Right-lateral Shear Research Articles

Shortening and asthenospheric flow drove Miocene vertical-axis rotations in the northern Tibetan Plateau: A palaeomagnetic study of the northeast Qaidam Basin of China

The timing and magnitude of upper crustal deformation in the northern Tibetan Plateau are important for testing models of plateau uplift and for understanding Cenozoic environmental variations and monsoon evolution in central and eastern Asia. Most published studies focus on the upward and outward growth of the northern Tibetan Plateau, whereas its tectonic, vertical-axis rotation history has received less attention. Here we evaluate paleomagnetic data from six stratigraphic sections in the northeastern Qaidam Basin of China, revealing a phase of clockwise rotation from ca. 20–11 Ma followed by counterclockwise rotation from ca. 11–5 Ma. We propose that differential crustal shortening and right-lateral shear drove early to middle Miocene clockwise rotation, whereas eastward flow of the Tibetan asthenosphere induced the post-middle Miocene counterclockwise rotation. These processes coincide with increased activity of first-order strike-slip faults in the northeastern plateau, emphasizing the important role of strike-slip faults in accommodating crustal deformation in northern Tibetan Plateau during the late Cenozoic.

Effect of pH on primary and secondary crack propagation in sandstone under constant stress (creep) loading

Time-dependent, chemically assisted crack propagation behavior in rocks is fundamental to understanding the long-term stability of civil engineering structures. In this study, we investigated the propagation of primary and secondary cracks in macrofractured sandstone in distilled water (pH = 5.6) and in an aqueous solution of hydrochloric acid (HClaq, pH = 1.82) using digital image correlation (DIC), comparing the results to those from experiments performed in air. Results show that for the macrofracture angles γ = 0 and 90°, the nucleation position r of the primary crack is not affected by pH, and always occurs at the fracture tip (r = 0) and fracture center (r = 1), respectively. For γ = 30, 45, and 60°, r increases quasi-linearly with the increase of γ in semi-log plots, and an increase in pH moves the primary crack away from the fracture tip. For a given γ, an increase in pH produces an increase in the kink angle k1 of the secondary crack. The influence of pH on secondary crack propagation is most pronounced at high γ values. As the pH increases, the location of the secondary cracks gradually turns counterclockwise with a decreasing swing range, from 37° at pH 1.82–35° at pH 5.6. The secondary crack is inclined at a maximum of 30° and 32° to the maximum principal stress direction at pH 1.82 and pH 5.6, respectively. Right-lateral shear secondary cracks with well-developed tensile tail fractures are more prevalent in higher pH environments. The comparison with dry experiments indicates that distilled water is more aggressive than HClaq. The significant loss of calcium ions in distilled water and the reduction of large pore volume fraction in HClaq are considered the main reasons for the deterioration of the pH effect.

Determining the slip rate of the Kendeng Fault in East Java through GPS observations

The Kendeng Fault is recognized as an extension of the Flores back-arc thrust, primarily exhibiting thrust fault behavior. However, prior research has indicated the presence of a significant right-lateral shear component along this fault. While no significant seismic events have been documented in recent years, historical records suggest that the region experienced significant seismic events prior to the 20th century, necessitating a careful consideration of the earthquake hazard associated with this fault. The question arises whether the absence of recent seismic activity is a consequence of energy accumulation or gradual energy release. Our investigation utilizes GPS data collected over a four-year period starting in 2016 to determine the slip rate of the Kendeng Fault. Our findings indicate that the slip rate along the Kendeng Fault is approximately 0.45 ± 0.07 cm/year. These results provide valuable insights into the behavior of the Kendeng Fault and contribute to a better understanding of its seismic potential and associated risks.

Seismic Sequence Analysis of the Arraiolos Zone, South Portugal, and Its Seismotectonic Implications: A Detailed Analysis of the Period 15 January–30 June 2018

The Arraiolos Zone has been affected by the persistent superficial seismicity (focal depth < 20 km) of a weak magnitude (M < 4) and some events of a higher magnitude (M > 4), and is mainly located around the Aldeia da Serra village. On 15 January 2018, at 11:51 UTC, the largest instrumental earthquake recorded in that area occurred, with a magnitude (ML 4.9) located northeast of Arraiolos, near the Aldeia da Serra village. This event was followed by a sequence of aftershocks with a magnitude (ML) ≤ 3.5. This seismic sequence was monitored by the designated temporary seismic network of Arraiolos, comprising 12 broadband seismic stations (CMG 6TD, 30 s) from the ICT (Institute of Earth Sciences, Évora) and 21 short-period stations (CDJ 2.0 Hz) from the IDL (Instituto Dom Luiz), distributed around the epicenter, within a radius of approximately 25 km. To infer the structure and kinematics of faults at depth and to constrain the crustal stress field in which the earthquakes occur, we use the polarities of the first P-wave arrivals and the S/P amplitude ratios to better constrain the focal mechanisms of 54 events selected, and apply the HASH algorithm. Overall, the good-quality (defined by the HASH parameters) focal solutions are characterized by a mixture of reverse and strike-slip mechanisms in our study area (AZS). Our seismicity and focal mechanism results suggest that the horizontal stress is more dominant than the vertical one and oriented in the NW–SE direction, parallel with the strike of the main faults. This analysis leads us to affirm that the ASZ is an active right-lateral shear zone.

GEOMORPHOLOGICAL INDICATORS OF SHEAR DISPLACEMENTS ON THE URUP ISLAND (GREAT KURIL RIDGE)

The Urup Island, located in the southern part of the Great Kuril ridge, is an interesting object of study because its relief is very contrasting and has a significant depth of vertical dissection. It was greatly influenced by the recent tectonic processes. Judging by the geological data, the island at the present stage is located in a shear stress field with the southeastern orientation of the axis of maximum compression. It can be explained by the subduction interaction of the Pacific and Sea of Okhotsk lithosphere plates at an acute angle of about 55°. Visual and automated decoding of satellite images and digital terrain models, and the analysis of the erosion network pattern made it possible to identify geomorphologic signs of shear displacements. These include the echeloned arrangement of lineaments as separation cracks in right-lateral shear zone in the area of the Rybnaya River and on the Skvoznyakovyj isthmus, where displacements of unidentified kinematics were previously revealed. The zones, which are oriented in the north-east direction and differ in the asymmetric pattern of the drainage network, were considered: within their limits, small tributaries are located on one side of the watercourse of the highest order as megatracks of separation. In the area of the epicenter of the earthquake of 1989, which occurred under the latitudinal horizontal stretching, a linear lowering of the relief with a feathery pattern of watercourses was recorded, interpreted as a morphostructure of stretching. Similar probably stretching zones, extending mainly along the azimuth of 130-140°, were recorded on the Pacific coast of the island. Their orientation, as well as the kinematics of probable shear displacements identified from the geomorphologic data, correlates with the general ideas about the stress-strain state of the Southern Kuriles, as well as the reconstruction of the main normal stress axes of the Kastricum Peninsula performed by us from the fracture measurements. All this prove that the suggested structural-geomorphologic interpretation of the Urup Island territory is quite reasonable. In general, our new data on the configuration and kinematics of probable active shear faults and extensional morphostructures complement the existing ideas about the latest deformations of the study area.

The atypical Gaoligong orocline: Its geodynamic origin and evolution

Various orocline systems around the India–Eurasia collision zone have long been recognized and studied. Different portions of the India–Eurasia boundaries represent various scales and models of orocline-forming processes, such as the Baluchistan orocline formed by multiple deformation events and the Himalayan orocline formed by a mixture of complex structural mechanisms. The curvature from the eastern Himalayan syntaxis through east Burma to west Yunnan showed a unique convex curvature toward the mantle wedge. This is different from the concave Baluchistan orocline and the Himalayan orocline. The unique geometry of the Gaoligong orocline shows an N-S trend for the northern section and a NE-SW trend for the southern section. This curvature also marks the boundary between the Tengchong and Baoshan blocks along the Santaishan suture in western Yunnan, China. Our structural reconstruction identified five deformation events: 1) D1 is km-scale upright folding, which only affected the Neoproterozoic meta-sedimentary unit, 2) D2 recumbent folding, which only developed in the southern section of the Gaoligong orocline, 3) D3 large-scale gently westward-inclined thrust folding, 4) D4 right-lateral shear belt, and 5) the D5 normal faults. Since the D3 structure is the earliest event that shows penetrative foliation development along the orocline, we consider D1 and D2 as pre-orocline-forming events. The geometry of the Gaoligong orocline is controlled by the distribution of the Ordovician basement between the Tengchong and Baoshan blocks. Both north and south sections experienced the same structural evolution since D3 (a fault-propagation fold system occurred between 40 Ma and 28 Ma), D4 (steep right-lateral shear belt occurred between 28 Ma and 15 Ma), and D5 (normal faults after 15 Ma). The curvature first developed as a shovel-like top-to-the-NE thrust plane (S3) that formed under amphibolite-facies conditions between 40 Ma and 28 Ma. The following deformation events (D4 and D5) show orocline parallel foliation development under lower metamorphic conditions, indicating that the curvature of the Gaoligong orocline is not generated by additional rotation along multiple deformation events. However, due to the lack of orocline parallel foliation development for S3, and the lack of a proper position of the indenter, the Gaoligong orocline cannot be classified as a primary orocline nor a rotational orocline. The curved geometry is an interference pattern of topography relief to the shovel-like thrust plane that developed during D3. Our new reconstructed structural evolution concludes that the Gaoligong orocline is an “atypical” orocline.

Time-dependent cracking and brittle creep in macrofractured sandstone

Time-dependent cracking and brittle creep of rock is fundamental to understanding the long-term evolution and dynamic failure in underground rock engineering. In the present paper, we present a systematic laboratory investigation into the control of single open macrofractures of differing orientations on time-dependent cracking and brittle creep in sandstone using digital image correlation (DIC). For a given macrofracture inclination angle β, we find that the failure of macrofractured sandstone under a constant stress is accompanied by the generation of more tensile fractures (wing cracks and secondary cracks) than in our constant strain rate experiments. This result differs from experiments performed on intact sandstones, for which macroscopic failure under a constant stress and constant strain rate were essentially identical. The nucleation site for the wing cracks is β-dependent. As β is increased, the crack nucleation position gradually moves from the center of the fracture towards the fracture tip with a decreasing speed. The kink angle of the secondary crack (k1) increases quasi-linearly with the increase of fracture angle γ. With the increase of the secondary crack angle θ, the kink angle of the tail crack (k2) shows different increasing trends for the left-lateral and right-lateral shear. The secondary crack could be inclined at a maximum of 26° (θ) to the maximum principal stress direction. Creep bursts—transient accelerations in deformation—are more easily triggered in macrofractured rock than in initially intact rock, and coincide in time with the coalescence of secondary cracks. The likelihood of occurrence of a creep burst was found to be higher for lower values of β. Finally, we present a secondary crack location map that defines the swing interval and identifies the fracture mechanism. The influence of a macrofracture, and its orientation, on damage evolution and the likelihood of creep bursts during interseismic periods provides crucial information for those tasked with monitoring hazards associated with the dynamic failure of crustal rocks.

Reorganization of Northern Peri-Gondwanan Terranes at Cambrian–Ordovician Times: Insights from the Detrital Zircon Record of the Ossa-Morena Zone (SW Iberian Massif)

Abstract The Ossa-Morena Zone constitutes a fringe Gondwana-related terrane all along the Paleozoic. This continental block has been classically interpreted as being attached to a portion of the northern Gondwanan margin located close to the West African Craton. We report here the results of U-Pb dating on detrital zircon grains from 15 metasedimentary rocks collected in two well-exposed and well-dated Cambrian sections (Córdoba and Zafra) of the Ossa-Morena Zone. The studied samples show a dominant late Tonian-Ediacaran population peaked at c. 600 Ma. Secondary populations are Rhyacian-early Orosirian and late Orosirian-Statherian in age, with maxima at c. 2.1 and 1.9 Ga. Minor detrital zircon populations are Mesoarchean-early Siderian in age, with peaks between c. 3.05 and 2.45 Ga. Most of the studied samples lack a Stenian-early Tonian population, except for two of them with a minor peak at c. 1 Ga. Our results corroborate previous studies that locate the Ossa-Morena Zone close to the West African Craton and/or the Tuareg Shield (i.e., in a western position with respect to other Variscan zones of the Iberian Massif) at the onset of the rifting stage that opened the Rheic Ocean. Nevertheless, the absence of a significant Stenian-early Tonian population in the Cambrian Ossa-Morena rocks contrasts with the reported results on middle Ordovician-Lower Devonian rocks of this zone, which systematically contain an important population with a peak at c. 1 Ga. We relate this change to the latest Ediacaran-early Ordovician paleogeographic/paleotectonic evolution of the Ossa-Morena Zone, which might have recorded a significant eastward displacement, together with a reorganization of the drainage systems. Thus, the vanishing stages of the Cadomian orogeny could have translated eastward the Ossa-Morena Zone terrane at latest Ediacaran-earliest Cambrian time due to change in plate kinematics from subduction to right-lateral shearing. This translation would have shifted the Ossa-Morena Zone from an Ediacaran location close to the West African Craton to a Cambrian position close to the Tuareg Shield. Finally, the rift-to-drift transition occurred at late Cambrian-early Ordovician time along the northern Gondwanan margin would have reorganized the drainage systems, facilitating sediment supply from an eastern source with abundant Stenian-early Tonian detrital zircon grains, probably the Saharan Metacraton.

Thermochronological constraints on Eocene deformation regime in the Long-Men Shan: Implications for the eastward growth of the Tibetan Plateau

Understanding the spatio-temporal distribution of strain during Cenozoic growth of the Tibetan Plateau is important for constraining the geodynamic processes underpinning plateau formation. Offset Quaternary landforms and historic earthquake data suggest an along-strike change in deformation style for the eastern margin of the Tibetan Plateau, characterized by a transition from ESE-verging shortening to right-lateral shear from the southern to northern segment of the Long-Men Shan fault zone within a distance of ca. 500 km. When and how this along-strike deformation pattern formed is central to understanding the uplift history and spatio-temporal distribution of strain in the eastern margin of the Tibetan Plateau, and the underpinning geodynamics. To address this, we report a suite of low-temperature thermochronology data from the northern segment of Long-Men Shan fault zone that show a contrast in post late Cretaceous cooling and exhumation histories between the hinterland (west of the marginal Yingxiu-Beichuan fault) and foreland sides (east of the fault). Prior to the Eocene (ca. 40 Ma), the hinterland experienced significant exhumation in contrast to minor exhumation on the foreland side, but, post Eocene exhumation accelerated on the foreland side. This change reflects a switch in the deformation regime from shortening to strike-slip-dominated. This switch reduced hinterland rock uplift and tectonic and topographic loading over the foreland basin, leading to accelerated foreland exhumation through isostatic adjustments. A compilation of fault deformation history for the eastern Tibetan Plateau shows a second tectonic transition in the late Miocene, characterized by formation of the south-striking Huya and Minjiang faults. Our results highlight the importance of progressive late Eocene and late Miocene tectonic transitions in shaping the eastern margin of the Tibetan Plateau. • The hinterland and foreland sides of the eastern Tibetan Plateau margin experienced differential Eocene exhumation. • The differential evolution indicates a coeval tectonic transition from shortening-dominated to strike-slip-dominated. • A second tectonic transition is characterized by the late Miocene formation of the Huya and Minjiang faults. • The eastern margin of the Tibetan Plateau is shaped by progressive late Eocene and late Miocene tectonic transitions.

Three-dimensional displacement field of the 2010 Mw 8.8 Maule earthquake from GPS and InSAR data with the improved ESISTEM-VCE method

The 2010 Mw 8.8 Maule earthquake occurred offshore central Chile and ruptured ∼500 km along the megathrust fault resulting from the oceanic Nazca plate subducting beneath the continental South American plate. The Maule earthquake produced remnant crustal displacements captured by a vast set of geodetic observations. However, given the nature of the observational techniques, it is challenging to extract its accurate three-dimensional coseismic deformation field with high spatial resolution. In this study, we modified the extended simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements (ESISTEM) method with variance component estimation algorithm (ESISTEM-VCE) to retrieve the three-dimensional surface displacement field of this event by integrating the interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) measurements. The ESISTEM-VCE method accounts for the spatial correlation of surface displacement among the adjacent points and determine the accurate weight ratios for different data sets, but also uses the uncertainties of GPS data and considers the different spatial scales from the different datasets. In the simulation experiments, the RMSEs of the ESISTEM-VCE method are smaller than those of the ESISTEM and ESISTEM-VCE (same d0) methods, and the improvements of 97.1%, 3.9%, and 84% are achieved in the east-west, north-south, and vertical components, respectively. Then, we apply the proposed methodology to the 2010 Mw 8.8 Maule earthquake, to obtain a three-dimensional displacement field that could provide fine deformation information. In the east-west component, the significant deformation is in the north of the epicenter, closed to the Constitución, with a maximum westward displacement of 495.5 cm. The displacement in the north-south component is relatively small compared to that in the east-west component. The maximum uplift reaches 211.8 cm, located at the southwest of the Concepción. Finally, the derived vertical displacements are also compared with field investigations, indicating that the ESISTEM-VCE method can obtain more accurate weight of different datasets and perform better than the ESISTEM method. The results highlight that the earthquake ruptured along the NE-SW direction, with a dominant thrust and a relatively small component of right-lateral strike-slip, coinciding with the characteristics of subduction and right-lateral shear. The experiments with the simulated and real data suggest that the improved ESISTEM-VCE method in this study is feasible and effective.

Timing and origin of orogenic gold mineralization in the Kanggurtag area, NW China

The Kanggur, Kangxi and Matoutan deposits are three important orogenic gold deposits located in the Kanggurtag gold mineralization belt of Eastern Tianshan, NW China. These deposits are hosted in low-grade metamorphic (greenschist-facies) volcaniclastic rocks of the Early Carboniferous Yamansu Formation. The orebodies are distributed in approximately E–W-striking shear zones and show evidence of hydrothermal alteration and four deformation events. The first deformation (D1) is associated with progressive regional N–S shortening without growth of gold mineralization. The second deformation (D2) is manifested by right-lateral shear zones under ductile conditions, which triggered the initial enrichment of gold in the stage I pyrite. The third deformation (D3) is characterized by a transpressional event under ductile–brittle conditions and formed stage II–Ⅳ pyrites with dominant Au mineralization; the native gold is mainly distributed in the interstices or interface of the pyrites. The fourth deformation (D4) is archived by post-collision extensional structures accompanied by magma emplacement and numerous limonite and comb-like quartz veins.Six pyrite samples (stage III) separated from the Kanggur deposit yield a Re–Os isochron age of 321 ± 6.7 Ma (MSWD = 3.0, initial 187Os/188Os = 0.35 ± 0.22). Three pyrite samples (stage III) separated from the Kangxi deposit yield a Re–Os isochron age of 327 ± 4.5 Ma (MSWD = 1.2, initial 187Os/188Os = 0.94 ± 0.12). The δ34S value significantly increases from stage I pyrite (−37.3‰ to −25.5‰, average = −32.2‰) to stage II−Ⅳ pyrite (−0.90‰ to 6.30‰, average = 1.96‰), indicating that sulfur in stage I pyrite migrated from sedimentary sources whereas in stage II–Ⅳ pyrites it was derived from the pyroclastic rocks in the country rocks. The H–O isotopic compositions of quartz show that the fluid in stage I (δ18Ow = 2.25‰ to 5.15‰, δ2D = −66.0‰ to −42.5‰) was metamorphic water and the fluid in stage Ⅴ was mixed with a large proportion of meteoric water. The fluids in stages II–Ⅳ maybe a “second hand” magmatic origin related to the andesite in the country rocks. Therefore, we propose that Au mineralization in the Kanggur, Kangxi and Matoutan deposits derived mainly from a metamorphic fluid source associated with the pyroclastic rocks in the country rocks. In addition, we conclude that these deposits are orogenic gold deposits that formed during the Late Carboniferous collisional tectonism (ca. 320 Ma) and the epithermal Au deposits (ca. 280 Ma) maybe associated with the post-collision extensional setting.

Late Cenozoic to Present Kinematic of the North to Eastern Iran Orogen: Accommodating Opposite Sense of Fault Blocks Rotation

The opposite-sense fault block rotation across the continental strike-slip faulting plays an important role in accommodating crustal deformation in the north of the East Iran orogen. This research constrains the post-Neogene kinematics of the NW-SE to E-W left-lateral transpressional zones at the northern termination of the N-S striking right-lateral Neh fault system in the East Iran orogen. Using two case studies, we analyzed the NW-SE Birjand splay and the E-W Shekarab transpression zone by analysis of satellite images, structural features, fault geometry and kinematics, GPS (Global Positioning System) velocities, fault- and earthquake-slip stress inversion, and paleomagnetic data. Our results show two distinctive regions of opposite-sense fault block rotations and with different rotation rates. As an asymmetric arc, the Birjand splay displays a transition from the prevailing N-S right lateral shear in the east to NW-SE left lateral transpression in the middle and E-W left lateral shear in the west. In the east, with clockwise fault block rotation, the N-S right lateral faults and the NW-SE oblique left-lateral reverse faults constitute push-ups through the restraining fault bends. In the west, with counterclockwise fault block rotation, the Shekarab transpression zone is associated with the duplex, pop-up, and shear folds. Our suggested kinematic model reveals that the N-S right-lateral shear is consumed on the left-lateral transpressional zones through the vertical axis fault block rotation. This led to an E-W shortening and N-S along-strike lengthening in the East Iran orogen. This research improves our understanding of how opposite fault block rotations accommodate India- and Eurasia-Arabia convergence in the north of the East Iran orogen. The suggested model has implications in the kinematic evolution of intra-plate strike-slip faulting through continental collision tectonics.

Internal deformation of the Gorda plate and its tectonic significance within the Cascadia subduction zone

In this review paper, we compile observations that have been made over decades of research on the Cascadia convergent boundary and re-examine the deformation using structural kinematic models, thereby providing new insights. Deformation in the subducting Juan de Fuca slab and the overriding North American plate is primarily accommodated by fracturing. We show that the along-strike deformation patterns are a function of fracture density and activity, which influences coupling with the overriding North American plate and the underlying mantle wedge. We analyze the deformation within a fracture kinematics framework and show that the orientations of the fractures and motion history follow expected empirical fracture mechanics models and are used to provide a more detailed kinematic history of the Cascadia convergent boundary. We focus on the Gorda plate of the Juan de Fuca subducting slab and show that it is intensely fractured with curved magnetic anomalies such that the brittle deformation is collectively expressed macroscopically as a deforming continuum. The Gorda plate has changed shape via NS shortening and clockwise rotation without losing cohesion at the plate-scale, which suggests that the fractures form an interlocking network to accommodate ductile deformation via cataclastic flow. In the forearc, the fracture and velocity patterns suggest that deformation is accommodated by a continental-scale right-lateral shear zone, trending 320°/140° and is assisted via crustal-scale cataclastic flow. We suggest that ETS activity is strongly correlated to deformation in the over-riding plate. The Gorda plate is also used to better understand larger-scale plate dynamics. We demonstrate that the Gorda plate acts as a ductile deformation zone to assist global plate mobility. Additionally, details of the magnetic lineation patterns in the Gorda plate indicate that slab pull overtakes plate rotation post-subduction. This suggests that plate behavior is strongly influenced by its internal deformation and coupling with the mantle post-subduction.

Neotectonics of Turkey (Türkiye) and surrounding regions: a new perspective with block modelling

This paper aims to present a new neotectonic perspective concordant with the seismic activities in Turkey and surrounding regions. The neotectonic structures have been re-evaluated mainly by using focal mechanism solutions and high-resolution satellite (Google Earth) images. The Southeast Anatolian Wedge explains thrust/blind thrust and asymmetrical folding relationship in SE Turkey, Syria, and Northern Iraq. The neotectonic structures of the Turkish-Iranian Plateau are enlightened by the rhomboidal cell model which creates a base to determine multiple intersection points between the region-wide left- and right-lateral shear zones. The releasing stepover between the North Anatolian Fault Zone and Southeast Anatolian-Zagros Fault Zone plus their connections with the Northeast Anatolian Fault Zone and the East Anatolian Fault Zone are described in a more meaningful way with the Anatolian Diagonal concept. It also clarifies the role of left-lateral shear zone in the west-southwest movement of Anatolian plate and its relationship with the Aegean and Cyprus arcs. A neotectonic region under the influence of NW-SE contraction is determined between the North Anatolian, Eskişehir, and Kırıkkale-Erbaa fault zones in which the Elmadağ-Eldivan and Abdüsselam pinched crustal wedges and the Beypazarı Blind Thrust Zone are developed. A new route for the southern branch of the North Anatolian Fault Zone is determined between Bolu and Değirmenlik (Milos) Island in the Aegean Sea via Mudurnu, Bursa, Balıkesir, and İzmir. All main neotectonic structures mentioned in this paper are evaluated by the elastic dislocation modelling and new neotectonic provinces are suggested.

Structures and Kinematics of the Huanghua Depression in Bohai Bay Basin, East China: Implications for the Formation Mechanism of a Transtensional Basin

Abstract The Bohai Bay Basin in East Asia is a rift basin created by Cenozoic subduction of the oceanic Pacific plate beneath the Asia continent. Many prior studies suggest that the basin was initially formed in the Paleocene with the development of several NNE-trending extensional grabens, but subsequently impacted by right-lateral shear along these existing NNE-trending structures in the middle Eocene, transforming the Bohai Bay Basin into a transtensional basin and producing EW-trending grabens in the Bozhong and the northeastern Huanghua depressions. However, how this transformation occurred remains to be fully understood. Based on seismic and drilling data, we herein investigated the fault structures, basin architecture, and evolutionary stages of the Huanghua Depression in the central-west Bohai Bay Basin to examine the strain partitioning and evolution mechanism during the Paleogene syn-rifting stage. The results reveal that the Huanghua Depression is composed of three structurally distinctive zones, namely, a dextral transtensional, a NW-SE extensional, and a N-S extensional zones from southwest to northeast, which are separated from each other by two transfer zones. The NW-SE extensional zone is interpreted as a horsetail structure on the northern termination of the dextral transtensional zone. This dextral transtensional zone and the Tan-Lu Fault zone to the east served as strike-slip boundaries within which EW-trending depressions such as the northeastern Huanghua and Bozhong depressions formed in the middle Eocene.

Latest Quaternary Active Faulting and Paleoearthquakes on the Southern Segment of the Xiaojiang Fault Zone, SE Tibetan Plateau

AbstractThe Xiaojiang fault zone (XJFZ) is an important part of the Xianshuihe-Xiaojiang fault system, acting as the eastern boundary of the Chuan-Dian block on the southeastern margin of the Tibetan Plateau and accommodating the lateral extrusion of the block. The faulting activity and paleoseismic history on the southern segment of the XJFZ remain poorly understood. Here, trench excavations and radiocarbon dating revealed that four recent surface-rupturing paleoearthquakes have occurred on the Jianshui fault (JSF) in the southern segment of the XJFZ since ~15370 yr BP. The ages of these events, labeled E4-E1 from oldest to youngest, are limited to the following time ranges: 15360-12755, 10845-6900, 1455-670, and 635-145 yr BP, respectively. The most recent event E1 was most likely the 1606 Jianshui earthquake. These events appear to occur unregularly in time. The time interval between the last two events is 726±235 yr, and the average recurrence interval for all four events is 4589±3132 yr. The deformed strata show that the JSF is characterized kinematically by transtension, which likely respond to the apparent change in the direction of clockwise rotation of the Chuan-Dian block around the eastern Himalayan syntaxis. Combined with the analysis of the neighboring NW-striking faults, our study suggests that the south-southeastward motion of the Chuan-Dian block is likely to be firstly accommodated in part by the right-lateral shear and dip-slip motions of the Qujiang and Shiping faults and continues across the Red River fault zone, then is transmitted southward along the Dien Bien Phu fault. Therefore, the southern segment of the XJFZ plays a dominant role in the tectonic deformation of the southeastern Chuan-Dian block, with a high seismic hazard.

Microseismic Evidence for Bookshelf Faulting in Western Montana

AbstractOne of the most seismically active regions in the United States, located hundreds of kilometers inland from the nearest plate boundary, is the Intermountain Seismic Belt (ISB). The 6 July 2017 M 5.8 earthquake occurred 11 km southeast of Lincoln, Montana, within the ISB. This was the largest earthquake to rupture in the state of Montana since the 1959 M 7.3 Hebgen Lake earthquake. We use continuous seismic data from the University of Montana Seismic Network, the Montana Regional Seismic Network, and the U.S. Geological Survey to investigate the Lincoln aftershock sequence and to evaluate crustal stress conditions. We manually picked P- and S-wave arrival times, computed 4110 hypocenter locations and 2336 double-difference relocations, and generated focal mechanisms for 414 aftershocks (12+ polarities) in the 2 yr following the mainshock. Based on the alignment of aftershocks, we infer that the mainshock occurred on a north–northeast-trending left-lateral strike-slip fault. The orientation of the fault is unexpected, given that it strikes nearly perpendicular to the prominent Lewis and Clark line (LCL) faults in the area. Although most aftershocks concentrate near the mainshock, several distinct clusters of microseismic activity emerge along subparallel faults located primarily to the west of the mainshock. The subparallel faults also exhibit left-lateral strike-slip motion oblique to the LCL. We postulate that the aftershocks reveal the clockwise rotation of local-scale crustal blocks about vertical axes within a larger, right-lateral shear zone. The inferred block rotations are consistent with a bookshelf-faulting mechanism, which likely accommodates differential crustal motion to the north and south of the LCL region. The tension axes of well-constrained focal mechanisms indicate local northeast–southwest extension with a mean direction of N60°E.

Present-Day Crustal Deformation Within the Western Qinling Mountains and Its Kinematic Implications

The western Qinling Mountains, located between the East Kunlun fault and the West Qinling fault, hold the key to investigating the outward expansion of the Tibetan Plateau. We use up-to-date GPS observations to derive high spatial–temporal resolution crustal velocity and strain rate fields for this region. Our results suggest that a series of NEE-trending faults bounding the eastern margin of the western Qinling Mountains experience right-lateral strike slip with low rates of ~ 1 mm/yr. In addition, our results suggest that crustal deformation of the western Qinling Mountains could be governed by a right-lateral shear zone trending NNE at a rate of ~ 6 mm/yr, producing a clockwise rotation of subblocks: left-lateral and right-lateral strike slip on the NWW-trending and NEE-trending faults, respectively. The eastward extrusion of the Tibetan Plateau along the western Qinling Mountains is limited further east. In contrast, the NNE-ward expansion of the northeastern Tibetan Plateau is the primary motion which extends through the western Qinling Mountains and the Longxi block. The results of this study improve our understanding of the outward expansion of the Tibetan Plateau influenced by the Indian–Eurasian collision.

The Comoros archipelago: a right-lateral transform boundary between the Somalia and Lwandle plates

The origin of the Comoros archipelago in the Mozambique channel is the subject of a longstanding controversy. This volcanic chain has been successively interpreted as a hotspot track, as built on an intraplate fracture zone or a passive margin, or as the northern limit of a diffuse deformation zone in the Nubia-Somalia plate system, none of these interpretations being entirely satisfactory. It is also possible that the volcanism of the Comoros is a branch of the East African Rift System, and delineates the still poorly constrained northern boundary between the Lwandle and Somalia plates. To better understand the origin of the Comoros archipelago, we combined a formal stress inversion of earthquake focal mechanisms and deformation structures (faults and dykes) observed on three islands (Mayotte, Anjouan, and Mohéli) with a morphologic study of the repartition of onshore and offshore volcanic vents in the area.Earthquake focal mechanisms and field-observed deformation structures both yield a generalized strike-slip regime with a maximum horizontal stress oriented NW-SE. In tandem, the study of topographic and bathymetric features shows that volcanic vents follow a preferential repartition along two trends: N105 ± 10° lineaments up to 100 km-long, and N145 ± 10°, less than 60 km-long lineaments organized in an en échelon fashion. Earthquakes, deformation structures, and volcanic vents all consistently designate the Comoros archipelago as an E-W elongated, 150 km-wide zone of right-lateral tear in the lithosphere. Because the clockwise rotation of Somalia relative to Lwandle requires the existence of such a right-lateral shear zone, we interpret the Comoros archipelago as the northern boundary between the two plates. This plate boundary is still in an immature state, which is why it remained undetected until now.

Quaternary slip rates on the White Mountains fault zone, eastern California: Implications for comparing geologic to geodetic slip rates across the Walker Lane

AbstractThe White Mountains fault zone in eastern California is a major fault system that accommodates right-lateral shear across the southern Walker Lane. We combined field geomorphic mapping and interpretation of high-resolution airborne light detection and ranging (LiDAR) digital elevation models with 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene right-lateral slip rates on the White Mountains fault zone. Alluvial fans were found to have ages of 46.6 + 11.0/–10.0 ka and 7.3 + 4.2/–4.5 ka, with right-lateral displacements of 65 ± 13 m and 14 ± 5 m, respectively, yielding a minimum average slip rate of 1.4 ± 0.3 mm/yr. These new slip rates help to resolve the kinematics of fault slip across this part of the complex Pacific–North American plate boundary. Our results suggest that late Pleistocene slip rates on the White Mountains fault zone were significantly faster than previously reported. These results also help to reconcile a portion of the observed discrepancy between modern geodetic strain rates and known late Pleistocene slip rates in the southern Walker Lane. The total middle to late Pleistocene slip rate from the southern Walker Lane near 37.5°N was 7.9 + 1.3/–0.6 mm/yr, ∼75% of the observed modern geodetic rate.