WSW trending late Caledonian shear zone. (3) We correlate five ages between ca. 70 and 60 Ma with far-field effects and dynamic uplift related to the proto-Iceland mantle plume, the effect and extent of which is highly debated. (4) The five youngest ages (< 50 Ma) from distinct NE–SW trending faults are interpreted to represent several episodes of post-breakup fracture dilation, indicating a long-lived Cenozoic deformation history. Our new U–Pb data combined with structural and isotopic data show that much larger tracts of the uplifted continental margin of western Norway have been affected by far-field tectonic stresses than previously anticipated, with deformation continuing into the late Cenozoic.

Geología, alteración hidrotermal y mineralogía del sistema de vetas Sofía-Julia-Valencia, distrito minero Andacollo, Neuquén, Argentina

The Sofía-Julia-Valencia vein system, located in the Andacollo mining district in central west Argentina, is hosted by ENE-WSW oriented strike-slip faults which are the result of reactivation of normal faults affecting Carboniferous to Jurassic rocks during Upper Cretaceous-Paleogene. These veins contain a total resource of 22,900 Oz of gold with 5.5-6.7 g/t AuEq. Geologic mapping and a U-Pb age of 71±1Ma in zircon, obtained in an altered and mineralized dacitic dyke of the district, allowed to associate the mineralizing event to the Naunauco Andesitic belt magmatism (Upper Cretaceous-Paleogene) and to the Cretaceous-Paleogene Metallogenic Belt of the Andes in southwestern Argentina. The ore bodies are made up of multiple veins and veinlets that, from oldest to youngest, correspond to: (1) scarce early quartz+pyrite+molybdenite+iron poor-sphalerite veinlets, (2) quartz+epidote+calcite±albite (apatite+rutile+titanite+light rare earth elements bearing phosphates) associated with quartz+biotite, epidote (actinolite)+chlorite+calcite, with pyrite+pyrrhotite±chalcopyrite±(iron rich-sphalerite), marcasite veins. These veins are cut and reopened by (3) polymetallic veins and veinlets formed by quartz+sericite±carbonates (chlorite), with iron-gold rich sphalerite+silver rich-galena+chalcopyrite+pyrite, native gold±arsenopyrite±(pyrrhotite, bornite, argentite). Pyrite (4) and (5) carbonate+framboidal pyrite veinlets cuts all the previous ones. Multistage carbonate generation brecciate and cut previous veins and veinlets. Quartz shows granular, comb textures and some calcites developed platy textures. Four hydrothermal alteration types affected the veins host rock: (1) patches of early potassic alteration; (2) widespread propylitic alteration with disseminated sulfides; (3) later phyllic alteration overlapped to the previous ones; and (4) late supergene alteration. The sphalerite and chlorite composition in the veins (1 and 2) along with their mineral assamblages indicates they were formed by initially alkaline fluids (e.g., feldspar stable) with intermediate sulfur and oxygen fugacity and mesothermal temperature conditions (~400-240 °C), that evolved to conditions of lower sulfur (e.g., pyrrhotite stable) and oxygen fugacity, temperature

Selective reactivation of inherited fault zones driven by stress field changes: Insights from structural and paleostress analysis of the Pocuro Fault Zone, Southern Central Andes (32.8°S)

This study aims to explain the selective reactivation of normal faults during the Andean orogeny at the Southern Central Andes western flank. We conducted a structural mapping and paleostress field reconstruction in the regional-scale Pocuro Fault Zone (PFZ) at 32.8°S. Results reveal that the architecture of the PFZ results from at least two deformation phases, each revealing an individual progressive and gradual evolution. The earliest deformation phase is recorded by two ∼ NS-striking normal faults involving a 5 km wide damage zone characterized by quartz-laumontite and calcite veins that were developed under an extensional regime with a WNW-ESE-trending σ3-axis. The latest deformation phase is recorded by one NS-striking reverse-dextral fault with goethite-hematite syn-tectonic precipitation and two NW-striking reverse-sinistral faults. Reverse faults were developed under a compressional/transpressional regime characterised by an ENE-WSW-trending σ1-axis with a σ2-/σ3-axis permutation. From a geophysical data reassessment, we inferred that observed faults in the surface within the PFZ are regional-scale deep-seated structures. Considering previous geochronological data, we correlated the earliest and latest phases with the Abanico Basin extension (middle Eocene-early Miocene) and its subsequent inversion (Miocene). Given the neotectonic evidence (geomorphic markers and deformation of unconsolidated deposits), the latter phase likely remains active. Quartz-laumontite cementation of the fault core's cataclastic material promotes mechanical strengthening leading to negative feedback for the reactivation of inherited normal faults as reverse ones. Conversely, the concentration of fractures in the damage zone between the normal faults promotes mechanical weakening resulting in a preferential area for the propagation of reverse fault during the compressive/transpressional phase.

Variscan structures and their control on latest to post-Variscan basin architecture: insights from the westernmost Bohemian Massif and southeastern Germany

Abstract. The Bohemian Massif exposes structures and metamorphic rocks remnant from the Variscan orogeny in central Europe and is bordered by the Franconian Fault System (FFS) to the west. Across the FFS, Variscan units and structures are buried by Permo-Mesozoic sedimentary rocks. We integrate existing DEKORP 2D seismic reflection, well, and surface geological data with the newly acquired FRANKEN 2D seismic survey to investigate the possible westward continuation of Variscan tectonostratigraphic units and structures and their influence on latest to post-Variscan basin development. Subsurface Permo-Mesozoic stratigraphy is obtained from available wells and tied to seismic reflection profiles using a synthetic seismogram calculated from density and velocity logs. Below the sedimentary cover, three main basement units are identified using seismic facies descriptions that are compared with seismic reflection characteristics of exposed Variscan units east of the FFS. Our results show upper Paleozoic low-grade metasedimentary rocks and possible Variscan nappes bounded and transported by Variscan shear zones ca. 65 km west of the FFS. Basement seismic facies in the footwall of the Variscan shear zones are interpreted as Cadomian basement and overlaying Paleozoic sequences. We show that the location of normal fault-bounded latest to post-Variscan late Carboniferous–Permian basins are controlled by the geometry of underlying Variscan shear zones. Some of these late Carboniferous–Permian normal faults reactivated as steep reverse faults during the regional Upper Cretaceous inversion. Our results also highlight that reverse reactivation of normal faults gradually decreases west of the FFS.

Influence of Surface Processes on Postrift Faulting During Divergent Margins Evolution

AbstractWe used thermomechanical numerical models to simulate the formation and evolution of divergent continental margins since continental rifting, taking into account surface processes of erosion of the continental escarpment. We found that the degree of crust–mantle coupling, the magnitude and extent of erosion of the coastal landscape, and the preexistence of weakness zones in the continental crust are important elements that control the reactivation of faults along divergent margins during the postrift phase. The numerical experiments indicated that the presence of a lower crust with a relatively low viscosity, facilitating the decoupling of the upper crust and the development of hyperextended margins, could also contribute to the development and/or reactivation of normal faults in the interior of the continent when the margin is continually subjected to differential denudation and consequent flexural response of the lithosphere. This effect is suppressed in scenarios where the lower crust presents a relatively high viscosity, consequently inducing the coupling of the upper crust with the lithospheric mantle. In this case, the long wavelength of the flexural response of a coupled lithosphere to erosional unloading has a minor impact on the reactivation of faults in the upper crust. As an application of our model, we propose that the combination of a decoupled continental lithosphere, escarpment retreat due to erosion, and the preexistence of shear zones parallel to the coast contributed to the development of postrift Cenozoic tectonism in southeastern Brazil.

A new geological map and review of the Middle Devonian rocks of Westray and Papa Westray, Orkney, Scotland

The Middle Devonian lacustrine sediments of Orkney, off the NE Scottish mainland, are composed largely of the Lower and Upper Stromness formations and overlying Rousay Formation. These three formations have been subdivided and defined by vertebrate biostratigraphic biozones with recent division of the Rousay Formation into three further units based on characteristic fish fossils. The division of the Rousay Formation has enabled a map to be constructed of the solid geology of the island of Westray, Orkney, based on fish identification, detailed logging of sedimentary cycles throughout the Rousay succession, parameters of divisional boundaries, and a survey of faults marking sinistral transtensional movement parallel to the Great Glen Fault. Post-Carboniferous shortening and basin inversion led to uplift, folding and reactivation of normal faults as reverse faults, to form a positive strike-slip flower structure in Westray. A suite of Permian igneous dykes intruded across Orkney include three minor offshoots in Westray. The resulting map is the first to make use of biostratigraphic units within the Rousay Flagstone, which are now regarded as Members.

Normal fault reactivation during multiphase extension: Analogue models and application to the Turkana depression, East Africa

We present crustal scale physical analogue models of multiphase rifting to provide new constraints on the role exerted by faults formed during early extension on structures developed during later episodes of rifting. Our laboratory experiments (sand mixture is used to simulate the upper crust and PDMS-Corundum mixture is used to simulate the lower crust) are inspired by the Turkana depression in the East African Rift System, a natural case of rift zone developed through multiple extensional phases. The models reproduce a first rifting phase with NE-SW extension direction followed by a phase of W-E trending extension. In the experiments, we varied the amount of extension during the initial phase, which caused variations in the development and prominence of sets NW-SE normal faults, representing pre-existing faults to be potentially reactivated during the later rifting phase. Our model results indicate that the amount of extension in the early phase is critical in controlling the reactivation: the greater the amount of extension in the 1st-phase, the more important early faults are, and the easier these 1st-phase normal faults are reactivated during the 2nd-phase. At a local scale, fault reactivation may result in faults with a typical zigzag pattern, whose importance increases with the increase in the amount of extension during the early phase. Extrapolation of model results to the Turkana depression suggests that large-scale fault reactivation is unlikely to have occurred in the region, possibly implying that the Cretaceous-Early Paleogene extension was limited in the central part of Turkana depression. Results have been also extrapolated to other regions interested by multiphase rifting, such as basins in the North Sea rift.

Structural controls on Jurassic gold mineralization, and Cretaceous-Tertiary exhumation in the foreland of the southern Patagonian Andes: New constraints from La Paloma area, Deseado Massif, Argentina

Fluid-rock interaction occurring within high-permeability conduits represented by fault-fracture meshes is well known to control the development of hydrothermal ore deposits. In this study we document the fundamental role of four sets of precursor joints, reactivated as dilational faults displaying variable normal dip-slip and strike-slip components of motion, in the formation of epithermal Au-Ag deposits of economic interest. Paleostress analysis unravels how all the resulting four sets of hybrid extension-shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the σ3 and the σ2 axes. Such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension, and by orthogonal stretching produced by differential downthrow of the hanging-wall blocks of laterally-terminating Jurassic normal faults.Later fault reactivation appears to be associated with far field propagation of Andean stresses, marked by superposed strike-slip and reverse fault slip components recording roughly E-W horizontal compression. This late fault reactivation could be related with a late Early Cretaceous shortening event known to have affected the studied sector of the Deseado Massif. The subsequent tectonic evolution of the study area occurred within the general framework of a slow Cretaceous to Tertiary exhumation recorded by apatite fission track cooling ages. Our new thermochronometric results, recording steady-state cooling at a rate of c. 1 °C/Ma during the last 90 Ma, shed new light onto the post-Jurassic geodynamic evolution of the foreland of the southern Patagonian Andes.

Cenozoic thermal history reconstruction of the Dongpu Sag, Bohai Bay Basin: Insights from apatite fission‐track thermochronology

AbstractKnowledge of the thermal history of the onshore Dongpu Sag (DPS), Bohai Bay Basin is important for understanding its tectonic development within a broader regional context and for elucidating its poorly studied source‐rock maturation history. To unravel DPS time‐temperature development, apatite fission‐track data were acquired from eight sandstone samples in three deep wells. Thermal history modelling indicates continuous heating during the rifting stage (early Eocene to late Oligocene), followed by cooling attributed to tectonic uplift between ~27 and ~16 Ma, which resulted in the removal of ~1,400 to 1,800 m of section. Reheating occurred during the subsidence stage from middle Miocene until present. Unlike typical passive continental margin basins, the DPS experienced accelerated post‐rifting subsidence from ~5.8 Ma, which was especially marked since ~2.6 Ma. This phase was probably triggered by normal fault reactivation, in combination with a variation in the deep heat flow pattern.

Strike‐slip reactivation of segmented normal faults: Implications for basin structure and fluid flow

AbstractReverse reactivation of normal faults, also termed “inversion”, has been extensively studied, whereas little is known about the strike‐slip reactivation of normal faults. At the same time, recognizing strike‐slip reactivation of normal faults in sedimentary basins is critical, as it may alter and impact basin physiography, accommodation and sediment supply and dispersal. Motivated by this, we present a study of a reactivated normal fault zone in the Liassic limestones and shales of Somerset, UK, to elucidate the effects of strike‐slip reactivation of normal faults, and the inherent deformation of relay zones that separate the original normal fault segments. The fault zone, initially extensional, exhibits a series of relay zones between right‐stepping segments, with the steps between the segments having subsequently become contractional due to sinistral strike‐slip movement. The relay zones have therefore been steepened and are cut by a series of connecting faults with reverse and strike‐slip components. The studied fault zone, and comparison with larger‐scale natural examples, leads us to conclude that the relays turned contractional steps are associated with (a) complex fault and fracture networks that accommodate shortening, (b) anomalously high numbers of fractures and faults, (c) layer‐parallel slip and (d) folding and uplift. Comparison with published statistics from global relay zones shows that whereas the reactivated relay zones feature aspect ratios similar to those of unreactivated relay zones, bed dips within reactivated relay zones are significantly steeper than unreactivated relay zones. Given the potential of reactivated relay zones to form areas of local uplift, they may affect basin structure and may also form potential traps for hydrocarbon or other fluids. The elevated faulting and fracturing, on the other hand, means reactivated relays are also likely loci for enhanced up‐fault flow.

Interplay between inherited rift faults and strike-slip structures: Insights from analogue models and field data from Iceland

Abstract Although structural inheritance is a fundamental factor in the tectonic evolution of the crust, few studies have been aimed at understanding in detail the interaction between pre-existing normal faults in a rift zone and successive dominant strike-slip (transtensional) movements. Firstly, we present the complex pattern of faults and tension fractures of the western, pre-Holocene part of the N-S Theistareykir Fissure Swarm (ThFS - Northern Iceland Rift), crossed by the NW-SE striking, right-lateral, Husavik-Flatey Fault (HFF), and we integrate previous observations with new field data that reveal Holocene motions along the N-S faults near the HFF. Secondly, we propose a set of analogue experiments that reproduce the N-S rifting followed by superimposed transtensional faulting along the HFF. In a first set of experiments, we tested the effect of an initial extension at the shallow crustal level by forming a rift zone. During this rifting phase, we modelled the presence of a sub-orthogonal discontinuity that represents the structural inheritance of the HFF (that is older than the ThFS), but without any imposed “regional” transtensional movement along it. After that, we superimposed a dominant right-lateral movement along the HFF on the modelled rift faults in order to assess the effect of the inheritance of the rift plus the HFF. Our results show that during rifting, a series of elongated and depressed areas develop along the initial HFF discontinuity (that is locally reactivated by the rift opening), in agreement with field data. During the second experimental set of exclusively shear deformation along the HFF, the models show the development of Riedel shears at the HFF, contemporaneous to the local reactivation of normal faults and tension fractures in the previous rift zone. This reactivation occurs by block rotation, incremental dip-slip motions on pre-existing fault scarps, widening of tension fractures and development of new structures. We conclude that transversal basins can develop from the early stages of rift development, and that the structures of a pre-existing rift may reactivate under a local stress state induced by incremental transtensional motions.

Reactivation of normal faults as high-angle reverse faults due to low frictional strength: Experimental data from the Moonlight Fault Zone, New Zealand

Large normal faults are frequently reactivated as high-angle reverse faults during basin inversion. Elevated fluid pressure is commonly invoked to explain high-angle reverse slip. Analogue and numerical modeling have demonstrated that frictional weakening may also promote high-angle reverse slip, but there are currently no frictional strength measurements available for fault rocks collected from large high-angle reverse faults. To test the hypothesis that frictional weakening could facilitate high-angle reverse slip, we performed single- and double-direct friction experiments on fault rocks collected from the Moonlight Fault Zone in New Zealand, a basin-bounding normal fault zone that was reactivated as a high-angle reverse fault (present-day dip angle 60°–75°). The fault core is exposed in quartzofeldspathic schists exhumed from c. 4–8 km depth and contains a <20 m thick sequence of breccias, cataclasites and foliated cataclasites that are enriched in chlorite and muscovite. Friction experiments on water-saturated, intact samples of foliated cataclasite at room temperature and normal stresses up to 75 MPa yielded friction coefficients of 0.19<μ < 0.25. On the assumption of horizontal maximum compressive stress, reactivation analysis indicates that a friction coefficient of <0.25 will permit slip on high-angle reverse faults at hydrostatic (or even sub-hydrostatic) fluid pressures. Since foliated and phyllosilicate-rich fault rocks are common in large reactivated fault zones at basement depths, long-term frictional weakening is likely to act in concert with episodic build-ups of fluid pressure to promote high-angle reverse slip during basin inversion.

The role of basement-involved normal faults in the recent tectonics of western Taiwan

AbstractIn the foreland area of western Taiwan, some of the pre-orogenic basement-involved normal faults were reactivated during the subsequent compressional tectonics. The main purpose of this paper is to investigate the role played by the pre-existing normal faults in the recent tectonics of western Taiwan. In NW Taiwan, reactivated normal faults with a strike-slip component have developed by linkage of reactivated single pre-existing normal faults in the foreland basin and acted as transverse structures for low-angle thrusts in the outer fold-and-thrust belt. In the later stage of their development, the transverse structures were thrusted and appear underneath the low-angle thrusts or became tear faults in the inner fold-and-thrust belt. In SW Taiwan, where the foreland basin is lacking normal fault reactivation, the pre-existing normal faults passively acted as ramp for the low-angle thrusts in the inner fold-and-thrust belt. Some of the active faults in western Taiwan may also be related to reactivated normal faults with right-lateral slip component. Some main earthquake shocks related to either strike-slip or thrust fault plane solution occurred on reactivated normal faults, implying a relationship between the pre-existing normal fault and the triggering of the recent major earthquakes. Along-strike contrast in structural style of normal fault reactivation gives rise to different characteristics of the deformation front for different parts of the foreland area in western Taiwan. Variations in the degree of normal fault reactivation also provide some insights into the way the crust embedding the pre-existing normal faults deformed in response to orogenic contraction.

Reactivation, inversion and basement faulting and thrusting in the Sierras Pampeanas of Córdoba (Argentina) during Andean flat-slab deformation

AbstractThe Sierras Pampeanas of Córdoba are the easternmost uplifted blocks caused by Andean foreland deformation, over 700 km from the Chile trench. This deformation started atc.340 Ma through basement faults, thrusts and reactivation of normal faults of the Cretaceous rift during the opening of the Atlantic Ocean. Other older faults, major oblique lineaments, were also reactivated. Thermochronological and geothermobarometric data indicate that some topographic relief could have been Palaeozoic–Mesozoic relicts and not only produced by the Andean orogeny. Faults are partially controlled by the early Cambrian S2metamorphic foliation, coincident with the curved fault traces at map scale. During Pliocene time, two deformation phases post-dating Miocene–Pliocene magmatism are recognized. Shallow seismicity data (c.25 km depth) indicate that the Sierras de Córdoba accommodate Quaternary displacement. Magnetotelluric studies detect the interface between the Pampia terrane and the Río de la Plata craton. The role of the oblique lineaments in the nucleation and development of the Tertiary faulting has been little considered; they could be correlated with an old pan-Gondwanan trend. During the Cretaceous period these lineaments worked in a transtensive way, producing the uplift of high-grade rocks and segmentation of the mountain chain favouring the diachronous uplift along the ranges. Recently, both the brittle–ductile transition atc.23 km depth and the crustal thickness have been determined by seismicity analysis. The oblique lineaments displace normally the Mohorovicic discontinuity. Main basement thrusts were probably rooted in the suture between the Pampia terrane and the Río de la Plata craton.

Andean shortening, inversion and exhumation associated with thin- and thick-skinned deformation in southern Peru

AbstractA balanced cross-section spanning the Eastern Cordillera and Subandean Zone of southern Peru (13–15°S) constrains ~130 km (38%) of Cenozoic orogen-normal SW–NE Andean deformation accommodated by thick- and thin-skinned retro–arc fold–thrust belt shortening that overprinted pre-Andean Triassic normal faults. Zircon and apatite (U–Th)/He ages demonstrate continuous Oligocene to Miocene cooling of the Permo-Triassic Coasa pluton in the Eastern Cordillera. Zircon (U–Th)/He ages (~34–18 Ma) are reset and define a steep age versus elevation relationship. Apatite (U–Th)/He results reveal reset ages that define two spatially separated groups with ages of ~30–26 Ma and ~17–11 Ma. Detrital zircon U–Pb geochronologic results from Cretaceous–Cenozoic siliciclastic rocks from the Altiplano/Eastern Cordillera record Andean fold–thrust belt and magmatic-arc sediment sources. Correlative Subandean Zone rocks preserve a cratonic sediment contribution, with minor Andean sediment appearing in some Cenozoic rocks. We propose that earliest Andean deformation and structural compartmentalization of the Eastern Cordillera was linked to selective inversion of inherited Permo-Triassic basement-involved normal faults that guided subsequent thick- and thin-skinned deformation. Provenance variations between the hinterland and foreland depocentres reveal competing eastern and western sediment sources, reflecting an axial zone in the Eastern Cordillera that coincided with the inherited Triassic graben and impeded sediment source mixing. Our zircon and apatite (U–Th)/He ages are consistent with published constraints along strike and support pulses of Eocene to late Miocene exhumation that were likely driven by normal fault reactivation and protracted Eastern Cordillera deformation.

Far-field effects of Appalachian orogenesis: A view from the craton

The sedimentary cover of the North American craton preserved little evidence of the Paleozoic tectonic events that shaped the Appalachian orogen on its eastern side. A notable exception is the NNW-trending Hudson Bay central high, which corresponds to a normal-fault array extending for a minimum length of 500 km. A working hypothesis is proposed in which stresses applied to the continental margin during the Silurian–earliest Devonian Salinian orogeny were transmitted over a distance of >1400 km in the continental interior, where they induced the normal-fault reactivation of older structural discontinuities. The shutdown of tectonic activity along the Hudson Bay central high during the latest Early Devonian to earliest Middle Devonian is interpreted as resulting from a change in the direction of plate convergence during the Acadian orogeny.