Oblique Thrust Research Articles

The 2024 Mw 7.0 Wushi Earthquake in Southern Tianshan Convergent Zone: Finite-Fault Model for the Coseismic Rupture and Aftershock

Abstract On 22 January 2024, an Mw 7.0 earthquake struck the oblique thrust Maidan fault (MDF) at the boundary between the Southern Tianshan and Tarim basin, making it the largest earthquake on the fault in the last 100 yr. Here, we use Interferometric Synthetic Aperture Radar (InSAR) to retrieve coseismic and aftershock deformation caused by the earthquake and then constrain fault geometry and slip distribution. Line-of-sight InSAR observations revealed a coseismic deformation area of 70 km × 60 km with a maximum value of ∼0.8 m and also captured the deformation from the aftershock event seven days after the mainshock. Our preferred two-segment coseismic fault model ruptures the MDF locked segment and deeper regions, exhibiting a homogeneous elliptical rupture with a maximum slip of ∼2.7 m on a rectangular plane with a dip angle of ∼60°, a length of ∼55 km, and a depth between 5 and 20 km. The aftershock formed a rupture plane of 10×8 km2 with the maximum slip of ∼0.3 m, causing slip in the shallow area of the fault where the coseismic ruptures were smaller, and supplementarily released the stress in the shallow part of the fault. The strong earthquake (Mw 7) return period on MDF inferred from the interseismic slip rate is 170–200 yr. Oblique thrust slip revealed by the slip distribution model is the result of long-term absorption of oblique convergence strain in the Southern Tianshan by the MDF and then concentrated release. The 2024 event resulted in a 60 km unruptured segment on the MDF being in a stress-triggering zone, increasing the potential seismic hazard, in contrast to the delayed seismicities on 100 km region of the southern Kalping fault (KPF).

The 8 September 2023, MW 6.8, Morocco Earthquake: A Deep Transpressive Faulting Along the Active High Atlas Mountain Belt

AbstractOn 8 September 2023 an MW 6.8 earthquake struck the High Atlas Mountains of western Morocco, about 70 km southwest from Marrakesh, causing significant devastation and casualties. In this study, we investigate a comprehensive geodetic data set, employing interferometric synthetic aperture radar measurements to assess the fault segment responsible for the seismic event. Our findings suggest two potential fault scenarios: either a transpressive NNW‐dipping high‐angle (70°) fault related to the Tizi n'Test alignment or a transpressive SSW‐dipping low‐angle (22°) fault associated with the North Atlas Fault, with slip (up to 2.2 m) only occurring on deeper parts of the fault. While seismic catalogs couldn't definitively determine the dip direction of the fault, evidence from mainshock locations, gravity and heat flow data and modeling, and active shortening direction suggest the activation of a low‐angle south‐westerly dipping oblique thrust of the North Atlas fault during the 2023 Moroccan earthquake.

Garnet growth and mineral geochronology constrains the diachronous Neoproterozoic convergent evolution of the southern Dom Feliciano Belt, Uruguay

AbstractThe Dom Feliciano Belt of southern Brazil and Uruguay represents part of a larger Neoproterozoic orogenic system formed during the amalgamation of Western Gondwana. The hinterland and foreland domains in parts of the belt preserve deformation structures and metamorphic assemblages that developed during early crustal thickening from c. 650 Ma. However, the metamorphic history of the southern foreland, in Uruguay, and its relationship with the hinterland, is not so well understood. We show that metamorphism in the southern hinterland is characterized by near‐isothermal decompression from ~10 kbar (~770°C) down to ~6 kbar, reflecting exhumation from depths of ~40 km during convergent thrusting and crustal thickening. This metamorphic event and associated magmatism is constrained by garnet Lu–Hf and zircon U–Pb dating to c. 655–640 Ma, supporting age and P–T constraints from previous studies. In contrast, prograde metamorphism in the foreland supracrustal rocks reached maximum lower‐amphibolite facies conditions (~6–7 kbar and ~550–570°C) and is constrained by garnet Lu–Hf dating to 582 ± 23 Ma. An exposed sheet of imbricated foreland basement rocks reached partial melting at upper‐amphibolite facies conditions, and metamorphism is similarly constrained to c. 585–570 Ma by monazite U–Pb dating. The data indicate that metamorphism in the foreland occurred during a sinistral transpressional event c. 55–85 Ma after the start of crustal thickening recorded in the hinterland, whereby strain partitioning during sinistral transpression led to imbrication in the foreland and oblique thrusting of the basement over more distal supracrustal rocks. This event is coeval with transpressional deformation in the Kaoko and Gariep belts, indicating a distinct two‐stage tectonic history driven by the three‐way convergence between the Congo, Kalahari, and South American cratons.

Prospecting for ophiolite-type chromite deposit in Sartohay, West Junggar (NW China): Constraints from geological and geophysical data

Chromite is a critical mineral that is of great significance for the development of metallurgical, chemical, and other industries. In order to study the deep ore prospecting methods of ophiolite-type chromitite, we have carried out plane-profile geological and mineral surveys, as well as high-precision gravity, magnetic, and controlled source audio-frequency magnetotelluric (CSAMT) measurements in the Sartohay, West Junggar. Based on the analysis, interpretation, and inference of multi-source geological information, a three-dimensional (3D) geological model of the typical mining area in Sartohay District was established. The results show that Sartohay ophiolite was occurred in the form of rootless tectonic blocks and generally displayed deformation and metamorphism (e.g., oblique thrust, transverse extrusion, and lateral strike slip). Sartohay ophiolite is in tectonic contact with surrounding strata. Several northwest dipping high-angle thrust faults are developed at the contact sites and inside the ophiolite rock mass, showing a thrust imbricated structure. Meanwhile, ophiolite in the modeling area is a NW-trending monoclinic rock mass. The base surface of the ophiolite is obliquely distributed in an “N-like” shape, and the chromitite bearing lithofacies zones also have a NW-trending distribution. Geophysical data reveal that the studied mining area is characterized by a layered electrical structure in the vertical direction. The upper layer displays a high-resistivity agglomerate and a low-resistivity band distributed alternately, while the lower layer is a complete high-resistivity block. Combined with the deposit characteristics in Sartohay, it is considered that the morphological occurrence, geological structure, lithology, and lithofacies of ophiolite control the spatial distribution of related chromitites. The dunite-harzburgite lithofacies belts, highly strain zones, and strong tectonic alteration belts of the ophiolite rock mass are significant sites for the chromite ore bodies/groups occurrence. This study provides approaches for researching on the production laws of ophiolite-type chromitite and supplys a prospecting pattern for deep concealed orebodies.

Rupture Scenarios for the 3 June 1770 Haiti Earthquake

ABSTRACT The 2010 M 7.0 Haiti earthquake provided the impetus to reconsider historical earthquakes in Hispaniola (Bakun et al., 2012). That earthquake also shed new light on complex fault systems along Haiti’s southern peninsula (Douilly et al., 2013; Saint Fleur et al., 2015). Recently, the 2021 M 7.2 Nippes earthquake (Calais et al., 2022; Douilly et al., 2022), and a recent study reconsidering the 1860 sequence (Martin et al., 2022) further underscored the complexity of fault systems and large earthquake ruptures along the peninsula. Motivated by these studies and recent geological investigations (Prentice et al., 2010; Saint Fleur et al., 2020), we reconsider the 3 June 1770 Haiti earthquake to explore the conventional assumption that it was the last major (M ≥7.5) earthquake along the Enriquillo–Plantain Garden fault (EPGF). Accounts provide compelling evidence for substantial liquefaction in the Cul-de-Sac plain, one or more likely landslide-driven tsunami in Gonaïves Bay, and extensive landsliding that created at least three documented landslide dams. We consider three end-member rupture scenarios that are consistent with available constraints: two scenarios with M 7.7 and rupture lengths of 150–170 km, and one scenario with a ∼90 km rupture and M 7.5. Absent future work to identify and date paleoevents along the southern peninsula, none of these scenarios can be ruled out. Our preferred rupture model extends from the Miragoâne pull-apart to near la Selle mountain, with a rupture length of 127 km, M 7.6, and a high stress drop. Rupture could have been on the EPGF or on an oblique thrust fault associated with overthrusting of the Massif de la Selle. The results do support the conclusion that the 1770 earthquake was the last major earthquake in southern Haiti, with a magnitude upward of M 7.5 and significantly more severe shaking in southern Haiti than during the 2010 earthquake.

The effects of pre-stress assumptions on dynamic rupture with complex fault geometry in the San Gorgonio Pass, California, region

Abstract We use three-dimensional (3-D) dynamic finite-element models to investigate potential rupture paths of earthquakes propagating along faults through the western San Gorgonio Pass, a structurally complex region along the San Andreas fault system in southern California (USA). We focus on the right-lateral San Bernardino strand of the San Andreas fault system, the oblique thrust–right-lateral San Gorgonio Pass fault zone, and a portion of the right-lateral Garnet Hill strand of the San Andreas fault system. We use the 3-D finite-element method to model rupture propagation along a fault geometry that reflects current understanding of the local geometrical complexity and is consistent with long-term loading and observed surface deformation. We test three different types of pre-stress assumptions: (1) constant tractions (assuming pure right-lateral strike-slip motion on the San Bernardino and Garnet Hill strands and oblique thrust–right-lateral strike-slip motion on the San Gorgonio Pass fault zone), (2) a uniform regional stress regime, and (3) long-term (evolved) stress from quasi-static crustal deformation modeling. Our results imply that under the more realistic regional stress and evolved stress assumptions, throughgoing rupture propagation from the southeast to northwest (i.e., from the Garnet Hill to the San Bernardino strand) may be more likely than throughgoing rupture in the reverse direction (from the San Bernardino to the Garnet Hill strand). The results may have implications for the earthquake potential in the region as well as for ground motion in the Los Angeles Basin. The results also emphasize how fault geometry and stress patterns combine to influence rupture propagation on complex fault systems.

Design and motion control analysis of double helix wall climbing robot

For the detection environment of complex walls such as high-rise buildings, a double helix wall climbing robot (DHWCR) with strong adsorption force and good stability is designed and developed, which uses symmetrical propellers to provide adsorption force. The symmetrical driving structure can provide smooth thrust for the DHWCR, so that the robot can be absorbed to the wall surface with different roughness. A left and right control frame with multiple degrees of freedom is designed, which can adjust the fixed position of the brushless propeller motor in the front and back directions, realize the continuous adjustable thrust direction of the robot, and improve the flexibility of the robot movement. Using the front wheel steering mechanism with universal joint, the steering control of the DHWCR is realized by differential control. In the vertical to ground transition, the front and rear brushless motors can provide the pull up and oblique thrust, so that the DHWCR can smoothly transition to the vertical wall. The motion performance and adaptability of the DHWCR in the horizontal ground and vertical wall environment are tested. The results show that the DHWCR can switch motion between the horizontal ground and vertical wall, and can stably adsorb on the vertical wall with flexible attitude control. The DHWCR can move at a fast speed. The speed on the horizontal ground is higher than that on the vertical wall, which verifies the feasibility and reliability of the DHWCR moving stably on the vertical wall.

Shear zone fabrics and their significance in curved, inverted basin-derived thrust systems

In this study, we report how criteria inferred from structural and kinematic analysis of shear zone fabrics can be instrumental in deciphering events of positive or negative inversion tectonics. Mesoscopic structural and kinematic analyses are performed on well-exposed brittle-ductile thrust shear zones within curved thrust systems in the Central-Northern Apennines of Italy.The structural and kinematic features of thrust-related tectonites make it possible to outline a history of positive inversion. S/C- and S-tectonites/fabrics are distinctive of frontal or oblique thrust ramps, respectively. In particular, S-fabrics result from selective transpressional-reactivation of pre-thrusting normal faults. By contrast, more complex composite fabrics, characterized by folded cleavage, reveal a history of negative inversion tectonics. Mesocopic cleavage folding is ascribed to shear-sense reversal along the shear surfaces during extensional reactivation of pre-existing thrusts; or to synthetic reactivation by listric foreland-dipping normal faults, merging downwards on former thrust surfaces during protracted episodes of gravitational collapse.Shear zone fabrics identification criteria, here illustrated from thrusts of different size, if systematically used, may be diagnostic of different deformation paths, and hence provide useful tools to elucidate the tectonic history of curved thrust systems in orogens that have experienced multiple episodes of inversion.

Free‐Surface‐Induced Supershear Transition in 3‐D Simulations of Spontaneous Dynamic Rupture on Oblique Faults

AbstractSeveral observations and simulations suggested that the supershear rupture induced by free surface invariably occurs in strike‐slip faults rather than in dip‐slip faults. To explore the mechanism of supershear transition induced by free surface on oblique faults, we simulate spontaneous dynamic rupture propagation governed by slip‐weakening friction on nonvertical planar faults. We find that oblique thrust faults are more likely to produce free‐surface‐induced supershear transition than oblique normal faults and significant asymmetry arises between the two opposite directions along fault strike. The supershear rupture may only occur within shallow depth or over the entire seismogenic depth, depending on geometric relations between the slip and rupture propagation vectors. Our results help explain the lack of universal observation of supershear earthquakes. Besides, the asymmetric ground shaking on the oblique faults allows us to predict the possible spatial distribution of the high seismic risk.

Regional subsurface mapping and 3D flexural modeling of the obliquely converging Putumayo foreland basin, southern Colombia

The Putumayo foreland basin (PFB) is an underexplored hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250 km long segment of the 7000 km long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We have used approximately 4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across strike, structural zones include (1) the 20 km wide, western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the Late Miocene, (2) the 45 km wide, central structural zone characterized by moderately inverted Jurassic half-grabens, and (3) the 120 km wide, eastern structural zone (ESZ) characterized by the 40 km wide, north–south-trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include (1) Lower Cretaceous pre-foreland basin deposits, (2) upper Cretaceous-Paleocene foreland basin deposits, (3) Eocene foreland basin deposits related to the early uplift of the eastern Cordillera, (4) Oligocene-Miocene underfilled, foreland basin deposits, and (5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness ([Formula: see text]) of the lithosphere below the PFB, and to model the location of the sedimentary-related and tectonically related forebulges of Cretaceous to Oligocene age. Flexural analysis finds two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in ESZ and controls a basement high that forms the eastern updip limit for most hydrocarbons found in the PFB.

Possibility of an earthquake prediction based on monitoring crustal deformation anomalies and thermal anomalies at the epicenter of earthquakes with oblique thrust faulting

Is it possible to predict location, time and magnitude of earthquakes through identifying their precursors based on remotely sensed data? Earthquakes are usually preceded by unusual natural incidents that are considered as earthquake precursors. With the recent advances in remote sensing techniques which have made it possible monitoring the earth’s surface with different sensors, scientists are now able to better study earthquake precursors. Thus, the present study aims at developing the algorithm of classic PS-InSAR processing for obtaining crustal deformation values at the epicenter of earthquakes with magnitude larger than 5.0 on the Richter scale and with oblique thrust faulting and then after calculating temperature values using remotely sensed thermal imagery at the epicenter of same earthquakes; thermal and crustal deformation anomalies were calculated using data mining techniques before earthquake occurrence. In the next stage, taking the correlation between thermal anomalies and crustal deformation anomalies at the epicenter of the study earthquakes into account, an integrated technique was proposed to predict probable magnitude and time of oblique thrust earthquakes occurrence over the earthquake-prone areas. Eventually, the validity of the proposed algorithm was evaluated for an earthquake with a different focal mechanism. The analysis results of the thermal anomalies and crustal deformation anomalies at the epicenter of April 16, 2016, Japan-Kumamoto earthquake of magnitude 7.0 with strike-slip faulting, showed completely different trends than the suggested patterns by the proposed algorithm.

Oblique Thrust of the Maidan Fault and Late Quaternary Tectonic Deformation in the Southwestern Tian Shan, Northwestern China

AbstractThe Maidan fault, which is an east‐northeast trending fault in the southwestern Tian Shan, is a sinistral reverse fault that extends more than 400 km in length and constitutes the boundary between the southwestern Tian Shan and the Tarim Basin. Here we quantify its late Quaternary activity based on the interpretations of high‐resolution remote sensing images and detailed field investigations. In the Aheqi valley, an ~150‐km‐long active fault can be divided into northeastern and southwestern segments based on variations in its strike and geometry. Based on the analysis of its offset geomorphological features and the dating of Quaternary sediments, we estimate the late Quaternary shortening rate across the fault to be 1.19 ± 0.25 mm/year, the sinistral strike‐slip rate to be 1.56 ± 0.64 mm/year, and the oblique thrust rate to be 1.96 ± 0.69 mm/year. Active tectonics, GPS crustal deformation data, and seismic activity indicate that the deformation in the southwestern Tian Shan is characterized by out‐of‐sequence thrust faulting and folding. Late Quaternary deformation has been partitioned into low‐angle thrust faulting along the Kalpin Tagh foreland fold and thrust system and sinistral reverse faulting along the high‐angle range‐front Maidan fault. The sinistral Maidan fault acts as a nucleation point for slip partitioning system, which can be viewed as positive flower structure with its surrounding thrust faults.

The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt

SUMMARYIn this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (SH) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

3D geometric modeling for the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, China: Oblique thrusting kinematic implications

Three-dimensional (3D) subsurface seismic data acquisition and modeling are rarely undertaken; consequently, the complex effects of oblique thrusting on the 3D morphology and kinematics of folds are still poorly understood. We use 3D seismic data to produce five 3D geometric models and compare our results with structural models of the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, to investigate the effects of oblique thrusting on 3D fold evolution. 3D geometric modeling results reveal that oblique thrusting may cause fold asymmetry, vertical axis rotation, and a curved fold axis. 3D seismic interpretations and structural modeling suggest that the Yanjinggou anticline developed as a trishear fault-propagation fold and experienced a maximum shortening amount of 1.2 ± 0.1 km. The 3D structural and geometric models are consistent with paleomagnetic results, suggesting that the anticline was affected by oblique thrusting at an angle of 55° above a listric thrust fault with three fault ramps (19°, 30°, and 40°), causing the fold to rotate and develop a curved morphology. The integrated method used in this study to investigate the effects of oblique thrusting on 3D fold morphology and kinematics can be applied to regional oblique thrusting in the Longmen Shan and other fold-and-thrust belts worldwide.

Mesoscopic, magnetic and petrofabric study of the High Himalayan gneisses and leucogranite along oblique and frontal ramps of the Vaikrita Thrust in Satluj and Bhagirathi valleys: Thrust locking and superposed folding

Field, microstructural and anisotropy of magnetic susceptibility (AMS) studies were performed along the oblique thrust ramp of the Satluj river valley and frontal thrust ramp of the Bhagirathi river valley. The presence of asymmetric and sheath folds indicates intense shearing near the thrust. The simple shear gradually decreases with an increase in distance from the thrust and pure shear with upright folds becomes prominent. The AMS studies suggest slightly higher pure shear in the frontal ramp region as compared to the oblique ramp region where oblique slip was predominant. The sense of shear criteria and orientation of superposed folds indicate locking of the thrust. Normal faults are the youngest structure.

Deformation within the Cycladic subduction–exhumation channel: new insights from the enigmatic Makrotantalo nappe (Andros, Aegean)

New detailed geological/structural mapping coupled with microstructural investigations and amphibole chemistry analyses are used to elucidate the enigmatic tectonic evolution of the Makrotantalo nappe on northern Andros (Aegean region). On the basis of the new geological map, we re-evaluate the structural position of samples with previously published metamorphic ages to show that the Makrotantalo nappe has been affected by the early Eocene high-pressure (HP) metamorphism. Therefore, the Makrotantalo nappe is an integral part of the Cycladic Blueschist Unit (CBU). The early, main ductile deformation phase recognized in the mapped area, is associated with the emplacement of the Makrotantalo nappe over the North Cyclades nappe at deep subduction levels during early Eocene. The subsequent deformation phase occurred at the middle Eocene–Oligocene synchronous with the decompression of blueschist rocks from the stability field of glaucophane to that of actinolite. During this phase, the nappe pile was folded by map-scale folds, which were locally transposed by ductile shear zones. These shear zones were related with NE-directed oblique thrusting and, locally, either restacked the nappe pile or reworked the early nappe contact. Our results in combination with previous studies in the northern Cyclades show that the top-to-the-NE shearing was dominant at a large part of the CBU during exhumation. Opposite, SW-directed shearing was restricted at the upper structural levels of the CBU. We suggest that the observed spatial variation in shear sense is compatible with NE- rather than SW-directed ductile extrusion of the CBU.

A case study of compression to escape tectonic transition: Tectonic evolution of the Nallıhan Wedge and comparison with the Tercan Wedge (Eastern Mediterranean, Turkey)

The Nallıhan and Çayırhan regions in western Anatolia host the stratigraphically and tectonically important features of Nallıhan Wedge. The stratigraphic relationships and structural architecture of the wedge are not well understood, which are essential for better understanding of the timing and tectonic evolution of western Anatolia. Today, the Nallıhan Wedge is bounded by the North Anatolian Fault and the Thrace-Eskişehir Fault in the northwest and southwest, respectively. The existence of Dağküplü Mélange within the wedge represents the closure of the İzmir-Ankara Ocean. The occurrence of the Lower Jurassic shelf, slope, reef and deep sea successions transitioning upward into the Cretaceous shelf and slope successions represent the opening and closing cycles of the Intra-Pontide Ocean. The tight and asymmetric folds (steep in the west, gentle in the east) within this stratigraphic sequence and the development of the Eocene units reflect the closure of the Intra-Pontide and İzmir-Ankara oceans. The geometric structure and evolution of the Nallıhan Wedge is directly related to the final continental collision. W-E and SW-NE striking large-scale oblique thrusts also exhibit notable strike-slip components within the study area. The principal direction of the fold axes are W-E and WNW-ESE oriented for the Eocene and Neogene units. During the Eocene, the depositional environment of the study area was a ramp basin mainly dominated by a braided river, when a portion of the Intra-Pontide Ocean was situated on the northern side of the study area. The compressional tectonic regime continued to the end of the Oligocene, which is followed by a new tectonic regime during the Miocene, when the triangular shape region between the Thrace-Eskişehir Fault and the North Anatolian Fault developed as an intermontane basin. The evolution of this basin can be compared with the Tercan Wedge, bordered by the North Eastern Anatolian Fault and the North Anatolian Fault.In the Nallıhan and Çayırhan regions the N-S-trending compressional regime started during the Upper Cretaceous and continued until the final continental collisional during the Upper Eocene. The compressional structures in the area became reactivated during the post-collisional escape tectonic phase in western Anatolia, leading to the development of the prominent Isparta Angle during the Miocene. During this period, the Thrace-Eskişehir Fault and Burdur-Fethiye Shear Zone also formed as intracontinental transform fault zones in western Anatolia. The Nallıhan Wedge started to form in a wedge-shaped area between the Thrace-Eskişehir Fault and the zone of strike-slip faults with thrust component that later developed into the North Anatolian Fault. Thus, Eocene and pre-Eocene structures obtain strike-slip component by an inversion during the Miocene. During and post Pliocene, the Thrace-Eskişehir Fault lost its tectonic activity as the North Anatolian Fault propagated westward into the region previously controlled by the Thrace-Eskişehir Fault. During this interval the Nallıhan Wedge entered a new stage of deformation, which is also observed today.

From widespread Mississippian to localized Pennsylvanian extension in central Spitsbergen, Svalbard

Abstract. In the Devonian–Carboniferous, a rapid succession of clustered extensional and contractional tectonic events is thought to have affected sedimentary rocks in central Spitsbergen, Svalbard. These events include Caledonian post-orogenic extensional collapse associated with the formation of thick Early–Middle Devonian basins, Late Devonian–Mississippian Ellesmerian contraction, and Early–Middle Pennsylvanian rifting, which resulted in the deposition of thick sedimentary units in Carboniferous basins like the Billefjorden Trough. The clustering of these varied tectonic settings sometimes makes it difficult to resolve the tectono-sedimentary history of individual stratigraphic units. Notably, the context of deposition of Mississippian clastic and coal-bearing sedimentary rocks of the Billefjorden Group is still debated, especially in central Spitsbergen. We present field evidence (e.g., growth strata and slickensides) from the northern part of the Billefjorden Trough, in Odellfjellet, suggesting that tilted Mississippian sedimentary strata of the Billefjorden Group deposited during active (Late/latest?) Mississippian extension. WNW–ESE-striking basin-oblique faults showing Mississippian growth strata systematically die out upwards within Mississippian to lowermost Pennsylvanian strata, thus suggesting a period of widespread WNW–ESE-directed extension in the Mississippian and an episode of localized extension in Early–Middle Pennsylvanian times. In addition, the presence of abundant basin-oblique faults in basement rocks adjacent to the Billefjorden Trough suggests that the formation of Mississippian normal faults was partly controlled by reactivation of preexisting Neoproterozoic (Timanian?) basement-seated fault zones. We propose that these preexisting faults reactivated as transverse or accommodation cross faults in or near the crest of transverse folds reflecting differential displacement along the Billefjorden Fault Zone. In Cenozoic times, a few margin-oblique faults (e.g., the Overgangshytta fault) may have mildly reactivated as oblique thrusts during transpression–contraction, but shallow-dipping, bedding-parallel, duplex-shaped décollements in shales of the Billefjorden Group possibly prevented substantial movement along these faults.

The 2014 Cephalonia Earthquakes: Finite Fault Modeling, Fault Segmentation, Shear and Thrusting at the NW Aegean Arc (Greece)

We present refined finite fault models (FFM) for the 2014 Cephalonia (Keffalinia, Kefalonia) seismic sequence (Mw ~ 6.0), at the NW edge of the Aegean Arc (Ionian Sea). The area represents the seismically most active part of Europe and a continental promontory in which fault modeling is a challenge because of structural complexity and poor coverage by seismological, GPS and InSAR data. Inversion was based on GPS data and a new algorithm permitting fusion of slip vectors of individual earthquakes and of their cumulative dislocation and accepting constraints and collocation-type analysis of uncertainties. Computed FFM, which correspond to an essentially strike-slip fault and a blind, shallow oblique slip thrust, were assessed by sensitivity analysis and InSAR data and are consistent with the tectonic fabric of the area. They can also explain the observed extreme peak ground accelerations. The 2014 faults, in combination with FFMs of the 2003 and 2015 Leucas (Lefkada, Lefkas) earthquakes farther NE and of the 1983 M7.0 earthquake farther SW, constrain a > 100 km long immature, strike-slip fault zone along/close to the Cephalonia–Leucas coasts. This fault pattern, previously regarded as a poorly documented Cephalonia Transform Fault, consists of occasionally overlapping oblique slip segments with variable geometric and kinematic characteristics in a shear zone landwards of the plate interface, as evidence from seismic profiles reveals. This pattern may explain the enigmatic superimposition of shear and compression in the NW edge of the Aegean Arc.

Oblique Thrusting and Strain Partitioning in the Longmen Shan Fold‐and‐Thrust Belt, Eastern Tibetan Plateau

AbstractEastern Tibet is an important example of oblique convergence and associated strain partitioning, as suggested by recent 2‐D and 3‐D structural interpretations, yet the nature and evolution of oblique strain partitioning of this region remain poorly known. Here we use seismic reflection profiles, borehole data, and field investigations in the Longmen Shan piedmont to determine the subsurface structural architecture, and we observe several nearly N‐S striking thrusts and reactivation of NE striking preexisting faults. We interpret that this behavior is due to a regional principal compressional stress oriented in the E‐W direction, oblique to the NE striking Longmen Shan. Using the records of fault activity and related Late Pliocene and Quaternary foreland sediments and growth strata, as well as the coseismic rupture of the 2008 Mw 7.9 Wenchuan earthquake, we demonstrate that the Longmen Shan has experienced E‐W crustal shortening and oblique motion since ~5–2 Ma. We present two strain partitioning models arising from oblique thrusting in eastern Tibet and suggest that the eastward extrusion from Tibet is mainly accommodated on the strike‐slip Longriba fault and the dip‐slip Longmen Shan‐Min Shan fault zones. These results enhance our understanding of the tectonic relationship between the Songpan‐Ganzi terrane and the Sichuan basin and provide additional constraints for studies of the geodynamic response of eastern Tibet to the ongoing India‐Eurasia collision.