Sinistral Strike-slip Movement Along Western Terminal Segment of the Active Daqingshan Piedmont Fault, Inner Mongolia, China

There are 18 gullies displaying sinistral contortions to different degrees along the western terminal segment about 10 km long of the active Daqingshan piedmont fault near the Donghe District, Baotou City. The contortion amount of gullies ranges from 20 m to 300 m. The contortion and length of the gullies are in direct proportion. The relation between piedmont terraces and gullies indicates that the gullies with upper reaches of about 1 ～ 5 km long and those smaller than one kilometer were formed at the end of Late Pleistocene and Holocene.Meanwhile, sandy gravel layer of alluvial-proluvial sediment on the upthrown wall is directly in contact with yellow clayey sand of the downthrown wall. During the Holocene, the sinistral strike-slip rate along the western terminal segment of the active Daqingshan piedmont fault reached 5 mm/a from age data of dislocated sediments. The evolutional mechanism of the active Daqingshan piedmont fault is also discussed in the paper.

The Altun Fault: Its Geometry, Nature and Mode of Growth

The Altun (or Altyn Tagh) fault displays a geometry of overlapping of linear and arcuate segments and shows strong inhomogeneity in time and space. It is a gigantic fault system with complex mechanical behaviours including thrusting, sinistral strike slip and normal slip. The strike slip and normal slip mainly occurred in the Cretaceous—Cenozoic and Plio-Quaternary respectively, whereas the thrusting was a deformation event that has played a dominant role since the late Palaeozoic (for a duration of about 305 Ma). The formation of the Altun fault was related to strong inhomogeneous deformation of the massifs on its two sides (in the hinterland of the Altun Mountains contractional deformation predominated and in the Qilian massif thrust propagation was dominant). The fault experienced a dynamic process of successive break-up and connection of its segments and gradual propagation, which was synchronous with the development of an overstep thrust sequence in the Qilian massif and the uplift of the Qinghai-Tibet plateau. With southward propagation of the thrust sequence and continued uplift of the plateau, the NE tip of the Altun fault moved in a NE direction, while the SW tip grew in a SW direction.

Deformation at the Easternmost Altyn Tagh Fault: Constraints on the Growth of the Northern Qinghai–Tibetan Plateau

How the Altyn Tagh fault(ATF) extends eastwards is one of the key questions in the study of the growth of the Qinghai–Tibetan Plateau. Detailed fieldwork at the easternmost part of the ATF shows that the ATF extends eastward and bypasses the Kuantan Mountain;it does not stop at the Kuantan Mountain, but connects with the northern Heishan fault in the east. The ATF does not enter the Alxa Block but extends eastward along the southern Alxa Block to the Jintanan Mountain. The Heishan fault is not a thrust fault but a sinistral strike-slip fault with a component of thrusting and is a part of the ATF. Further to the east, the Heishan fault may connect with the Jintananshan fault. A typical strike-slip duplex develops in the easternmost part of the ATF. The cut and deformed Quaternary sediments and displaced present gullies along the easternmost ATF indicate that it is an active fault. The local highest Mountain(i.e., the Kuantan Mountain) in the region forms in a restraining bend of the ATF due to the thrusting and uplifting. The northward growth of the Qinghai–Tibetan Plateau and the active deformation in South Mongolia are realized by sinistral strike-slipping on a series of NE–SW-trending faults and thrusting in restraining bends along the strike-slip faults with the northeastward motion of blocks between these faults.

Holocene activity evidence on the southeast boundary fault of Heqing basin, middle segment of Heqing-Eryuan fault zone, West Yunnan Province, China

The Heqing-Eryuan fault is an important part of the active fault system in the Northwestern Yunnan Province, China. Thus, the study on the nature, characteristics and activity history of this fault can provide not only the basis for seismic safety and engineering evaluation, but also the important information for the characteristics, history and patterns of the structural deformation of the southeastern margin of the Tibetan Plateau. Trench and faulted landforms investigations could provide effective paleoseismic methods to obtain the recent parameters of active faults. Using these methods, this study makes some breakthroughs on the recent activity of southeast boundary fault of the Heqing basin, middle segment of Heqing-Eryuan fault zone. Results indicate that the average vertical slip rate and left-lateral slip rate of the segment are about 0.28 mm/a and 1.8o mm/a respectively since the Late Pleistocene. The trench near the Beixi Village at the .outhea.t houndnrv fault of the lqaaing basinreveals that there have been at least three paleoearthquake events during the Holocene （8 ka BP）. The vertical displacement and sinistral strike-slip distance of a single paleoearthquake are ~20 cm and -1.2 m, respectively. The estimated paleoearthquake magnitudes with Ms7.o, and the recurrence interval at 2-5 ka, as well as the latest activity time during 800- 290 ca1 yr BP, are of great significance for preventing and mitigating regional earthquake disasters.

Tancheng－Lujiang Fault System： Its Characters and Control on Oil－Gas Distribution in Eastern China

Tancbeng-Lujiang fault system is one of the largest strike-slip fault systems in eastern Asia.It extends southward to Beibuwan Bay to the west of Hainan Island and northward through Lujiang of Anhui Province, Tancheng of Shandong Province and Luobei of Heilongjiang Province in China to the territory of Russia. Its formation is related to the subduction of Kula-Pacific plate to the Asian continent. It is oriented approximately parallel to the eastern edge of Asia. It is dominated by the sinistral translation from Jurassic to Eocene and then by dextrose strike-slip. It has the following characters: (1)clear linear character; (2)sharp dip angle, usually changing between normal and reverse faults; (3)showing braided structure on the plan and flower structure in section;(4)alternated by uplifts and sags along the fault belt; (5)many stages of the eruptions of alkaline to calc-alkaline basalt magma along the fault belt; and (6) frequent activities of earthquakes along the fault belt. Its control over the oil-gas distribution is shown by the following racts: (1) the formation of many oil-bearing fault depressions; (2) the increase of the basin area it has passed through, thus increasing the basin's subsiding quantity and the oil reservoirs; and (3)the formation of many kinds of oil-gas trap structures.

Morphodynamic development of the Terkhiin Tsagaan Lake Depression,Central Mongolia:Implications for the relationships of Faulting,Volcanic Activity,and Lake Depression Formation

Data on the origin and morphology of lake depressions caused by volcanism are scarce in Mongolia.Previous studies focused on climate change patterns based on Terkhiin Tsagaan Lake sediment.We present a result of existing reconstructions of lake depression development and changes in the hydrology system during the Khorgo volcanic activation and the Holocene environmental change.A depression of the Terkhiin Tsagaan Lake is formed by a lava flow barrier from the Khorgo volcano.However,the Khorgo volcanic eruption and the lake depression that could shape a large lake have arisen instead from a fault.The morphometric analysis and field measurements indicate that the derivation of the Terkhiin Tsagaan Lake depression and Khorgo volcano may have evolved from movement on a sinistral strike-slip fault,which is about 70 km long.The southern mountains and rivers were displaced from northwest to southeast along the Terkh Fault.The offset along Terkh Fault is 4.02-5.28 km in the depression of the Terkhiin Tsagaan Lake.After movement,a wide valley of the Terkh River developed in the present landscape.The active Khorgo Volcano formed along the Khorgo Fault.The Terkhiin Tsagaan Lake is formed by blocked water from the PaleoTerkh River after lava damming from the Khorgo Volcano.The initial paleo-lake area was about 195.7km^(2),which was three times larger than the modern lake.The current water volume of the Terkhiin Tsagaan Lake is 0.351 km^(3) while the volume of the paleo-lake was 2.248 km^(3).Based on this volume indicator the paleo-lake was 6.4 times larger than the current lake.Overflowing water from the lake depression formed the Suman River by a drying canyon through the lava plateau,but the canyon is along the Terkh Fault.Changes in the water volume of Terkhiin Tsagaan Lake and erosion of Suman River canyon are inversely related to each other.We present the morphometric relationships between the lava plateau of Khorgo Volcano and development of Terkhiin Tsagaan Lake depression.

Late Quaternary sinistral slip rate along the Altyn Tagh fault and its structural transformation model

Based on technical processing of high-resolution SPOT images and aerophotos, detailed mapping of offset landforms in combination with field examination and displacement measurement, and dating of offset geomorphic surfaces by using carbon fourteen (14C), cos- mogenic nuclides (10Be+26Al) and thermoluminescence (TL) methods, the Holocene sinistral slip rates on different segments of the Altyn Tagh Fault (ATF) are obtained. The slip rates reach 17.5 ±2 mm/a on the central and western segments west of Aksay Town, 11±3.5 mm/a on the Subei-Shibaocheng segment, 4.8±1.0 mm/a on the Sulehe segment and only 2.2±0.2 mm/a on the Kuantanshan segment, an easternmost segment of the ATF. The sudden change points for loss of sinistral slip rates are located at the Subei, Shibaocheng and Shulehe triple junctions where NW-trending active thrust faults splay from the ATF and propagate southeastward. Slip vector analyses indicate that the loss of the sinistral slip rates from west to east across a triple junction has structurally transformed into local crustal shortening perpendicular to the active thrust faults and strong uplifting of the thrust sheets to form the NW-trending Danghe Nanshan, Daxueshan and Qilianshan Ranges. Therefore, the eastward extrusion of the northern Qing- hai-Tibetan Plateau is limited and this is in accord with “the imbricated thrusting transforma- tion-limited extrusion model”.

2022年1月8日青海门源M_(S) 6.9地震地表破裂考察的初步结果及对冷龙岭断裂活动行为和区域强震危险性的启示

2022年1月8日青海门源M_(S) 6.9地震发生在青藏高原东北缘的祁连山断块内部,仪器震中位于海原活动断裂系西段的冷龙岭断裂带上,是该断裂系自1920年海原8.5级大地震后再次发生M>6.5的强震。考察结果的初步总结表明,此次门源地震产生了呈左阶斜列分布、总长度近23 km的南北两条破裂,在两者之间存在长约3.2 km、宽近2 km的地表破裂空区。南支破裂(F_(1))出现在托来山断裂的东段,走向91°,长约2.4 km,以兼具向南逆冲的左旋走滑变形为主,最大走滑位移近0.4 m。北支主破裂(F_(2))出现在冷龙岭断裂的西段,总长度近20 km,以左旋走滑变形为主,呈整体微凸向北东的弧形展布,包含了走向分别为102°、109°和118°的西、中、东三段,最大走滑位移出现在中段,为3.0±0.2 m。此外,在北支主破裂中—东段的北侧新发现一条累计长度约7.6 km、以右旋正断为主的北支次级破裂(F_(3)),累计最大走滑量约0.8 m,最大正断位移约1.5 m。综合分析认为,整个同震破裂以左旋走滑变形为主,具有双侧破裂特点,宏观震中位于北支主破裂的中段,其地表走滑位移很大可能与震源破裂深度浅有关,其中的右旋正断次级破裂可能是南侧主动盘向东运移过程中拖曳北侧块体发生差异运动所引起的特殊变形现象。印度与欧亚板块近南北向强烈碰撞挤压导致南祁连断块沿海原左旋走滑断裂系向东挤出,从而引发该断裂系中的托来山断裂与冷龙岭断裂同时发生破裂,成为导致此次强震的主要动力机制。在此大陆动力学背景下,以海原左旋走滑断裂系为主边界的祁连山断块及其周边的未来强震危险性需得到进一步重视。

Characteristics,timing,and offsets of the middle-southern segment of the western boundary strike-slip fault of the Songliao Basin in Northeast China

As the west boundary fault of the Songliao Basin and the eastern margin of the Da Hinggan Mountains, the Nenjiang-Balihan fault is located in the central part of Northeast China. It is traditionally considered to be a huge deep-seated NNE-striking fault, characterized by a normal fault or detachment fault displacement. The field investigation resulted in the finding of ductile shear zones in the Lingxia and Louzidian areas, the middle and southern sectors of the fault system. The authors conducted meas- urements of structural elements in the field, micro-structural studies, finite strain measurements, a study on preferred crystal orientations of quartz determined by Electron Back Scatter Diffraction and muscovite 40Ar/9Ar chronology of the deformed rocks in the ductile shear zone. The results show that the deformation features of the Lingxia and Louzidian ductile shear zones are similar, and that they represent one continuous fault, i.e., the middle-southern segment of the Nenjiang-Balihan Fault, which experienced a sinistral strike-slip ductile shearing in the Early Cretaceous （-130 Ma）. By measuring the displacement of the Xar Moron River suture and Wolegen Group on both sides of the Nenjiang-Balihan Fault, it is found that the cumulative strike-slip offset of the fault is about 40-50 km.