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.

中国东北完达山地区早白垩世同构造岩浆侵位——对晚中生代左行走滑作用的响应

分布于中国东北完达山地区的饶河花岗岩岩体中暗色矿物和斑晶钾长石定向排列,呈北北东走向,其中透镜状闪长质捕掳体近水平排列,局部具有左行剪切的特点。岩体中发育石香肠状石英脉,表明岩体在侵位过程中受到左行剪切作用的影响或制约。对出露的花岗岩进行LA-ICP-MS锆石U-Pb定年,获得年龄121±1Ma和119±1Ma,表明该岩浆流动形成于早白垩世。同时对围岩辉长岩、侵入岩体中的正长岩脉和辉绿岩脉进行锆石U-Pb年龄分析,分别获得160±1Ma、109±2Ma、124±1Ma的年龄结果。根据各样品中继承锆石的特征,围岩辉长岩的年龄数据很集中,不存在古老锆石的年龄信息。岩浆流动岩体及岩脉中都有太古宙、元古宙等各时代的锆石年龄数据,可能表明完达山地区在约120Ma之前已完成古太平洋板块的俯冲拼贴,饶河岩体形成于走滑环境下的陆内变形,为同构造侵入岩。

走滑作用对渤海凸起区油气聚集的控制作用:以沙垒田凸起为例

渤海海域已发现油气中绝大部分受NE向走滑性质断层的控制,但对其具体控制过程缺乏深入分析。本研究选取NE向断层发育、且有相当油气发现的沙垒田凸起为解剖实例,从构造演化的角度对NE向断层控藏过程与机理进行了分析。结果表明,研究区NE向断层对油气运聚的控制作用是一个长期演化的累积效果,主要通过两个重要过程实现:中生代左旋活动分割基底,改造基底高点,控制油气运移路径;新近纪晚期右旋活动分割披覆构造,改造盖层高点,控制油气聚集场所。这一认识可为渤海其他地区油气勘探提供借鉴。

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”.

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.

Shallow Structure of the Crust in the Sulu-Dabie Region,China and its Seismotectonic Implication

The P-wave velocity structure in the shallow crust is investigated in and around the Sulu-Dabie region by using seismic reflection data for deep soundings in 48 survey profiles and from rock velocity determinations.The observed velocity distributions show obvious heterogeneities in this region.The low velocity anomalies are observed mainly in the west of the Dabie region and the East Sea regions.The high velocity anomalies emerge in the shallow crust of the Sulu and Dabie orogeny.These high-velocity anomalies can be attributed to the ultra-high pressure metamorphosed(UHPM)rock formed by exhumation motion of mantle materials during the orogeny.The high-velocity anomalies in the different shallow layers beneath the Sulu region are located to the northeast of the Tan-Lu fault.The high-velocity anomalies beneath the Dabie region are located southwest of the Tan-Lu fault.Such a distribution pattern of velocity anomaly zones may reveal historical motion of a left-lateral strike-slip for the Tan-Lu fault,which differs from the result of a right-lateral strike-slip motion regime known from modern seismology,indicating a more complex tectonic motion along the Tan-Lu fault.

湖南邵阳—郴州北西向断裂左旋走滑暨水口山—香花岭南北向构造成因

湖南大义山断裂属邵阳—郴州NW向断裂的南东段,该断裂南、北两侧发育水口山—香花岭南北向构造带。根据构造变形特征并结合区域构造背景和构造演化分析,确定大义山断裂中新生代至少经历了3次构造活动,从早至晚依次为印支期左旋走滑、燕山期右旋走滑和喜山期(?)左旋走滑。对SN向构造与区域NNE向构造的时代与相互作用关系进行了深入分析,并提出SN向构造成因新认识:由于大义山NW向断裂和香花岭—阳明山NW向断裂在印支运动中产生基底左旋走滑,走滑所派生的近EW向挤压应力场以及走滑所产生的牵引作用,使区域NNE向构造线产生逆时针旋转而形成SN向构造。进一步分析认为,印支期可能为湘东主要NW向断裂在中生代活动最强烈的时期,多条NW向断裂的左旋走滑,可能是雪峰山弧形构造和祁阳“山”字型构造的形成机制之一。

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.

Surface Rupture and Coseismic Displacement of the M_S7.1 Yushu Earthquake

On April 14, 2010, a devastating earthquake measured 7.1 on the Richter scale struck Yushu county, Qinghai Province, China. Field geological investigation and remote sensing interpretation show that this earthquake generated an inverse ＂L-shaped＂ surface rupture zone, approximately 50km long. The surface rupture zone can be divided into three segments. Between the northern and middle segments of the surface rupture, there is a 16km-long segment, where no rupture was observed. The middle and the southern segments are arranged in a left-step manner, and there are right-step en echelon ruptures developed in the stepovers. The seismogenic structure is the Yushu fault, which is dominated by strike-slip with a small amount of thrust component. The earthquake results from the differential movements between the southern Qiangtang Block and northern Bayan Har Block. The earthquake recurrence interval is 185a^108a. Along an approximately 20km-long part of the Garze-Yushu fault, between the southern surface rupture of Yushu Ms7. 1 earthquake and the 1896 earthquake, there is no surface rupture, its seismic risk needs further research.

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.

Formation and evolution of the South China Sea since the Late Mesozoic:A review

The existing genetic models of the South China Sea(SCS)include an extrusion model of the Indochina Peninsula,a back-arc extension model,and a subduction and dragging model of the Proto-South China Sea(PSCS).However,none of these models has been universally accepted because they do not fully match a large number of geological phenomena and facts.By examining the regional tectonics and integrating them with measured data for the SCS,in this study,a back-arc spreading-sinistral shear model is proposed.It is suggested that the SCS is a back-arc basin formed by northward subduction of the PSCS and its formation was triggered by left-lateral strike-slip motion due to the northward drift of the Philippine Sea Plate.The left-lateral strike-slip fault on the western margin caused by the Indo-Eurasian collision changed the direction of the Southwest Sub-basin's spreading axis from nearly E–W to NE–SW,and subduction retreat caused the spreading ridge to jump southward.This study summarizes the evolution of the SCS and adjacent regions since the Late Mesozoic.

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.

^(40)Ar-^(39)Ar ages of the Louzidian-Dachengzi ductile shear zone near Chifeng,Inner Mongolia and their tectonic significance

The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone developed in its lower-plate was genetically related to sinistral strike-slips and extensional faulting. Two samples from this ductile shear zone yield 40Ar-39Ar plateau ages of 133 Ma (Bi) and 126 Ma (Kp), which are concordant with their isochron ages. The plateau age of 133 Ma (Bi) records the formation age of the ductile shear zone. The inconsistent relationship between the earlier strike-slip ductile shear zone and the later normal fault makes the Kalaqin Quasi-metamorphic core complex distinctive from Cordil-leran nietamorphic core complex. These ages provide important geochronological data for putting constraints on the formation age and genesis of such ductile shear zones.

Investigation of geothermal structure of the Sulawesi,using gravity and magnetic method

The Sulawesi Sea and Sulawesi Island are located in the western Pacific area where volcanic activity,plate subduction,and seismic activity are very active.The Sulawesi basin formed during the Middle Eocene-Late Eocene and nearly half of the Eocene oceanic crust has subducted below the North Sulawesi Trench.The Sulawesi Island was spliced and finalized in the Early Pliocene-Pleistocene during volcanic activity and is recently very active.This area is an optimal location to study volcanic geothermal conditions and subduction initiation mechanisms in the southern part of the western Pacific plate margin,which are important in geothermal and geodynamic research.In this study,we combined 133 heat flow data with gravity and magnetic data to calculate the Moho structure and Curie point depth of the Sulawesi Sea and periphery of the Sulawesi Island,and analyze the distribution characteristics of the geothermal gradient and thermal conductivity.The results show that the average depths of the Moho and Curie surfaces in this area are 18.4 and 14.3 km,respectively,which is consistent with the crustal velocity layer structure in the Sulawesi Basin previously determined by seismic refraction.The average geothermal gradient is 4.96°C(100 m)-1.The oceanic area shows a high geothermal gradient and low thermal conductivity,whereas the land area shows a low geothermal gradient and high thermal conductivity,both of which are consistent with statistical results of the geothermal gradient at the measured heat flow points.The highest geothermal gradient zone occurs in the transition zone from the Sulawesi Sea to Sulawesi Island,corresponding to the spreading ridge of the southward-moving Sulawesi Basin.Comprehensive gravity,magnetic,and geothermal studies have shown a high crustal geothermal gradient in the study area,which is conducive to the subduction initiation.The northern part of the Palu-koro fault on the western side of Sulawesi is likely the location where subduction initiation is occurring.During the process of moving northwest,the northern and eastern branches of Sulawesi Island have different speeds;the former is slow and the latter is fast.These branches also show different deep tectonic dynamic directions;the northern branch tilts north-up and the eastern branch tilts north-down.

Compressive Tectonics around Tibetan Plateau Edges

Various earthquake fault types, mechanism solutions, stress field, and other geophysical data were analyzed for study on the crust movement in the Tibetan plateau and its tectonic implications. The results show that numbers of thrust fault and strike-slip fault type earthquakes with strong compressive stress near NNE-SSW direction occurred in the edges around the plateau except the eastern boundary. Some normal faulting type earthquakes concentrate in the Central Tibetan plateau. The strikes of fault planes of thrust and strike-slip faulting earthquakes are almost in the E-W direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. This implies that the dislocation slip vectors of the thrust and strike-slip faulting type events have quite great components in the N-S direction. The compression motion mainly probably plays the tectonic active regime around the plateau edges. The compressive stress in N-S or NE-SW directions predominates earthquake occurrence in the thrust and strike-slip faulting event region around the plateau. The compressive motion around the Tibetan plateau edge is attributable to the northward motion of the Indian subcontinent plate. The northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins.