As the key area of interaction between land and sea, continental shelf is important for the tectonic evolution of continent, sea-land change, sea level eustacy and climate change. Due to the limits of different methods, the understanding of the chronology and potential geological information of the sediments on the continental shelf is not enough. The South China Sea, as the largest marginal sea of the West Pacific, is not only one of the most active areas of marine sedimentation in the world, but also the typical region of the interaction between land and sea. As the main sedimentary area of the East Asia, the South China Sea has received increasing academic research attention. At present, the researches mostly focus on the deep-sea sediments because they are continuous and can record stable signals, even though the relative slow deposition and low resolution. Comparatively, the shallow continental shelf deposits with faster sedimentary rate and higher resolution can provide important geological materials for studying the high-resolution chronology and paleoenvironment. However, the sedimentary signals recorded by the continental shelf sediments are unstable and even missing due to the turbulence of the sedimentary environment of the continental shelf. There are relatively few studies on the continental shelf sediments of the South China Sea, especially the high-resolution chronology of cores, thus limiting the understanding of tectonic and climate evolution of the South China Sea. In order to better constrain the geological chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, study the paleoenvironmental signals in the continental shelf sediments and discuss the driving mechanism of the climate changes in East Asia and provide the chronological framework for the study of marine active tectonics in the South China Sea, the comparison between magnetic susceptibility and Marine Oxygen Isotope based on microscopic paleonotological fossils and carbon isotopic age(¹⁴C)was studied on the Core DG in this paper. Additionally, the results of sediments color and pollens were used to study the paleoclimatic implications. The results of magnetic susceptibility suggest that the chronology of the sediments of Core DG can be constrained from MIS 1 to MIS 9, with the age of the bottom being about 300ka. The relative high and low values of magnetic susceptibility correspond to interglacial and glacial periods, respectively. This is consistent with the paleoclimatic signals evidenced by the changes of pollen and color parameters in the DG core sediments. Therefore, we suggest that the magnetic susceptibility of continental shelf sediments can be affected by the changes of climate. During glacial periods, the relative cold weather, shallow water and increased transportation distance of the sediments resulted in the enhanced oxidation and the formation of minerals with weak magnetic susceptibility(such as hematite), thus the magnetic susceptibility decreased and the redness increased in the sediments. However, during interglacial periods, the relative warm and wet climate, together with the decreased transportation distance of the sediments, led to the formation of minerals with strong magnetic susceptibility(such as magnetite), thus the magnetic susceptibility enhanced significantly and the redness decreased in the sediments. Therefore, the variations of the magnetic susceptibility in the continental shelf sediments in the northern part of the South China Sea can reflect the glacial-interglacial cycles in the East Asia since the late Pleistocene. In conclusion, as a relative dating method used in the unconsolidated sediments in the late Quaternary, the comparison between magnetic susceptibility and Marine Oxygen Isotope is applicative and reliable in constraining the chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, thus providing a new reference for studying and correlating the continental shelf sediments, which can be used reasonably in the Quaternary chronology.

At 02:04 a.m. on May 22, 2021, a M_S7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.
This preliminary study finds that the seismogenic fault of the Maduo M_S7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands.
Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs.

The deformation pattern in the northeastern margin of Tibetan plateau is characterized by NE compression, clockwise rotation and eastward extrusion, forming the NNE trending dextral strike-slip faults which further divide the region into several sub-blocks. The deformation of Qaidam secondary block is dominant by northwestward extrusion and rotation, which is controlled by the Elashan and East Kunlun faults. However, the deformation style of Dulan area, the junction of these two faults, remains unclear. We discovered a new active fault zone with a length of 60~70km west to Elashan Fault during our recent field geological survey around Dulan area, named Xiariha fault zone(XFZ), which is a dextral strike-slip fault zone trending NW, consisting of the Xiariha and Yingdeerkang faults. According to the remote sensing interpretation and field investigation, it is found that the Xiariha fault zone showed distinct linear characteristics, reverse scarp, sag pond and ridge dislocation on the satellite images and displaced multi-levels of alluvial fans and river terraces. According to previous studies, the exposed age of T1 terraces is Holocene in the Elashan area, which is located at east of Dulan. During the field investigation, we used the unmanned aerial vehicle(UAV)to get the fine geomorphology features along the XFZ. Also, to define the active era, we tried to find the fault section of the XFZ that could provide the information of the contact between the fault and late Quaternary strata. Based on the high-resolution DEM obtained by UAV, the offset of T1 is about 2.5m, indicating its activity in Holocene compared with the Elashan area. Along the XFZ, the fault displaced late Quaternary strata revealed on the section. The geomorphic features and fault section show that the XFZ is a late Pleistocene to Holocene active fault. The Dulan area is located at the convergence of East Kunlun Fault and Elashan Fault, the southeastern end of Qaidam secondary block, which is affected by the regional NE and SW principal compressive stress and shear stress. Under this circumstance, the Qaidam block is experiencing extrusion and rotation and there are a series of NW-trending dextral strike-slip faults parallel to the Elashan Fault and EW-trending sinistral strike-slip faults parallel to the East Kunlun Fault, such as Reshui-Taosituo River Fault, developed in the Dulan area. Therefore, we suggest that the Xiariha Fault and the nearly EW trending, Holocene sinistral Reshui-Taosituo River Fault adjust the extrusion rotation deformation jointly at the southeast end of the Qaidam block under the control of the Elashan Fault and the East Kunlun Fault, respectively. Meanwhile, the new discovery of Xiariha Fault and its activity in Holocene is not only of great significance to understand the regional tectonic deformation model, but also leads to a great change in the understanding of regional seismic risk because of its capabliliby of generating strong earthquakes. Therefore, it is urgent to carry out further research work in this area, improve the understanding of regional strain distribution mode, and provide reference for regional seismic safety issues.

The 40km-long, NEE trending Reshui-Taostuo River Fault was found in the southern Dulan-Chaka highland by recent field investigation, which is a strike-slip fault with some normal component. DEM data was generated by small unmanned aerial vehicle(UAV)on key geomorphic units with resolution<0.05m. Based on the interpretation and field investigation, we get two conclusions: 1)It is the first time to define the Reshui-Taostuo River Fault, and the fault is 40km long with a 6km-long surface rupture; 2)There are left-handed dislocations in the gullies and terraces cut by the fault. On the high-resolution DEM image obtained by UAV, the offsets are(9.3±0.5)m, (17.9±1.5)m, and(36.8±2)m, measured by topographic profile recovery of gullies. The recovery measurements of two terraces present that the horizontal offset of T₁/T₀ is(18.2±1.5)m and the T₂/T₁ is (35.8±2)m, which is consistent with the offsets from gullies. According to the historical earthquake records, a M53/4 earthquake on April 10, 1938 and a M_S5.0 earthquake on March 21, 1952 occurred at the eastern end of the surface rupture, which may be related to the activity of the fault. By checking the county records of Dulan and other relevant data, we find that there are no literature records about the two earthquakes, which is possibly due to the far distance to the epicenter at that time, the scarcity of population in Dulan, or that the earthquake occurred too long ago that led to losing its records. The southernmost ends of the Eastern Kunlun Fault and the Elashan Fault converge to form a wedge-shaped extruded fault block toward the northwest. The Dulan Basin, located at the end of the wedge-shaped fault block, is affected by regional NE and SW principal compressive stress and the shear stress of the two boundary faults. The Dulan Basin experienced a complex deformation process of compression accompanying with extension. In the process of extrusion, the specific form of extension is the strike-slip faults at each side of the wedge, and there is indeed a north-east and south-west compression between the two controlling wedge-shaped fault block boundary faults, the Eastern Kunlun and Elashan Faults. The inferred mechanism of triangular wedge extrusion deformation in this area is quite different from the pure rigid extrusion model. Therefore, Dulan Basin is a wedge-shaped block sandwiched between the two large-scale strike-slip faults. Due to the compression of the northeast and southwest directions of the region, the peripheral faults of the Dulan Basin form a series of southeast converging plume thrust faults on the northeast edge of the basin near the Elashan Fault, which are parallel to the Elashan Fault in morphology and may converge with the Elashan Fault in subsurface. The southern marginal fault of the Dulan Basin(Reshui-Taostuo River Fault)near the Eastern Kunlun fault zone is jointly affected by the left-lateral strike-slip Eastern Kunlun Fault and the right-lateral strike-slip Elashan Fault, presenting a left-lateral strike-slip characteristic. Meanwhile, the wedge-shaped fault block extrudes to the northwest, causing local extension at the southeast end, and the fault shows the extensional deformation. These faults absorb or transform the shear stress in the northeastern margin of the Tibet Plateau. Therefore, our discovery of the Dulan Reshui-Taostuo River Fault provides important constraints for better understanding of the internal deformation mode and mechanism of the fault block in the northeastern Tibetan plateau.
The strike of Reshui-Taostuo River Fault is different from the southern marginal fault of the Qaidam Basin. The Qaidam south marginal burial fault is the boundary fault between the Qaidam Basin and the East Kunlun structural belt, with a total length of ~500km. The geophysical data show that Qaidam south marginal burial fault forms at the boundary between the positive gravity anomaly of the southern East Kunlun structural belt and the negative gravity anomaly gradient zone of the northern Qaidam Basin, showing as a thrust fault towards the basin. The western segment of the fault was active at late Pleistocene, and the eastern segment near Dulan County was active at early-middle Pleistocene. The Reshui-Taostuo River Fault is characterized by sinistral strike-slip with a normal component. The field evidence indicates that the latest active period of this fault was Holocene, with a total length of only 40km. Neither remote sensing image interpretation nor field investigation indicate the fault extends further westward and intersects with the Qaidam south marginal burial fault. Moreover, it shows that its strike is relatively consistent with the East Kunlun fault zone in spatial distribution and has a certain angle with the burial fault in the southern margin of Qaidam Basin. Therefore, there is no structural connection between the Reshui-Taostuo River Fault and the Qaidam south marginal burial fault.

In the interior of the Tibetan Plateau, the active tectonics are primarily marked by conjugate strike slip faults and north-trending rifts, which represent the E-W extension since late Cenozoic of the plateau. The conjugate faults are mainly composed of NE-trending left-lateral strike-slip faults in Qiangtang terrane and NW-trending right-lateral strike-slip faults in Lhasa terrane. While, the rifts mainly strike N, NNW and NNE within southern Tibet. However, it is still a debate on the deformational style and specific adjustment mechanism of E-W extension. One of key reasons causing this debate is the lack of detailed investigation of these active faults, especially within the northwestern plateau. Recently, we found a 20km long, NNW-trending active fault at Bero Zeco in northwestern Tibet. This fault is presented as fault sag ponds, channel offsets and fault scarps. Displacement of channels and geomorphic features suggested that the Bero Zeco Fault(BZF)is a dextral strike-slip fault with a small amount of normal slip component, which may result from the E-W extensional deformation in the interior of Tibet. BZF strikes N330°~340°W, as shown on the satellite image. The main Quaternary strata in the studied area are two stages alluvial fans around the Bero Zeco. From the satellite images, the old alluvial fans were cut by the lake shoreline leaving many of lake terraces. And the young fans cut across the lake terraces and the old fans. By contrasting to the "Paleo-Qiangtang Huge Lake" since late Quaternary, these old alluvial fans could be late Pleistocene with age ranging from 40ka to 50ka. And the young fans could be Holocene. The sag ponds along the BZF are distributed in the late Pleistocene alluvial fans. Also, the BZF displaced the late Pleistocene fans without traces within Holocene fans, suggesting that the BZF is a late Pleistocene active fault. The fault scarps are gentler with the slope angle of around 10° and the vertical offset is about 2m by field measurement. Reconstruction of the offset of channels suggested that the accumulated dextral offset could be about 44m on the late Pleistocene alluvial fans. Therefore, we infer that the dextral slip-rate could be around 1mm/a showing a low-rate deformation characteristic. The angle between the strike of BZF and principal compressive stress axis(σ₁)is around 30°, which is significantly different to the other faults within the conjugate strike-slip fault zones that is 60°~75°. Now, the deformation mechanisms on these conjugate faults are mainly proposed in the studies of obtuse angle between the faults and σ₁, which is likely not applicable for the BZF. We infer that the BZF could be the northward prolongation of the north-trending rifts based on the geometry. This difference suggests that the conjugate strike-slip faults may be formed by two different groups:one is obtuse angle, which is related to block extrusion or shear zones in Lhasa and Qiangtang terranes possibly; the other is acute angle, which may represent the characteristics of new-born fractures. And more studies are needed on their deformation mechanisms.