Subvertical Faults Research Articles

Geophysical Constraints on the Crustal Architecture of the Transtensional Warm Springs Valley Fault Zone, Northern Walker Lane, Western Nevada, USA

AbstractThe Walker Lane is a zone of distributed transtension where normal faults are overprinted by strike‐slip motion. We use two newly acquired, high‐resolution seismic reflection profiles and a reprocessed Consortium for Continental Reflection Profiling (COCORP) deep crustal reflection profile to assess the subsurface geometry of the Holocene‐active, transtensional Warm Springs Valley fault zone (WSVFZ) near Reno, Nevada, USA. Our multiscale observations extend to 12 km depth and suggest that the WSVFZ is more complex in the subsurface than implied by late Pleistocene surface fault traces. Two 4‐km‐long high‐resolution profiles image to a depth of ∼2 km and reveal moderately dipping reflections and truncations, some of which project to mapped scarps formed in late Pleistocene surfaces. The shallow lines are collocated with COCORP profile NV 08 along ∼40°N latitude. Reanalysis of the COCORP data reveals previously unidentified coherent reflections to a depth of ∼12 km and a previously mapped ∼30° west‐dipping fault at 8–12 km. From these seismic profiles, the WSVFZ is not a simple, subvertical fault zone extending through the entire seismogenic crust. Instead, the reflections are consistent with a zone of steeply and moderately dipping faults that simplify and steepen with depth before intersecting a mid‐crustal, low‐angle (∼25–30°) fault. The complex fault geometry of the WSVFZ implies that crustal shear is accommodated by a mix of dipping and subvertical faults in the transtensional northern Walker Lane. If so, transtensional fault zones may present challenges to paleoseismic and geodetic studies and require careful treatment when included in seismic hazard analyses.

Variable vertical movements and their deformation behaviors at convergent plate suture: 14-year-long (2004–2018) repeated measurements of precise leveling around middle Longitudinal Valley in eastern Taiwan

• We provide 14-year-long leveling results across plate suture of Philippine Sea and Eurasia. • Along the LVF, shallow locked in north, and rapid creeping without locking in south. • The leveling data imply blind slip on the deeper part of the Central Range fault. • Active creeping occurs on a nearly vertical contact along the Chimei fault. To better characterize the vertical movements and the deformation behaviors across the plate suture of an arc-continent collision, we conducted annual repeated measurements on two precise leveling routes in a length of 34 and 37 km, respectively, across the middle part of the Longitudinal Valley in eastern Taiwan in 2004–2018. The 14-year-long results showed that the Longitudinal Valley fault (LVF) dominates the surface deformation: a) the middle LVF (Juisui fault) exhibited partially locked in the upper few kilometers, with a cumulative uplift rate of 9–10 mm/yr in a distance of 4 km; b) the southern LVF (Chihshang fault) showed a creeping behavior with a vertical rate of 24–27 mm/yr. In addition, we are able to characterize other features, including 1) tilting upward to the west in the eastern Central Range, suggesting activity on the west-dipping Central Range fault; 2) the hanging wall of the LVF showed tilting downward behavior to the east; 3) the Chimei fault, a suspected active fault, revealed active slip on the sub-vertical fault plane, that caused a vertical rate of 8–9 mm/yr. Putting the results under global ITRF system, the whole Juisui route was moving downward, supporting the notion that NNW subduction of the Philippine Sea plate starts around the latitude of the middle of the Longitudinal Valley. Finally, the co-seismic vertical deformation of the 2013 M L 6.4 Juisui earthquake was characterized by tilting upward to the west, consistent with stick–slip on the deeper part of a west-dipping interface of the forearc basement.

Birth of a large volcanic edifice offshore Mayotte via lithosphere-scale dyke intrusion

Volcanic eruptions shape Earth’s surface and provide a window into deep Earth processes. How the primary asthenospheric melts form, pond and ascend through the lithosphere is, however, still poorly understood. Since 10 May 2018, magmatic activity has occurred offshore eastern Mayotte (North Mozambique channel), associated with large surface displacements, very-low-frequency earthquakes and exceptionally deep earthquake swarms. Here we present geophysical and marine data from the MAYOBS1 cruise, which reveal that by May 2019, this activity formed an 820-m-tall, ~5 km³ volcanic edifice on the seafloor. This is the largest active submarine eruption ever documented. Seismic and deformation data indicate that deep (>55 km depth) magma reservoirs were rapidly drained through dykes that intruded the entire lithosphere and that pre-existing subvertical faults in the mantle were reactivated beneath an ancient caldera structure. We locate the new volcanic edifice at the tip of a 50-km-long ridge composed of many other recent edifices and lava flows. This volcanic ridge is an extensional feature inside a wide transtensional boundary that transfers strain between the East African and Madagascar rifts. We propose that the massive eruption originated from hot asthenosphere at the base of a thick, old, damaged lithosphere.

New insights on the geometry and kinematics of the Shunbei 5 strike-slip fault in the central Tarim Basin, China

Integrated geological-geophysical methods have been used to depict the geometric characteristics and kinematic evolution of the Shunbei 5 (SB5) and adjacent Shunbei1 (SB1) strike-slip faults in the central Tarim Basin, China. Geometric evidence for strike-slip faulting includes the occurrence of positive and negative flower structures in stepover zones, oblique secondary splay faults, en echelon folds, and adjacent secondary faults. The multi-humped along-strike and along-dip displacement variations of the SB5 fault provide insights into the initial segmentation, interaction and polycyclic growth of the fault zones both in plan and section views: The SB5 fault initiated as an apparent “X-type” conjugate pure shear fault system in the north and a simple shear fault system in the south respectively. At depth (mainly in Cambrian to Middle Ordovician rocks), the northern and southern parts of the SB5 fault exhibit a typical dextral strike-slip architecture consisting of multiple fault segments connected via transpressional push-up or transtensional pull-apart stepovers or bends. The southern part has a linear, narrow sinistral ridge system. At shallower depths (in Upper Ordovician to Middle Devonian rocks), the lower subvertical faults propagated upwards as en echelon normal faults in the north, and as a two-phase, partitioned system consisting of boundary grabens and en echelon normal faults that dissected the early-formed border grabens in the south. With progressive deformation, the northern and southern parts of the SB5 fault reactivated and transferred into one large, sinistral fault in Upper Devonian to Permian rocks. Four major tectonic phases matching the kinematic evolutions of regional fault systems and uplifts in the study region, have also been recognized in the Middle Ordovician to Cretaceous. The pull-apart stepover and single fault zones developed in Middle Ordovician rocks are favorable fracture-related reservoirs.

Lateral fluid propagation and strike slip fault reactivation related to hydraulic fracturing and induced seismicity in the Duvernay Formation, Fox Creek area, Alberta

SUMMARY Fault shear slip potential is analysed in the area where induced earthquakes (up to 3.9 Mw) occurred in May–June 2015 approximately 30 km south of Fox Creek, Western Canada Sedimentary Basin, Canada. The induced earthquakes were generated by the hydraulic fracturing of the Upper Devonian Duvernay Formation. Interpretation of a 3-D seismic survey and analysis of the ant tracking attribute identifies a linear discontinuity that likely represents a subvertical fault with strike length of 1.4 km, which is aligned with the zone of induced earthquake hypocentres. 1-D–3-D geomechanical modelling is conducted to characterize mechanical rock properties, initial reservoir pressure and stress field. Hydraulic fracture propagation and reservoir pressure buildup simulations are run to analyse lateral fluid pressure diffusion during well treatment. The interaction of natural fractures introduced as Discrete Fracture Network and hydraulic fractures is tested. 3-D poroelastic reservoir geomechanical modelling is completed to simulate slip reactivation of the identified fault zone. The obtained results support that additional pressure buildup of 20 MPa in treatment wells can propagate laterally along hydraulic fractures (and potentially natural fracture network) for about 550 m and reach the fault zone. The increase of fluid pressure by 20 MPa in the fault zone results in dextral slip along the fault, mostly in the interval of the Duvernay and overlying Ireton Formations, corroborating prior focal mechanism results and hypocentral depths. The simulations indicate that lateral transmission of additional fluid pressure from the fracturing stimulation area to the fault zone could happen in a few days after the treatment of lateral wells that is supported by the observed induced earthquakes. This study helps to quantify changes in fluid pressure and stresses that may result in fault shear slip during hydraulic fracturing and predict the potential of induced seismicity connected to hydrocarbon production from the Duvernay Play.

Radon concentration in caves as a proxy for tectonic activity in The Cantabrian Mountains (Spain)

Radon (Rn) constitutes a good geochemical tracer for neotectonic activity in faults since associated fracturing near the surface favours fluid escape to the atmosphere. In this contribution, we measured the Rn concentration in the air inside karst caves to constraints the recent fault activity in the Cantabrian Mountains (N Spain). Rock formations exhumed during the uplifting of the Cantabrian Mountains record a long history of fracturing, which has the potential to connect deeper sources of Rn with the surface. In this regional study, we correlate Rn measurements with cave survey data and geological structures using a Geographic Information Systems. Thirty-four Rn average concentration was recorded by CR-39 detectors during 8 integrated months. The method is applied to the central part of the Cantabrian Mountains that is built on sedimentary and low-grade metamorphic rocks relatively poor in U. Dominant tectonic structures and Rn concentration are examined in 28 cavities. The concentration of Rn values is higher than 0.5 kBq·m-3 in caves developed preferably following fractures with the direction N30oW, being the concentration greater than 0.8 kBq·m-3 in cavities located less than 200±50 m from subvertical faults with such orientation. Rn anomalies point to relative high connectivity along subvertical fault zones NW-trending, preserving fracture connectivity in the most recent structures in the Cantabrian Mountains. Finally, in the study area there is a low but significant radioactive hazard which is associated to fault zones in a fractured rock massif. It contrasts with other active tectonic settings where the radioactive hazard may come from fault movements.

Seafloor Methane Seepage Related to Salt Diapirism in the Northwestern Part of the German North Sea

This study focuses on seafloor methane seep sites and their distribution in the northwestern part of the German North Sea. Methane seepage is a common phenomenon along marine shelves and known to occur in the North Sea, but proof of their existence was lacking in the study area. Using a ship-based multibeam echosounder we detected a minimum of 166 flares that are indicative for free gas releases from the seafloor in the German “Entenschnabel” area, which are not related to morphologic expressions at the seafloor. However, a group of small depressions was detected lacking water column anomalies but with indications of dissolved fluid release. Spatial analysis revealed that flares were not randomly distributed but show a relation to locations of subsurface salt diapirs. More than 60% of all flares were found in the vicinity of the salt diapir “Berta”. Dissolved methane concentrations of ∼100 nM in bottom waters were ten times the background value in the “Entenschnabel” area (CH4< 10 nM), supporting the finding of enhanced seepage activity in this part of our study area. Furthermore, locations of flares were often related to acoustic blanking and high amplitude reflections in sediment profiler echograms, most prominently observed at location Berta. These hydroacoustic signatures are interpreted to result from increased free gas concentrations in the sediments. Electromagnetic seabed mapping depicts local sediment conductivity anomalies below a flare cluster at Berta, which can be explained by small amounts of free gas in the sediment. In our area of interest, ten abandoned well sites were included in our mapping campaign, but flare observations were spatially not related to these wells. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Due to the shallow water depths of 30 to 50 m in the study area, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters with methane in the flare-rich northeastern part of the working area indicates that part of the released methane through seepage may contribute to the atmospheric inventory.

New insights from 2- and 3-D numerical modelling on fluid flow mechanisms and geological factors responsible for the formation of the world-class Cigar Lake uranium deposit, eastern Athabasca Basin, Canada

The Cigar Lake uranium deposit in the Athabasca Basin is the world’s second largest high-grade unconformity-related uranium deposit. Its distinct geological architecture includes heterogeneous basement lithologies, a local basement high and sub-vertical faults. The sub-vertical faults are classified further into two sub-types: faults that are restricted to the basement, termed ‘basement faults’, and faults that are distinctly related to post-Athabasca fault reactivation, in that they extend upward into the sandstone, termed ‘extended basement faults’. This study aims to evaluate the effects of the aforementioned geological factors on the fluid flow patterns under two different driving forces, buoyancy due to variation of fluid density or ‘thermal convection’ and deformation or a combination of them, and to determine the most probable fluid flow scenarios for the formation of the deposit. The numerical results show that fluid flow is strongly affected by the two types of faults. While the basement faults represent fluid paths with complex fluid flow patterns, depending on the driving forces, providing favourable physical conditions for different chemical processes, such as fluid-fluid and fluid-rock interactions, the extended basement faults enhance the permeability of an E-W corridor within the sandstone and significantly strengthen the overall upwelling flow above the basement faults, promoting sandstone-hosted mineralization. The numerical results also suggest that the main deposit likely formed via fluid convection during tectonically quiet periods, although faulting played a critical role in increasing permeability, in turn, enhancing thermal convection.

Potential of Surface-to-Tunnel Seismic Tomography to Detect Vertical Faults: Application to the Tournemire Underground Research Laboratory, France

The Tournemire Underground Research Laboratory (Aveyron, France), developed by the French Institute for Radiological Protection and Nuclear Safety, is composed of a tunnel excavated in an argillaceous geological layer belonging to a limestone–argillite–limestone sub-horizontal sedimentary sequence. Sub-vertical fault zones are intercepted by drifts and boreholes excavated from the tunnel in the argillaceous layer at a depth of about 250 m. These fault zones are characterized by velocities lower than those of the embedding rocks. In this study, we assess the capacity of first arrival traveltime tomography to detect and characterize such fault zones in a surface-to-depth transmission configuration with a limited angle aperture. The source at the surface is a sledgehammer and turns out to be very repetitive. The source trigger is transmitted to the acquisition bay via a VHF transmitter. Receivers are located at depth in underground works. First arrivals are clear and traveltime picking is straightforward. Before inverting real data, we conduct synthetic tests with the real acquisition geometry to assess limits and validity of first arrival traveltime tomography to detect a lower velocity fault zone using different initial configurations and inversion constraints during the inversion. We show that introducing a priori information on velocities in the shallow subsurface, limestones and argillaceous layers and constraints on maximum velocities greatly helps improving results and subsequent interpretation. Velocities obtained from the real data display a main lower velocity zone that extends from the galleries in the argillaceous layer right up to the surface in limestones. The location and extension of this lower velocity zone are consistent with fault and fracture zones mapped from geological observations.

Orthogonal Fault Rupture and Rapid Postseismic Deformation Following 2019 Ridgecrest, California, Earthquake Sequence Revealed From Geodetic Observations

AbstractWe studied the 2019 Mw6.4 and Mw7.1 Ridgecrest, California, earthquake sequence, using Sentinel‐1 and ALOS‐2 coseismic interferograms and subpixel offsets to retrieve the three‐dimensional (3‐D) surface displacements. By inverting the 3‐D displacements, optimal dip angles of the earthquake faults and the slip model were obtained. The interferometric synthetic aperture radar‐based slip model supplemented with the analysis of GPS data shows that the Mw6.4 event ruptured two orthogonal faults and its major geodetic moment was released by sinistral motion on a NE‐SW trending fault that dips 78° NW. Right‐lateral slip on NW‐SE trending subvertical faults was responsible for the Mw7.1 earthquake. Postseismic analysis with U.S. Geological Survey earthquake catalog and GPS time series at site P595 shows that the postseismic surface deformation following the Mw7.1 event has similar temporal patterns with the postseismic moment release, but requires more energy. This implies that the early aftershocks were likely controlled by rapid afterslip following the mainshock.

Fault Tectonics and Deep Structure of Seismic Zones in the Eastern Priamurye Region

This paper is concerned with spatial relationships between seismic zones in the eastern Priamurye region (М ≥ 5) on the one hand and, on the other, regional faults and zones of hidden faults as indicated by axes of gravity and magnetic anomalies. The seismoactive zones where М ≥ 5 earthquakes occurred are mostly confined to regional faults, while there are two cases where no such relationship has been detected. Seismic zones are observed both at intersections of regional faults and at intersections of regional faults with hidden faults of different rank. The seismic zones comprise inclined and subvertical deep-seated faults as identified by deep seismic sounding (DSS), the earthquake converted wave method (ECWM), and magnetotelluric sounding (MTS) surveys. The seismoactive zones were studied by geophysical techniques to identify indication of fluid saturation, as follows: the seismoactive faults frequently control low velocity and low resistivity inhomogeneities in the Earth’s crust and upper mantle; some seismoactive faults have displaced the Moho interface; there are dome-shaped flexures of crustal interfaces and of the Moho interface; and the crust is found to contain numerous conversion interfaces. The deep occurrence of seismoactive faults and the indications of fluid saturation suggest that the seismoactive faults in the eastern Priamurye region provide supply of fluids from the mantle to the crust.

Evolution of ice shelf rifts: Implications for formation mechanics and morphological controls

Ice shelf rifts are predecessors to iceberg calving events. Observations of their morphology over time and interaction with their surroundings are presented. Newly-available Landsat-derived annual velocity data and laser altimetry from NASA's ICESat, ICESat-2 and Operation IceBridge missions were used to characterize several ice shelf rift systems. Actively propagating ice shelf rifts exhibit uplift on both sides, with an offset in height between the two rift walls, suggesting an asymmetric buoyancy force resulting from the rifting process itself. It is observed that the direction of the asymmetry is the same for rifts within a single ice shelf, but not between ice shelves. This finding suggests a rheological or basal properties influence on rift formation and buoyancy asymmetry. It is shown that this asymmetry can be accounted for by modeling ice shelf rifts as subvertical faults. With time and inactivity, rift walls eventually relax and become rounded off. Measurements of widening suggest that rift infilling is active on many active rifts. Active rifts experience higher spreading rates than do dormant rifts. Lastly, two candidate processes are identified for rift nucleation (subvertical basal crevassing and localized melt). It is found that more than any other ice shelf in this study, rifting in Pine Island Glacier is driven by ocean heat variations and is showing signs of increasing destabilization.

The features of fault tectonics and deep structure of the seismoactive zones in Eastern Priamurye

The paper examines the spatial relationship between the seismoactive zones in eastern Priamurye (М ≥ 5) and the regional faults and hidden fault zones identified from the gravity and magnetic anomaly axes. The seismoactive zones where earthquakes with М ≥ 5 occurred are mostly confined to the regional faults, though such a relationship has not been validated in two cases. The seismoactive zones are detected both at the regional fault intersection and in areas where the regional faults intersect with the hidden faults of various ranks. According to the data obtained by deep seismic sounding (DSS), earthquake converted wave method (ECWM) and magnetotelluric sounding (MTS), the seismoactive zones are formed by deep inclined and subvertical faults. The indications of fluid saturation are found in the seismoactive zones from geophysical data which show that the seismoactive faults often control low-velocity and low-resistivity anomalies in the crust and upper mantle. In some cases, the Moho displacement and the dome-like flexures of the crustal and Moho boundaries are observed along these seismoactive faults and the abundance of the conversion boundaries in the crust is also noted. The deep pattern of the seismoactive faults and the revealed indications of fluid saturation allow us to consider the seismoactive zones in eastern Priamurye as the channels providing fluid supply from the mantle to the crust.

Fluid inclusion and stable isotope constraints on the heavy rare earth element mineralisation in the Browns Range Dome, Tanami Region, Western Australia

This study reports on fluid inclusion and oxygen isotope compositions of mineralised and barren hydrothermal quartz veins and hosting metasedimentary rocks associated with the heavy rare earth element (HREE) mineralisation in the Browns Range Dome of the Tanami Region, Western Australia. The HREE mineralisation consists of quartz and xenotime-bearing hydrothermal veins and breccias that occur along sub-vertical faults within Archean to Paleoproterozoic metasedimentary rocks. Based on analysis of nearly 550 quartz-hosted primary fluid inclusions, three fluid inclusion types were identified in the mineralised samples: type I low-salinity H2O-NaCl (largely <5 wt% NaCl; consistent with meteoric water), type II medium-salinity H2O-NaCl (12–18 wt% NaCl), and type III low- to high-salinity H2O-CaCl2-NaCl (1 to ca. 24 wt% NaCl + CaCl2). Homogenisation temperatures of all fluid inclusion types vary over a relatively wide range from 100 to 250 °C. Barren quartz veins contain only type I low-salinity H2O-NaCl fluid inclusions, with homogenisation temperatures extending from 170 to 350 °C. Raman analyses of all three fluid inclusion types confirmed their aqueous nature with no carbon-bearing fluid species identified. The three fluid inclusion types indicate mixing of three hydrothermal fluids: a low-salinity H2O-NaCl meteoric fluid (<5 wt% NaCl), a medium-salinity H2O-NaCl (12–18 wt% NaCl) fluid, and a high-salinity H2O-CaCl2-NaCl (ca. 24 wt% NaCl + CaCl2) fluid.Limited LA-ICP-MS analysis found detectable Y, Ce, U and Cl only in the type III fluid inclusions, which indicates that transport of ore metals was (at least partly) by Cl complexes in the type III fluid. The δ18Ofluid values calculated from quartz from mineralised samples are in the range defined by the Archean metasedimentary host rocks of the Browns Range Metamorphics (δ18Ofluid = +1.8 to +5.2‰) and the unconformably-overlying Paleoproterozoic Birrindudu Group sandstones (δ18Ofluid = +8‰). Collectively, our fluid inclusion and oxygen isotope data, together with other field, mineralogical and geochemical data, support an ore genesis model involving mixing of the three hydrothermal fluids in fault zones and along unconformity surfaces in, and around, the Browns Range Dome. The meteoric low-salinity H2O-NaCl fluid potentially carried P from the Birrindudu Group sandstones, and the high-salinity H2O-CaCl2-NaCl fluid leached HREE + Y from metasedimentary rocks of the Browns Range Metamorphics. Ore deposition occurred following mixing of the P-bearing and HREE + Y-bearing fluids, and was associated with a widespread white mica alteration. The temperature and pressure during the fluid-fluid mixing and mineralisation was between 100 and 250 °C, and 0.4 and 1.6 kbar, respectively.

Source characteristics of the March 16, 2014 Mw 6.7 earthquake and its implications for the Mw 8.2 Pisagua mainshock

The March 16, 2014 Mw 6.7 earthquake played an important role in the complex seismic sequence leading to the nucleation of the April 1st, 2014 Mw 8.2 Pisagua earthquake in northern Chile. The Mw 6.7, an upper plate reverse faulting event with nodal planes highly rotated counterclockwise with respect to the strike of the megathrust at its location, is analyzed. For each nodal plane of the regional W-phase centroid moment tensor solution, the spatiotemporal slip distribution is inverted using near-field records. The optimal space and time smoothing constraints are determined objectively based on the Akaike's Bayesian Information Criterion. We analyze seismicity on the region affected by the Mw 6.7 and Mw 8.2 Pisagua mainshock, with accurate hypocenter locations obtained using a 3D heterogeneous medium. We also performed regional moment tensor inversion of events (M≥4.0) occurred before the Mw 8.2. Our results suggest the sub-vertical fault plane dipping southward (dip∼70°) as being the causative fault of the March 16 Mw 6.7 earthquake. The rupture propagated to the northwest, lasted 15s, and yielded a total seismic moment of 1.36×1019Nm (Mw 6.7). Estimated slip is characterized by total rupture length of ∼22km, and suggest shallow slip distributed between ∼ 3km, down to 17km depth. A comparison between coastal tide gauge records and forward tsunami modeling show similarity of the time series, thus supporting the plausibility of our estimated slip models. Our interpretation suggests that the rupture of the Mw 6.7 intraplate could be explained by stress accommodation between two segments along-dip with different mechanical properties at the interplate boundary, and inside the upper plate.

The nature of the Paleoproterozoic orogen in the Jacobina Range and adjacent areas, northern São Francisco Craton, Brazil, based on structural geology and gravimetric modeling

Defintion of the geometry and tectonic style of Paleoproterozoic mobile belts at the Paleoarchean cratonic margin of the Gavião Block brings new insights into the early evolution of the São Francisco Craton. This paper presents the regional structural framework and tectonic stratigraphic 3D models made from geological mapping and gravimetric modeling of the Jacobina Range. The Jacobina Range is composed of an Archean greenstone belt, Au-U detrital pyrite-bearing sedimentary rocks and Paleoproterozoic granite-gneissic terranes deformed between 2.1 and 1.9 Ga as the result of collision of the Archean Gavião, Serrinha, and Jequié blocks. Two deformational phases and three subdomains were described in the field area. The deformational phases include (i) a compressional phase with a westward tectonic convergence that built an imbricated fault system and (ii) a sinistral transpressional phase with a northwestward tectonic convergence, which forms a subvertical fault system with sinistral-oblique to lateral displacement. The southern subdomain is characterized by an imbricated tectonic system overprinted by transpressional faults. The central subdomain contains western-verging meso- to megascopic, slightly asymmetric folds, preserved as evidence of high strain. The northern subdomain is characterized as a fold and thrust system that was tilted by the intrusion of late granitic bodies. The modeling of two gravimetric sections identified deep structures that suggest sutures between the Gavião and Serrinha crustal blocks. Here, we propose a model for the Archean evolution of the Mundo Novo Greenstone Belt and the subsequent deposition of sediments in the Jacobina Basin, followed by intrusion of mafic-ultramafic sills and dikes and Paleoproterozoic deformation.

The interaction between strike-slip dominated fault zones and thrust belt structures: Insights from 4D analogue models

Strike-slip motion is a fundamental tectonic process around the globe often resulting in prominent surface expressions. The full reconstruction and comprehension of strike-slip dominated structures can be a major challenge due to the complexity of the associated fault patterns. Further complexities occur when strike-slip dominated structures in the foreland interact with thrust belt fronts. Starting from a well-studied natural example (the Sciacca Fault Zone in the Sicilian Channel, Italy) we ran a set of analogue models simulating the interaction between strike-slip dominated fault zones and thrust structures using quartz sand as the analogue material and X-Ray Computed Tomography as a technique to carry out a 4D analysis of model internal structures. During a first phase, a thrust belt was created in our models, which was subsequently affected by a perpendicular fault system that underwent either pure strike-slip, 10°-, 20°-, 30°-transtensional, or 10°-, 20°-, 30°-transpressional motion. In general, transpressional models form pop-up structures, while the pure strike-slip model develops one sub-vertical fault bounded by two converging reverse faults. The 10˚-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. This model also shows a positive elevation change along the strike-slip dominated structures near the intersection with the thrust belt. The 20˚-transtensional model contains some Riedel shear faulting as well, but is dominated by two normal faults with steep dip angles and some minor sub-vertical strike-slip faults in between. This fault architecture is also observed in our 30˚-transtensional model. Three different tectonic regimes interacting with pre-existing and newly formed thrust front led to very different structural settings. These results were of help in better defining the tectonic regime that controlled the strike-slip dominated part of the Sciacca Fault Zone, but they show a fair match with other natural examples as well.

Evidence for Rupture Through a Double Benioff Zone During the 2017 Mw 8.2 Chiapas, Mexico Earthquake

AbstractWe combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

Structural characterization of intracratonic strike-slip faults in the central Tarim Basin

The strike-slip fault systems in the central Tarim Basin, China, afford an exceptional opportunity to document the structural characteristics and evolution process of small displacement intracratonic strike-slip faults using three-dimensional seismic reflection data. These strike-slip faults display subvertical segments at depth and en echelon normal fault zones where relatively shallow. Fault segmentation and flower structures can be commonly observed in plan view and cross-section view, respectively. Consistent with the notion that segment coalescence is the fundamental process for fault evolution, the mean segment length of representative strike-slip faults examined in this study is positively correlated to the measured fault offset. The width of the en echelon normal fault zone is positively correlated with the estimated maximum overburden thickness. The integrated data sets suggest that the evolution of the conjugate fault array followed a sequential evolution process instead of forming simultaneously. The switch in slip direction of the master fault of the conjugate fault array is attributed to the change of stress orientation. Regarding individual strike-slip faults, increase in displacement induces the formation of faults with lower fault-array angles linking initially formed en echelon normal faults. In cross sections, throughgoing fault surfaces can also form, connecting the lower subvertical fault segment and the upper en echelon normal faults. The presented data sets and evolution models established in this study can be used as tools to better predict the structural attributes of subsurface strike-slip fault systems with important consequences for reservoir formation and hydrocarbon accumulation in the Tarim Basin in particular, and in ancient marine basins in general.

Structural setting of the Koum sedimentary basin (north Cameroon) derived from EGM2008 gravity field interpretation

Abstract This study is a contribution to the planning of hydrocarbon exploration program of the Koum sedimentary basin in North Cameroon. 3D modeling of WGM2012 gravity data derived from EGM2008 geopotential model in the Koum basin was used together with existing geological and spectral analysis information to give structural picture of the basin. The 3D model of the Koum basin confirms that the basin is developed as a half graben bounded by sub-vertical faults. The thickness of the Cenozoic sediments is about 1.5 km in the eastern part and reaches 4.5 km in the western part of the basin. Gravity lineaments computed by multi-scale analysis revealed structural trends in the E–W, NW–SE, NE–WS and N–S directions. The faults in the sedimentary terrain reach 6 km depth and have a predominant NW–SE trend with E–W trending faults along the contact between the sedimentary section and the basement complex in the northern edge.