Southern Segment Research Articles

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.

Exploring structured scripting cartographic technique of GMT for ocean seafloor modeling: A case of the East Indian Ocean

This paper examines spatial variations in the geomorphology of the Ninety East Ridge (NER), located in the Indian Ocean. The NER is an extraordinary long linear bathymetric feature with topography reflecting complex geophysical setting and geologic evolution. The research is based on a compilation of high-resolution bathymetric, geological, and gravity datasets clipped for the study area extent (65° - 107°E, 35°S - 21°N): General Bathymetric Chart of the Oceans (GEBCO), Earth Gravitational Model (EGM2008, EGM96). The submarine geomorphology of the NER was modeled by digitized cross-sectional profiles using Generic Mapping Tools (GMT). The availability of the method is explained by 1) the free datasets; 2) the open source GMT toolset; 3) the available tutorials of the GMT and the codes explained in this work. Three segments of the NER were selected, digitized, and modeled: 1) northern 89°E, 7°S to 90°E, 7°N; 2) central 88.4°E, 14.7°S to 88.8°E, 8.2°S; 3) southern 87.9°E, 17°S to 87.5°E, 27°S. Measured depths were visualized in graphs, compared, and statistically analyzed by the histograms. The northern segment has a steepness of 21.3° at the western slopes, and 14.5° at the eastern slope. The slopes on the eastern flank have dominant SE orientation. The central segment has a bell-shaped form, with the highest steepness comparing to the northern and southern segments. The eastern flank has a steepness of 49.5°. A local depression at a distance of 50 km off from the axis (90°E) continues parallel to the NER, with the shape of the narrow minor trench. The western slope has a steepness of 57.6°, decreasing to 15.6°. The southern segment has a dome-like shape form. Compared to the northern and central segments, it has a less pronounced ridge crest, with a steepness of 24.9° on the west. The eastern flank has a steepness of 36.8° until 70 km, gradually becoming steeper at 44.23°. A local minor trench structure can be seen on its eastern flank (100 km off the axis). This corresponds to the very narrow long topographic depressions stretching parallel to this segment of the NER at 90.5°E. The study contributes to regional geographic studies of Indian Ocean geomorphology and cartographic presentation of GMT functionality for marine research and oceanographic studies.

Crustal thickness beneath the Tanlu fault zone and its tectonic significance based on two-layer H-κ stacking

Tanlu fault zone (TLFZ) is the largest active fault zone in eastern China. It is characterized by complex tectonic evolution and multiple faults and marks the boundary between the North and South China blocks. An in-depth understanding of the distinct crustal structures of both parts of the TLFZ will provide valuable insights into the lithospheric and crustal thinning in eastern China, extensive magmatism since the Mesozoic, and formation mechanisms of metallogenic belts along the Yangtze River. In this study, a two-layer H-κ stacking approach was adopted to estimate the thicknesses of the sediment and crystalline crust as well as the corresponding νP/νS ratios based on high-quality teleseismic P-wave receiver functions recorded by permanent and temporary stations in and around the TLFZ. The geological units in the study region were delineated, especially the crustal structures beneath extensive sedimentary basins on both sides of the TLFZ. The following conclusions can be drawn: (1) The crustal thickness in and around the TLFZ greatly varies depending on the segment. In the northern segment, the crust is relatively thin beneath the eastern part of the Songliao Basin, a broad uplift of the Moho can be observed, and the Moho descends from south to north. The crust below the central and southern segments becomes thinner from west to east. The thickness of the crust is less than 30 km toward the eastern side of the boundary between the Jiangsu and Anhui provinces, that is, significantly thinner than in other areas. In terms of the νP/νS ratios, high anomalies were detected in the central-southern segments of the TLFZ, indicating the upwelling of deep mantle magma via deep faults. (2) Positive isostatic gravity anomalies were observed in the eastern part of the northern segment of the TLFZ and in the eastern part of the Suwan segment. The crustal thickness is smaller than that obtained from the Airy model of isostasy. This suggests that the lower crust in this area may have experienced intensive transformation processes, which may be related to crustal thinning (caused by crustal extension) and the strong uplift of the mantle in eastern China. The isostatic gravity anomalies between the eastern and western parts of the TLFZ indicate that the fault zone plays a dominant role in controlling the development of the deep crustal structure. (3) Significant crustal thinning was observed beneath the eastern part of the boundary between the Jiangsu and Anhui provinces in the southern segment of the TLFZ, suggesting that this area is prone to lithospheric thinning of the North China Craton. Due to the subduction, compression, and retreat of the Paleo-Pacific Plate during the Yanshanian Period as well as the dehydration of subducting oceanic crust (within subduction zones), the asthenosphere and oceanic crust in eastern China partially melted, resulting in mantle enrichment. The basic magma from the mantle is accumulated at the base of the crust, leading to magmatic underplating. In areas with weak topography toward the east of the TLFZ, magma rises to the upper crust and surface, resulting in the enrichment of multiple metal deposits in this area.

Fault Pattern and Seismotectonic Style of the Campania – Lucania 1980 Earthquake (Mw 6.9, Southern Italy): New Multidisciplinary Constraints

New fault trace mapping and structural survey of the active faults outcropping within the epicentral area of the Campania-Lucania 1980 normal fault earthquake (Mw6.9) are integrated with a revision of pre-existing earthquake data and with an updated interpretation of the CROP-04 near-vertical seismic profile to reconstruct the surface and depth geometry, the kinematics and stress tensor of the seismogenic fault pattern. Three main fault alignments, organized in high-angle en-echelon segments of several kilometers in length, are identified and characterized. The inner and intermediate ones, i.e. Inner Irpinia (InIF) and Irpinia Faults (IF), dip eastward; the outer Antithetic Fault (AFA) dips westward. Both the InIF and the IF strike NW-SE along the northern and central segments and rotate to W-E along the southern segments for at least 16 km. We provide evidence of surface coseismic faulting (up to 1 m) not recognized before along the E-W segments and document coseismic ruptures with maximum vertical displacement up to ∼1 m where already surveyed from other investigators 40 years ago. Fault/slip data from surface data and a new compilation of focal mechanisms (1980 – 2018) were used for strain and stress analyses to show a coherent NNE-directed least principal stress over time and at different crustal depths, with a crustal-scale deviation from the classic SW-NE tensional direction across the Apennines of Italy. The continuation at depth of the outcropping faults is analyzed along the trace of the CROP-04 profile and with available hypocentral distributions. Integrating all information, a 3D seismotectonic model, extrapolated to the base of the seismogenic layer, is built. It outlines a graben-like structure with a southern E-W bend developed at depth shallower than 10–12 km, at the hanging wall of an extensional NE- to E-dipping extensional basal detachment. In our interpretation, such a configuration implies a control in the stress transfer during the 1980 earthquake ruptures and provides a new interpretation of the second sub-event, occurred at 20 s. Our reconstruction suggests that the latter ruptured a hanging wall NNE-dipping splay of the E-W striking main fault segment and possibly also an antithetic SSW-dipping splay, in two in-sequence episodes.

The Caroline Ridge fault system and implications for the bending-related faulting of incoming oceanic plateaus

The latest multichannel seismic data from the Yap subduction zone is presented to investigate the Caroline Ridge fault system, developed by faulting and bending processes within the incoming oceanic plateau. We reveal that horst and graben structures exist along the entire incoming Caroline Ridge adjacent to the trench and that pervasive bending-related faults trending ~N30°E have developed subparallel to the trench. However, the widths of the horst and graben structures decrease gradually from north to south. The two-stage ancient abyssal hill fabrics overlap the incoming Caroline Ridge. Abyssal hill faults trending ~N65°W present large trench-ridge angles (85–90°); in the southern segment, abyssal hill faults trending ~N70°E have relatively low trench-ridge angles (~35–40°). From the Caroline Ridge to the horst and graben structures, the topography decreases, and slump bodies with deformed internal reflections have developed above an irregular reflector. Little stable pelagic sediment with horizontal reflectors accumulates on the horst and graben structures, suggesting that the faulting and bending processes involve long and intense tectonic processes that affect the sedimentary environment. The sedimentary strata and fault distributions differ between the North and South Caroline Ridge, which is primarily attributable to the different rifting processes. Furthermore, we propose that N65°W-trending ancient faults sufficiently decrease the yield strength of the incoming Caroline Ridge to facilitate the bending-related faulting. During subduction, the incoming North Caroline Ridge that has a large degree of rifting and the Sorol Trough, which has a thin crust, may resist less flexural bending and generate the wide horst and graben structures. The ancient faults and rifting model of incoming oceanic plateaus play a significant role in bending-related faulting

Slip Rates and Seismic Potential Along Main Faults in the Eastern Mediterranean and Caucasus from dense GPS Observations and Seismic Data

The Eastern Mediterranean and Caucasus regions are two of the most seismically active deforming continental regions. Monitoring and studies of the extensive deformation processes in this comparatively limited region enhance our understanding of the continental collision in the region. In this paper, we built a new elastic block model to constrain present-day plate motions and regional deformation in the intraplate zone and estimate fault slip rates from a dense global positioning system (GPS) velocity field. We additionally considered differences in moment accumulation rate per unit fault length using seismogenic and geodetic depth along the main faults. The right-lateral slip rates were estimated at between 24.33 ± 0.47 and 28 ± 0.5 mm/year along the North Anatolian Fault Zone (NAFZ) and the left-lateral slip at 10.6 ± 0.9 to 12 ± 1 mm/year on the East Anatolian Fault Zone (EAFZ). The Dead Sea Transform Fault (DSTF) slip rate ranges between 4.1 ± 0.95 and 6 ± 1 mm/year. The slip rate for the Main Caucasus Thrust Fault (MCTF) is 6.5 ± 0.2 to 14.3 ± 0.7 mm/year from west to the east. A shallow locking depth of 9 ± 3.2 km was found in the Marmara region. The locking depth along the DSTF is 10.9 ± 5.5 km in the northern segment, 9 ± 6.7 km in the central section and 11.8 ± 4 km in the southern section. The locking depth of the EAFZ is 16.7 ± 4.4 km in the northern segment, 19.8 ± 7.4 km in the middle segment and 12.5 ± 4.2 km in the southern segment. The locking depths along the MCTF range between 12.7 ± 5.8 and 17.8 ± 4.9 km from west to east. Our new block model has the best fit to the observed velocities. The results provide an overall understanding of fault activity and have implications for the earthquake hazard along the main faults in the region.

The visualization of geophysical and geomorphologic data from the area of Weddell Sea by the generic mapping tools

The paper concerns GMT application for studies of the geophysical and geomorphological settings of the Weddell Sea. Its western part is occupied by the back-arc basin developed during geologic evolution of the Antarctic. The mapping presents geophysical settings reflecting tectonic formation of the region, glaciomarine sediment distribution and the bathymetry. The GlobSed grid highlighted the abnormally large thickness of sedimentary strata resulted from the long lasting sedimentation and great subsidence ratio. The sediment thickness indicated significant influx (>13,000m) in the southern segment. Values of 6,000–7,000 m along the peninsula indicate stability of the sediments influx. The northern end of the Filchner Trough shows increased sediment supply. The topography shows variability -7,160–4,763 m. The ridges in the northern segment and gravity anomalies (>75 mGal) show parallel lines stretching NW-SE (10°–45°W, 60°–67°S) which points at the effects of regional topography. The basin is dominated by the slightly negative gravity >-30 mGal. The geoid model shows a SW-NE trend with the lowest values <18 m in the south, the highest values >20m in the NE and along the Coats Land. The ripples in the north follow the geometry of the submarine ridges and channels proving correlation with topography and gravitational equipotential surface.

Геоморфология хребта Девяностого градуса

This paper explores the geomorphological features of the Ninety East Ridge (NER), Indian Ocean. NER presents especially long and linear topographic structure formed as a result of complex regional geophysical and geologic development. The research is based on high precision bathymetric, geological and gravity data. The submarine geomorphology of NER was digitized as three cross-sectional profiles. The profiles were selected in northern, central and southern segments. The depths were visualized in graphs, compared and statistically analyzed by histograms. The study contributes to the geophysical studies of the Indian Ocean.

Germany and «Libyan problem» during 2010-s

Germany has been growing political and military activity in Northern Africa and Sahel region by the mid 2010s. FRG and its EU partners faced the great number of instability risks which are projected from zones of armed conflicts located in northern part of Africa. The key elements of the corridor of instability which has connected the fragile states in Sahel and Northern Africa were the «Libyan door» (the upper part) and «Malian gates» (the lower one). But in the 2010s FRG faced the absence of opportunities for itself to be directly involved in the resolution of «Libyan problem». That is why in 2012-2019 Germany had been trying only to fence «Libyan problem» in. This perimeter has four segments. German contribution to the creation of Western one (the strengthening of Tunisian and Algerian borders with Libya) and especially Eastern segment (the same with Egypt) was rather limited and consisted of arms export to these countries. The article explores the evolution of German participation to the resolution of Mali armed conflict (first of all FRG`s military contribution to EUTM Mali and MINUSMA missions). This one and also German participation in the reform of Niger`s security sector was the creation of Southern segment of the perimeter. By 2020 Germany has deepened cooperation on «Libyan problem» not only with regional players but also world powers. During Merkel`s visit to Moscow (January 11, 2020) the scheme of future Berlin conference on Libya was declared. This format was established on January 19, 2020. Germany became the coordinator of inter-Libyan dialogue (between the Government of national consensus in the West of the country and Libyan national army in the East) and supported it by the launch of the EU mission «IRINI». The article concludes about the perspectives of German policy towards Libya considering COVID-19 pandemics.

Crustal velocity, density structure, and seismogenic environment in the southern segment of the North-South Seismic Belt, China

The southern segment of the North-South Seismic Belt in China is a critical region for earthquake preparedness and risk reduction efforts. However, limited by the low density of seismic stations and the use of single-parameter physical structural models, the deep tectonic features and seismogenic environment in this area remain controversial. Thus, a comprehensive analysis based on high-resolution crustal structures and multiple physical parameters is required. In this study, we applied the ambient noise tomography method to obtain the three-dimensional (3D) crustal S-wave velocity structure using continuous waveform data from 112 permanent stations and 350 densely distributed temporary stations in the southern segment of the North-South Seismic Belt. Then, we obtained the high-resolution 3D density structure through wavenumber-domain 3D gravity imaging constrained by the velocity structure. The low-velocity and low-density anomalies in the upper crust of the study area were mainly distributed in the Sichuan Basin and around Dali and Simao, while the high-velocity and high-density anomalies were primarily distributed in the Panxi region, corresponding to the surface geological features. Two prominent low-velocity and low-density anomalies were observed in the middle and lower crust: one to the west of the Songpan-Garzê block and Sichuan-Yunnan diamond-shaped block, and the other near the Anninghe-Xiaojiang fault. Combined with the spatial distribution of seismic events in the study area, we found that previous earthquakes predominantly occurred in the transition zones between high and low anomaly regions and in the low-velocity and low-density zones in the upper crust. In contrast, moderate-to-strong earthquakes mainly occurred within the transition zones between high and low anomaly regions and close to the high-velocity and high-density regions, often with low-velocity and low-density layers below their hypocenters. Fluids play a critical role in the seismogenic process by reducing fault strength and destabilizing the stress state, which may be a triggering factor for earthquakes in the study area. Additionally, the upwelling of molten materials from the mantle may lead to energy accumulation and stress concentration, providing an important seismogenic background for moderate-to-strong earthquakes in this area.

Oligocene-Early Miocene exhumation and shortening along the Anninghe fault in the southeastern Tibetan Plateau: insights from zircon and apatite (U-Th)/He thermochronology

ABSTRACT The growth mechanisms of the southeastern Tibetan Plateau are debated between models of Oligocene shortening and lateral extrusion of coherent crustal blocks versus Late Miocene flow of low-viscosity lower crust. To test these models, we examine brittle deformation of the north-striking Anninghe fault, which has been acting as a left-lateral strike-slip fault since Late Miocene time, using structural and bedrock low-temperature thermochronologic methods. Different from previous interpretations of Late Miocene strike-slip motion, our structural observations indicate westward contractional dip-slip along the southern segment of the fault. New zircon and apatite (U-Th)/He thermochronologic data along a vertical profile of Precambrian volcanoclastic cover both sides of the Anninghe fault. The age-elevation relationships and inverse thermal modelling suggest a phase of Oligocene-Early Miocene (~24-18 Ma) differential rock exhumation and cooling across the fault, with a mean erosion rate of ~0.30 ± 0.12 km/m.y. and ~0.007 ± 0.007 km/m.y. in the hanging wall and footwall, respectively. Consistent with the structural observations, the higher rate of rock exhumation in the hanging wall indicates an Oligocene-Early Miocene phase of west-verging reverse faulting. Combined with previously published data from adjacent sites, we suggest that the Anninghe fault has experienced a tectonic transition from Oligocene-Early Miocene east-west contraction to Late Miocene strike-slip motion. The early to Mid-Cenozoic crustal shortening contributed to the uplift of the southeastern Tibetan Plateau, supporting the crustal shortening and extrusion model.

Spatial variation of nearly NS-trending normal faulting in the southern Yadong-Gulu rift, Tibet: New constraints from the Chongba Yumtso fault, Duoqing Co graben

The Yadong-Gulu rift is the largest active rift within the southern Tibet Plateau. Although tectonic characteristics and geodynamic implications of normal faulting have been widely discussed, precise constraints on the fault slip rates and their spatial distribution are still scarce to date, hindering full exploration of the deformation mechanism of active rifts in the southern Tibet. In this study, we integrated the interpretations of high-resolution satellites and unmanned aerial vehicle (UAV)-acquired images, and topography with the results of field surveys and radiocarbon dating to demonstrate the slip distribution along the Chongba Yumtso fault (CYF) bounding the northeastern sector of the Duoqing Co graben and then discussed their implications for the seismic risk and regional deformation. Since the late Pleistocene, the vertical slip rate and extension rate across the CYF are about 1.2 ± 0.1 and 1.7 (+0.7/−0.5) mm/a, respectively, and the average extension rate across the Yadong-Gulu rift zone is about 2.1 (+0.5/−0.6). According to the activity rate, plane size, basin-mountain height difference, and sediment thickness of the grabens in the southern segment of the rift zone, it is considered that the extension of the main boundary normal fault in the southern segment of the rift has the spatial variation characteristics of decreasing from south to north. According to the comparison of the total extension rate on different time scales, it is considered that the total extension rate of the rift zone in southern Tibet is approximately stable in different time periods, which is ~16 (+5/−4) mm/a. According to the active rate and historical seismic records of the Duoqing Co graben in the Yadong-Gulu Rift Valley, it is considered that the CYF has potential to produce earthquakes with Mw 6.6–7.4.

New 40Ar/39Ar geochronology data of the Fuping and Wutai Complexes: Further constraints on the thermal evolution of the Trans-North China Orogen

The Trans-North China Orogen has witnessed the assembly of the Western and Eastern blocks of the North China Craton during Late Paleoproterozoic. Numerous studies have focused on the subduction/collision timing and metamorphic evolution of the Trans-North China Orogen, whereas the thermal evolution and exhumation history are still poorly understood. The Fuping and Wutai Complexes are widely exposed of middle- to high-grade metamorphic rocks, which are excellent regions to investigate the thermal evolution and exhumation history of the Trans-North China Orogen. Previous U-Pb geochronological data of metamorphic zircon and monazite suggest that the peak granulite facies metamorphism in the Fuping Complex occurred during 1.92–1.80 Ga, and peak amphibolite facies metamorphism in the northeastern part of the Wutai Complex happened at 1.85–1.82 Ga. Here we report newly obtained hornblende 40Ar/39Ar age of 1813–1806 Ma in the Fuping Complex, and biotite 40Ar/39Ar age of 1802–1786 Ma in the northeastern part of the Wutai Complex, which representing the cooling ages. Combined with previous thermochronological data, the cooling rate of 2.0–8.5 °C/Ma was obtained for the middle to southern segments of the Trans-North China Orogen, possibly suggesting a moderate to fast exhumation relative to other Proterozoic orogens.

Zircon age depth-profiling sheds light on the early Caledonian evolution of the Seve Nappe Complex in west-central Jämtland

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths. The earliest of the deep subduction events has been dated to ca. 500–480 Ma. Evidence of this event has been reported from the northern exposures of the SNC. Farther south in the central and southern segments of the SNC, (ultra)high-pressure rocks have yielded younger ages in the range of ca. 470–440 Ma.This study provides the first record of the early Caledonian event in the southern SNC. The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tväråklumparna microdiamond-bearing gneiss. These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6 ± 3.8 Ma. A second, chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3 ± 3.6 Ma, which corroborates previous geochronological evidence for granulite facies metamorphism at this time. Based on these results, we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician, but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.

A geophysical investigation of the 2018 Lake Muir earthquake sequence: reactivated Precambrian structures controlling modern seismicity

Seismicity in the intraplate southwest of Western Australia is poorly understood, despite evidence for potentially damaging earthquakes of magnitude > M6. Identifying stress-focusing geological structures near significant earthquake sequences assists in understanding why these earthquakes occur in seemingly random locations across a region of more than 250 000 km2. On 16 September 2018, an ML5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an ML5.4 aftershock. The main earthquake formed a mainly north-trending fault scarp ∼5 km in length and with a maximum vertical displacement of ∼40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of ML5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major ∼ east–west-trending faults segmenting eight distinct geological domains, as well as a network of less prominent northwest-trending faults, one of which aligns with the southern segment of the scarp. Surface faulting, surface deformation and earthquake focal mechanism studies suggest movements on north- and northeast-trending structures. The main shock, the aftershocks, surface faulting and changes in InSAR-derived surface elevation all occur in a region bounded to the south by a prominent northwest-trending fault and to the north by a west-northwest-trending domain-bounding structure. Thus, we interpret the north-trending thrust fault associated with the main Lake Muir event as due to local stress concentration of the regional east–west stress field at the intersection of these structures. Further, we propose that a particularly large west-northwest-trending structure may be broadly focusing stress in the Lake Muir area. These findings encourage similar studies to be undertaken in other areas of Australia’s southwest to further the current understanding of seismic release in the region. KEY POINTS A large ML5.7 earthquake occurred in southwest Western Australia in September 2018, damaging infrastructure and forming a 40 cm-high thrust fault scarp. An aeromagnetic interpretation of the Lake Muir region revealed that the new scarp aligned with pre-existing faults that were reactivated. Local and regional structures likely acted as regional stress focusers in the Lake Muir area. Identifying regional stress-focusing structures is important for the purpose of understanding seismic risk in intraplate areas such as Western Australia.

Differential crustal rotation and its control on giant ore clusters along the eastern margin of Tibet

AbstractControls on the formation and distribution of mineralization in continental collisional settings remain unclear. However, our synthesis of diverse geophysical data sets from the eastern margin of Tibet revealed that differential crustal rotation played a key role in the production of a variety of mineralization types. Due to Cenozoic continental collision between India and Eurasia, the elongated continental blocks in the eastern margin of Tibet were extruded and reoriented. Prior to block extrusion in the Eocene, two giant porphyry-skarn ore clusters formed at the boundaries between the central segment and both the northern and southern segments of the Jinshajiang-Ailaoshan suture zone. These crustal segment boundaries displayed counterclockwise rotation, due to clockwise rotation of the central segment relative to both the essentially immobile northern and southern segments, combined with crust-mantle decoupling. This is considered to have induced crustal friction and resultant generation of fertile magmas that formed the porphyry-skarn Cu-Au deposits. During Oligocene–Miocene block extrusion, differential rotation of upper crust occurred on the western and eastern sides of the north-northwest–trending Central Axis fault in the Lanping-Simao basin. Two Oligocene–Miocene Mississippi Valley–type ore clusters occur on fault segments with anomalous differential rotation of 70° to 80°, suggesting that this differential rotation resulted in local extension with consequent ore-fluid influx.

Complex co- and postseismic faulting of the 2017–2018 seismic sequence in western Iran revealed by InSAR and seismic data

The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran–Iraq border on 12 November 2017. This Mw 7.3 earthquake was followed by an intense seismic sequence. Implementing the double-difference earthquake location technique, we relocate 1069 events recorded by our local seismic network, deployed after the mainshock. The spatial distribution of the epicenters indicates linear alignments of the events nucleated along at least four notable clusters. The clusters are characterized by at least one significant earthquake, such as the Tazehabad earthquake of 25 August 2018 (Mw 5.9) along a dense, east–west trending cluster and the Sarpol-e Zahab earthquake of 25 November 2018 (Mw 6.3) along the cluster with a northeast–southwest trend. We use two-pass differential SAR interferometry (DInSAR) and Small BAseline Subset (SBAS) methods to study the coseismic permanent displacements of the Azgeleh, Tazehabad and Sarpol-e Zahab events as well as the one-year postseismic deformation field of the 2017–2018 seismic sequence, respectively. We use non-linear and linear optimization algorithms to derive the source geometry and the slip distribution along the fault planes. The inversion is conducted by introducing also seismological constraints, leading to the definition of a listric geometry for the Azgeleh mainshock rupture that accommodates the slip area at depth of 10–16 km along a sub-horizontal plane (dipping ~3°) and a low-angle (~16°) ramp. The thrust and dextral movements along this NNW-striking (~345°) fault have triggered a tear fault responsible for the Tazehabad event ruptured an east–west trending (~267°), north-dipping (~78°) sinistral shear fault. We present the dextral slip distribution of the Sarpol-e Zahab event along a NE-striking (~34°) fault, as a synthetic Riedel structure for the southern segment of the Khanaqin fault, dipping 63° to the southeast. We find the postseismic deformation field associated with the seismic sequence is not confined only to the mainshock source (the Azgeleh fault), but also develops along the Tazehabad and Sarpol-e Zahab faults. We additionally propose afterslip along a duplex, flat-ramp-flat structure down-dip and up-dip of the Azgeleh coseismic slip area. The up-dip afterslip develops onto the shallow detachment (~3°) at depth of ~8 km and the down-dip afterslip propagate onto the mid-crustal décollement level within the Pan-African basement. The Azgeleh, Tazehabad, Sarpol-e Zahab and Khanaqin faults mark the Lurestan Arc–Kirkuk Embayment sharp margin in the Northwest Zagros and play a key role in the lateral escape of the Lurestan Salient and vertical strain partitioning in the Zagros front.

О тектоно-геодинамических взаимоотношениях Евразийского бассейна и хребта Ломоносова с континентальной окраиной Сибири по новым сейсмическим данным

Basing on the results of the interpretation of new seismic materials, the authors consider the structural features of the southern segments of the Eurasian Basin and the Lomonosov Ridge in the zone of junction with the continental margin of Siberia (the Laptev and East Siberian Seas). Interpretative analysis of the materials shows that the base of the sedimentary cover of the southern segment of the Eurasian Basin, where there are no regular linear magnetic anomalies, is predominantly represented by strongly stretched blocks of the continental basement. The formation the axial zone of spreading of the Gakkel Ridge here took place according to a three-stage development scheme: rifting in the Aptian-Alba and its telescoping development in the Late Cretaceous — Paleocene-Eocene, inherited valley formation in late neotectonic time. The development of the Severny Basin located in the zone of junction of the Lomonosov Ridge with the continental margin is similar to the scenario for the formation of pull-apart basins. Its formation was interconnected with the simultaneously opening adjacent extreme southeastern segment of the Amundsen Basin of the Eurasian Basin.

Geochronology and Petrogenesis of Granitoid Intrusions in the Feidong District, Southern Tan–Lu Fault Zone, China

AbstractThe Feidong district is located in the southern segment of the Tan–Lu fault zone that separates the South China Block (NCB) from the North China Craton (NCC). We report zircon U‐Pb geochronology and Hf isotope data, as well as whole‐rock geochemistry for Xishanyi granodiorite and Jianshan granite in the Feidong district. Zircon U‐Pb dating results show that the emplacement ages of the Xishanyi and Jianshan intrusions are 124 ± 3 Ma and 130 ± 1 Ma respectively, coeval with magmatic events linked to large‐scale lithospheric thinning in eastern China. The whole‐rock geochemistry of the Xishanyi and Jianshan intrusions demonstrate that they are peraluminous, high potassium calc‐alkaline I‐type granites with adakitic characteristics. Both intrusions underwent weak crustal assimilation during emplacement. The in situ zircon ∊Hf(t) values of the Xishanyi granodiorites range from –26.4 to –21.8, with TDM2 model ages of 2552 to 2841 Ma. The in situ zircon ∊Hf(t) values of the Jianshan granite are from –27.5 to –23.0 with TDM2 model ages of 2632 to 2904 Ma. The peak age of inherited zircon grains from the Xishanyi granodiorite and the Jianshan granite were ∼2.07 Ga and ∼1.94 Ga, respectively. After compared with the regional magmatism, we suggest that both the Xishanyi and Jianshan granitoid intrusions were derived from partial melting of the NCC lower crust.

Large-Scale Vortex Motion and Multiple Plasmoid Ejection Due to Twisting Prominence Threads and Associated Reconnection

We analyze the characteristics of a quiescent polar prominence using the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). Initially, small-scale barb-like structures are evident on the solar disk, which firstly grow vertically and thereafter move towards the south-west limb. Later, a spine connects these barbs and we observe apparent rotating motions in the upper part of the prominence. These apparent rotating motions might play an important role for the evolution and growth of the filament by transferring cool plasma and magnetic twist. The large-scale vortex motion is evident in the upper part of the prominence, and consists of a swirl-like structure within it. The slow motion of the footpoint twists the legs of the prominence due to magnetic shear, causing two different kinds of magnetic reconnection. The internal reconnection is initiated by a resistive tearing-mode instability, which leads to the formation of multiple plasmoids in the elongated current sheet. The estimated growth rate was found to be 0.02--0.05. The magnetic reconnection heats the current sheet for a small duration. However, most of the energy release due to magnetic reconnection is absorbed by the surrounding cool and dense plasma and used to accelerate the plasmoid ejection. The multiple plasmoid ejections destroy the current sheet. Therefore, the magnetic arcades collapse near the X-point. Oppositely directed magnetic arcades may reconnect with the southern segment of the prominence and an elongated thin current sheet is formed. This external reconnection drives prominence eruption.