The 2020 Mw 6.0 Jiashi Earthquake: Coinvolvement of Thin-Skinned Thrusting and Basement Shortening in Shaping the Keping-Tage Fold-and-Thrust Belt in Southwestern Tian Shan

The Keping-tage fold-and-thrust belt in southwest Tian Shan is seismically active, yet the most well-recorded earthquakes occurred south of the mountain front. The lack of large earthquakes beneath the fold-and-thrust belt thus hinders our understanding of the orogenic process to the north. The 2020 Mw 6.0 Jiashi earthquake is an important event with surface deformation in the fold-and-thrust belt well illuminated by Interferometric Synthetic Aperture Radar, providing an opportunity to study the present-day kinematics of the thrust front through the analysis of satellite measurements of surface deformations. Here, we employ the surface deformation and relocated aftershocks to investigate the fault-slip distribution associated to this event. Further added by an analysis of Coulomb stress changes, we derive a fault model involving slips on a shallow, low-angle (∼10°) north-dipping thrust fault as well as on a left-lateral tear fault and a high-angle south-dipping reverse fault in mid-crust. Aftershocks at depth reflect the basement-involved shortening activated by a thin-skinned thrust faulting event. In addition, this earthquake uplifted the southernmost mountain front with relatively low topography, indicating the basin-ward propagation of the southwest Tian Shan.

Spatio-Temporal Evolution of Intermediate-Depth Seismicity Beneath the Himalayas: Implications for Metamorphism and Tectonics

Intermediate-depth earthquakes (>40 km) have been observed beneath the central Himalayas over decades, with little known about their nature and characteristics. Here, we apply a state-of-the-art systematic processing routine, starting from continuous waveform data, to obtain the most comprehensive high-quality earthquake catalog with a focus on the intermediate-depth seismicity beneath the central Himalayas. We construct a catalog containing 414 robust earthquake locations with depths ranging from 40 to 110 km spanning from late 2001 till mid-2003. We calculate earthquake magnitudes in a consistent way and obtain values ranging between ML 0.8 and 4.5 with a magnitude of completeness of Mc 2.4. This information allows us to study the spatiotemporal characteristics of the seismicity in great detail. Earthquakes mainly take place in a cluster, consisting of two linear segments at ca. 35° azimuth difference, situated beneath the high Himalayas in NE Nepal and adjacent S. Tibet. Seismicity there does not feature any mainshock-aftershock patterns but presents a few sequences with potential seismicity migration rates compatible with linear or diffusive migration. This result, along with previous studies in the lower Indian crust, allows interpreting these events as related to metamorphic reactions involving dehydration processes. However, given the geodynamic context, a tectonic interpretation with a dextral basement fault zone propagating beneath the Himalaya and continuing as a westward propagating tear fault would also be possible. This represents a continuous fault zone from the deep crust in S. Tibet, across the Himalaya along the Dhubri-Chungthang fault zone (DCFZ) to the Shillong plateau, which could be an inherited tectonic feature.

U-Pb dating of middle Eocene-middle Pleistocene multiple tectonic pulses in the Alpine foreland

Foreland fold-and-thrust belts record long-lived tectonic-sedimentary activity, from passive margin sedimentation, flexuring, and further involvement into wedge accretion ahead of an advancing orogen. Therefore, dating fault activity is fundamental for plate movement reconstruction, resource exploration, or earthquake hazard assessment. Here, we report U-Pb ages of syntectonic calcite mineralizations from four thrusts and three tear faults sampled, at the regional scale, across the Jura fold-and-thrust belt in the northwestern Alpine foreland (eastern France). Four regional tectonic phases are recognized in the middle Eocene-middle Pleistocene interval: (1) pre-orogenic faulting at 44.7 ± 2.6 and 48.4 ± 1.5 Ma associated to the uplift of the Alpine forebulge, (2) syn-orogenic thrusting at 11.4 ± 1.1, 10.6 ± 0.5, 9.7 ± 1.4, 9.6 ± 0.3, and 7.5 ± 1.1 Ma associated to possible in-sequence thrust propagation, and (3) syn-orogenic tear faulting at 10.5 ± 0.4, 9.1 ± 6.5, 7.3 ± 1.9, 5.7 ± 4.7, 4.8 ± 1.7, and at 0.7 ± 4.2 Ma including the reactivation of a pre-orogenic fault as tear fault at 3.9 ± 2.9 Ma. Previously unknown faulting events at 44.7 ± 2.6 and 48.4 ± 1.5 Ma predate by ~ 10 Ma the accepted late Eocene age for tectonic activity onset in the Alpine foreland. In addition, we dated the previously inferred strike-slip faults re-activation as tear fault. The U-Pb ages demonstrate the long-lived tectonic history at the plate boundary between European and African plates and that the deformation observed in the foreland is directly linked to continental collision.

Forearc Crustal Structure of Ecuador Revealed by Gravity and Aeromagnetic Anomalies and Their Geodynamic Implications

Along the Western Cordillera of Ecuador, fault-bounded ophiolites derived from the Late Cretaceous Caribbean Large Igneous Province (CLIP) have provided key petrotectonic indicators that outline the nature and the mechanism of continental growth in this region. However, most of the forearc basement across Western Ecuador is buried under sediments impairing its crustal structure understanding. Here, we propose a first crustal model throughout the spectral analysis of gravity and aeromagnetic data, constrained by observations made both at the surface and at the subsurface. Three main geophysical domains, within the North Andean Sliver in Western Ecuador, have been defined based on spectral analysis and augmented by 2D forward models. An outer domain, characterized by magnetic anomalies associated with mafic rocks, coincides with evidence of a split intraoceanic arc system. An inner domain is governed by long-wavelength mid to deep crust-sourced gravity and magnetic anomalies possibly evidencing the root of a paleoisland arc and the residuum of a partial melting event with subsequent associated serpentinization, the latest possibly associated with an obduction process during the middle Eocene-Oligocene. In addition, our model supports the presence of a lithospheric vertical tear fault, herein the southern suture domain, inherited from an oblique arc-continent interaction. Our interpretation also brings new insights and constraints on the early geodynamic evolution of the Ecuadorian forearc and provides evidence on the structural style and preservation potential of the forearc basement, most likely the roots of a mature island arc built within an oceanic plateau.

Geologic Map of the Park Reservoir Quadrangle, Sheridan County, Wyoming

This project involved the construction of a detailed geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques were used in concert with isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. This research supports the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, two sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead Sandstone, two Quaternary tills, and Quaternary alluvium. The Archean rock units range in age from ca. 2.96–2.75 Ga, the oldest of which are the most ancient rocks yet reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek thrust block, is mapped into the Archean core as a mylonite zone. This relationship indicates that the bounding faults of the Piney Creek thrust block were controlled by weak zones within the Precambrian basement rocks.

Structure and genesis of the Boulder-Weld allochthon, Denver Basin, Colorado - Gravity slide or Laramide thrust sheet?

This study was undertaken to determine the structure and genesis of the Boulder-Weld allochthon (BWA), the 216 mi2 (559 km2) remnant of a once larger feature, that moved east from the flank of the Front Range into the western part of the Denver Basin. This review of surface and subsurface data revealed new aspects of the BWA, especially in its western part. There, the decollement of the BWA ramps 900 feet up-section to the east from a near bedding-parallel detachment low in the upper transition member of the Pierre Shale to a bedding-parallel detachment near the base of the Fox Hills Formation. Repeated sections found in wells east of the decollement ramp demonstrate up to two miles of translation in the system. Secondary faults in the hanging wall of the allochthon include antithetic thrusts bounding pop-up structures and occasional normal faults that almost exclusively overprint the decollement ramp. The hanging wall is also cut by a postulated tear fault separating areas exhibiting different amounts of translation. The western, trailing edge of the decollement shows attenuation in its hanging wall that increases to the west. This part of the decollement either represents a very low-angle breakaway normal fault or a thrust fault cutting slightly down-section in the direction of transport. Past studies perceived a southeast transport direction for the BWA in contrast to the northeast slip directions on nearby Laramide thrusts, a difference used to interpret the allochthon as a gravity slide. However, similar east-west oriented slickenlines on thrusts across the western part of the allochthon and into the neighboring Front Range leave open the possibility the BWA originated as a Laramide thrust sheet. Furthermore, both the BWA and Laramide thrusts in the neighboring Front Range utilized detachments near the top of the Pierre Shale, suggesting a possible common genesis. Given the available data, both the gravity slide and Laramide thrust models provide viable explanations for the BWA.

Evolution of tear faults in subduction zones: an analogue modelling perspective

<p>Tear faults are common structures in subduction zones, especially at slab edges, where they origin from differential forces applied to a subducting slab in areas close to the trench. Presence and geometry of tears have been sometimes inferred from bathymetric features, suggesting the abrupt lateral termination of the subduction zone.</p><p>Differential forces acting at the subduction boundaries can be related to different mechanisms, such as slab retreat, differential velocities along plate margins, complex mantle flow, differential lateral rheology. As a result, plates down-warp and tear in a scissor-like motion, with both strike-slip and dip-slip kinematics.</p><p>The goal of this work is to gain insights into the evolution of tear faults by adopting an analogue modelling approach and comparing the results with natural cases. In particular, we focus on the bathymetric observation made in subduction zones where the upper plate accretionary wedge is not well developed. Two scenarios were considered: 1) tear faults nucleating and evolving in a homogeneous setting, i.e. without large mechanical discontinuities (e.g., Tonga subduction zone); and 2) tear faults reactivating pre-existing strike-slip faults as an analogue of transform faults (e.g., South Sandwich subduction zone).</p><p>The experimental apparatus was designed to reproduce the lateral propagation of a tear fault using two blocks: one entirely flat and the other with an inclined plane. Wet kaolin acts as the analogue of the intact rocks above a propagating tear fault.</p><p>Our results revealed different evolutionary processes: in the homogeneous setting, the tear fault generates a symmetric subsidence zone with an axis perpendicular to the fault zone and a depocenter located in the centre; in the second case, the depocenter is located in front of the fault plane and the subsidence zone is asymmetric. Both cases depict a symmetrical Gaussian shape of the displacement profile, with the maximum displacement located at the centre of the fault. However, the maximum slip (D<sub>max</sub>) and the fault length (L)  are both larger in the experiment involving a strong re-activation of the strike-slip fault than those in the case of the homogeneous setting.</p>

Image of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

<p>We obtain P-wave receiver functions from recordings at a dense seismic broadband transect, deployed along 170 km across the eastern Betic orogen in south Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the ~50 km wide necking domain. Crustal thinning affects the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching ~45 km depth, and terminates abruptly at a major slab tear fault. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic Orogene. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust.</p>

Online measurement of torsional stiffness and fault analysis for flexible coupling under working condition

In this paper, a method of measuring the relative torsional angles of the flexible coupling under working condition was developed. A double-encoder measurement system was proposed to get the torsional angles. The torsional angles were divided into “constant torsional angle” and “alternating torsional angle”. The measured results were analyzed further, which were applied to a series of flexible couplings. Firstly, the torsional stiffness of D-type flexible coupling was obtained according to the relationship between torques and torsional angles. The proposed method overcomes the difficulty that the angular displacements of flexible coupling cannot be measured online under operating condition. The feasibility and accuracy of the method developed in this study was verified by comparing stiffness curve with unloading curve obtained from static torsional experiment and values from factory inspection report. Furthermore, in view of the tear fault of A-type flexible coupling in a diesel generator set, the maximum alternating torsional angles of A-type, B-type, and C-type flexible couplings were measured under working condition, then safety margins were obtained for the diesel generator set. According to the safety margins of the three kinds of couplings, C-type flexible coupling is chosen to replace A-type, which provided the basis for type selection of flexible coupling. Finally, the method developed in this study would also be applied to the operation monitoring and fault diagnosis of flexible coupling online.