Formation and evolution of the strike-slip faults in the central Sichuan Basin, SW China

Based on 3D seismic and drilling data, the timing, evolution and genetic mechanism of deep strike-slip faults in the central Sichuan Basin are thoroughly examined by using the U-Pb dating of fault-filled carbonate cement and seismic-geological analysis. The strike-slip fault system was initially formed in the Late Sinian, basically finalized in the Early Cambrian with dextral transtensional structure, was overlaid with at least one stage of transpressional deformation before the Permian, then was reversed into a sinistral weak transtensional structure in the Late Permian. Only a few of these faults were selectively activated in the Indosinian and later periods. The strike-slip fault system was affected by the preexisting structures such as Nanhuanian rifting normal faults and NW-striking deep basement faults. It is an oblique accommodated intracratonic transfer fault system developed from the Late Sinian to Early Cambrian to adjust the uneven extension of the Anyue trough from north to south and matches the Anyue trough in evolution time and intensity. In the later stage, multiple inversion tectonics and selective activation occurred under different tectonic backgrounds.

Control of strike-slip faults on Sinian carbonate reservoirs in Anyue gas field, Sichuan Basin, SW China

The largest Precambrian gas field (Anyue gas field) in China has been discovered in the central Sichuan Basin. However, the deep ancient Ediacaran (Sinian) dolomite presents a substantial challenge due to their tightness and heterogeneity, rather than assumed large-area stratified reservoirs controlled by mound-shoal microfacies. This complicates the characterization of “sweet spot” reservoirs crucial for efficient gas exploitation. By analyzing compiled geological, geophysical and production data, this study investigates the impact of strike-slip fault on the development and distribution of high-quality “sweet spot” (fractured-vuggy) reservoirs in the Ediacaran dolomite of the Anyue gas field. The dolomite matrix reservoir exhibits low porosity (less than 4%) and low permeability (less than 0.5×10^(-3) μm^(2)). Contrarily, fractures and their dissolution processes along strike-slip fault zone significantly enhance matrix permeability by more than one order of magnitude and matrix porosity by more than one time. Widespread “sweet spot” fracture-vuggy reservoirs are found along the strike-slip fault zone, formed at the end of the Ediacaran. These fractured reservoirs are controlled by the coupling mechanisms of sedimentary microfacies, fracturing and karstification. Karstification prevails at the platform margin, while both fracturing and karstification control high-quality reservoirs in the intraplatform, resulting in reservoir diversity in terms of scale, assemblage and type. The architecture of the strike-slip fault zone governed the differential distribution of fracture zones and the fault-controlled “sweet spot” reservoirs, leading to wide fractured-vuggy reservoirs across the strike-slip fault zone. In conclusion, the intracratonic weak strike-slip fault can play a crucial role in improving tight carbonate reservoir, and the strike-slip fault-related “sweet spot” reservoir emerges as a unique and promising target for the efficient development of deep hydrocarbon resources. Tailored development strategies need to be implemented for these reservoirs, considering the diverse and differential impacts exerted by strike-slip faults on the reservoirs.

LASER PROBE ^(40)Ar/ ^(39)Ar DATING OF MICAS ON THE DEFORMED ROCKS FROM ALTYN FAULT AND ITS TECTONIC IMPLICATIONS,WESTERN CHINA

Recently, the Altyn strike\|slip fault in western China has become a hot topic to the research on continental dynamics of Tibetan Plateau. The chronological research is very important to constrain the age of tectono\|thermal event within Altyn fault belt. Many isotopic dating researches, related to the ophiolites, high\|pressure metamorphic rocks and some granitic rocks, have been done by Chinese and foreign geologists in the area. There, however, are only few isotopic dating researches on the syntectonic\|growing minerals within Altyn fault. We collected a sample of Caledonian mylonitized granite (At3a) in the north of Dangjin pass and two samples of Jurassic meta\|sedimentary rocks in Qaidam gate fault\|valley (At37c) and Geshi fault\|valley (At30d). All the samples contain the syntectonic\|growing minerals such as white mica, chlorite, sericite and biotite etc. By dating these minerals, we can constrain the time of the tectonic events occurred in Altyn fault belt.Sample At3a, mylonitized granite, has been strongly deformed with undulating extinction of quartz. The plagioclase and quartz were elongated and surrounded by fine\|grained white micas and chlorites with strain shadow texture. These suggest that the sample has been suffered ductile deformation. The estimation of p\|T condition is 350℃, 180MPa. The isochronal age of (89.2±1.6)Ma has been obtained by laser probe 40 Ar/ 39 Ar dating analyses of six white mica grains.

The Dihedral Angle and Intersection Processes of a Conjugate Strike-Slip Fault System in the Tarim Basin, NW China

Recent studies, focused on dihedral angles and intersection processes, have increased understandings of conjugate fault mechanisms. We present new 3-D seismic data and microstructural core analysis in a case study of a large conjugate strike-slip fault system from the intracratonic Tarim Basin, NW China. Within our study area, ＂X＂ type NE and NW trending faults occur within Cambrian- Ordovician carbonates. The dihedral angles of these conjugate faults have narrow ranges, 19~ to 62~ in the Cambrian and 26~ to 51~ in the Ordovician, and their modes are 42~ and 44~ respectively. These data are significantly different from the ~60~ predicted by the Coulomb fracture criterion. It is concluded that： （1） The dihedral angles of the conjugate faults were not controlled by confining pressure, which was low and associated with shallow burial; （2） As dihedral angles were not controlled by pressure they can be used to determine the shortening direction during faulting; （3） Sequential slip may have played an important role in forming conjugate fault intersections; （4） The conjugate fault system of the Tarim basin initiated as rhombic joints; these subsequently developed into sequentially active ＂X＂ type conjugate faults; followed by preferential development of the NW-trending faults; then reactivation of the NE trending faults. This intact rhombic conjugate fault system presents new insights into mechanisms of dihedral angle development, with particular relevance to intracratonic basins.

RESEARCH PROGRESS OF ALTYN FAULT IN WESTERN CHINA

As the northern boundary of Tibetan Plateau, the Altyn strike\|slip fault in western China has a very important implication to the tectonic division in the middle Asia continent, and has become a hot topic for the research on continental dynamics of Tibetan Plateau. During the research of our project funded by NSFC, we collected a sample of Caledonian mylonitized granite in the north of Dangjin pass and two samples of Jurassic meta\|sedimentary rocks in Qaidam gate fault\|valley and Geshi fault\|valley. The texture study shows that all samples were reformed by ductile deformation with undulating extinction and elongated quartzes. The metamorphic p\|T condition are estimated to be 350～500℃ and 150～450MPa They contain the syntectonic\|growing minerals such as white mica, chlorite, sericite and biotite etc. By dating these minerals, we obtain a group white mica 40 Ar/ 39 Ar isochronal age of 89 2～91 7Ma and a (46 6±6 4)Ma sericite 40 Ar/ 39 Ar apparent age. These data are very important to determine the strike\|slip age of Altyn fault.The 40 Ar/ 39 Ar age data (91～46Ma), we obtained, are reported for the first time in the researches of Altyn fault belt. The ages of 91～89Ma indicate that Altyn fault began to slip with slight metamorphism around late Jurassic. These ages are consistent with the ages of the collision between Kohistan\|Ladakh massif (northwestern India) and Eurasian continent at 102～85Ma.This suggests that the strike\|slip movement of Altyn fault should be related to the formation of the so\|called“Western tectonic joint" in Nepal\|western Kunlun area. The age of 46Ma should represent the overprint age of the tectono\|thermal event during main collision between Indian and Eurasian continents along the Yarlung Zangbo River. With the continent\|continent collision of Indian and Eurasian Plates, Altyn fault underwent Multi\|phase strike\|slip events in late Paleocene—Oligocene (37～35Ma), Miocene (22～20Ma), Miocene—Pliocene (8～6Ma), Pliocene (2 5Ma) and early Pleistocene (1～0 7Ma), respectively. The eastern extension of Altyn fault has two branches, one to Xar Moron River with around 3500km length, and another to Okhotsk Ocean with about 4500km length. So Altyn fault is the biggest strike\|slip fault in Cenozoic in the Middle Asia continent, it is characterized by sinistral strike\|slip movement. The basins and tectonic belts, formed before late Jurassic, on the both side of Altyn fault should be displaced. Therefore, the tectonic outline of western China should be re\|recognized.

Timing,Displacement and Growth Pattern of the Altyn Tagh Fault:A Review

The Altyn Tagh Fault（ATF） is the longest, lithospheric scale and strike-slip fault in East Asia. In the last three decades, multidisciplinary studies focusing on the timing, displacement of strikeslip and growth mechanics of the ATF have made great progresses. Most studies revealed that the ATF is a sinistral strike-slip and thrust fault, which underwent multiple episodes of activation. The fault is oriented NEE with a length of 1600 km, but the direction, timing of activity and magnitude of its extension eastward are still unclear. The AFT was predominately active during the Mesozoic and Cenozoic, in relation to the Mesozoic collision of the Cimmerian continent（Qiangtang and Lhasa block） and Cenozoic collision of India with Asia. The AFT strike-slipped with a left-lateral displacement of ca. 400 km during the Cenozoic and the displacement were bigger in the western segment and stronger in the early stage of fault activation. The slip-rates in the Quaternary were bigger in the middle segment than in the western and eastern segment. We roughly estimated the Mesozoic displacement as ca. 150-300 km. The latest paleomagnetic data showed that the clockwise vertical-axis rotation did not take place in the huge basins（the Tarim and Qaidam） at both side of ATF during the Cenozoic, but the rotation happened in the small basins along the ATF. This rotation may play an important role on accommodating the tectonic deformation and displacement of the ATF. Even if we have achieved consensus for many issues related to the ATF, some issues still need to be study deeply; such as：（a） the temporal and spatial coupling relationship between the collision of Cimmerian continent with Asia and the history of AFT in the Mesozoic and（b） the tectonic deformation history which records by the sediments of the basins within and at both side of AFT and was constrained by a high-resolution and accurate chronology such as magnetostratigraphy and paleomagnetic data.

Uniform Strike-Slip Rate along the Xianshuihe-Xiaojiang Fault System and Its Implications for Active Tectonics in Southeastern Tibet

Recent studies on the Xianshuihe-Xiaojiang fault system suggest that the Late Quaternary strike-slip rate is approximately uniform along the entire length of the fault zone, about 15±2 mm/a. This approximately uniform strike slip rate strongly supports the clockwise rotation model of the southeastern Tibetan crust. By approximating the geometry of the arc-shaped Xianshuihe-Xiaojiang fault system as a portion of a small circle on a spherical Earth, the 15±2 mm/a strike slip rate corresponds to clockwise rotation of the Southeastern Tibetan Block at the （5.2±0.7）×10^-7 deg/a angular velocity around the pole （21°N, 88°E） relative to the Northeast Tibetan Block. The approximately uniform strike slip rate along the Xianshuihe-Xiaojiang fault system also implies that the Longmeushan thrust zone is not active, or at least its activity has been very weak since the Late Quaternary. Moreover, the total offset along the Xiaushuihe-Xiaojiang fault system suggests that the lateral extrusion of the Southeastern Tibetan Block relative to Northeastern Tibetan Block is about 160 km and 200-240 km relative to the Tarim-North China block. This amount of lateral extrusion of the Tibetan crust should have accommodated about 13-24% convergence between India and Eurasia based on mass balance calculations. Assuming that the slip rate of 15±2 mm/a is constant throughout the entire history of the Xianshuihe-Xiaojiang fault system, 11±1.5 Ma is needed for the Xianshuihe-Xiaojiang fault system to attain the 160 km of total offset. This implies that left-slip faulting on the Xianshuihe-Xiaojiang fault system might start at 11±1.5 Ma.

Late Quaternary Tectonic Deformation of the Eastern End of the Altyn Tagh Fault

The Quaternary activity of the faults at the eastern end of the Altyn Tagh fault, including the Dengdengshan-Chijiaciwo, Kuantanshan and Heishan faults, was studied on the basis of interpretation of satellite images, trenching, geomorphologic offset measurements and dating. The Altyn Tagh fault has extended eastwards to Kuantanshan Mountain. The left-slip rates of the Altyn Tagh fault decreased through the Qilianshan fault and were transformed into thrust and folds deformation of many NW-trending faults within the Jiuxi basin. Meanwhile, under NE-directed compression of the Tibetan plateau, thrust dominated the Dengdengshan-Chijiaciwo fault northeast of the Kuantanshan uplift with a rate lower than that of every fault in the Jiuxi basin south of the uplift, implying that tectonic deformation is mainly confined to the plateau interior and the Hexi Corridor area. From continual northeastward enlargement of the Altyn Tagh fault, the Kuantanshan uplift became a triangular wedge intruding to the east, while the Kuantanshan area at the end of this wedge rose up strongly. In future, the Altyn Tagh fault will continue to spread eastward along the Heishan and Jintananshan faults. The results have implications for understanding the propagation of crustal deformation and the mechanism of the India-Eurasian collision.

THE ALTUN—NORTH QAIDAM ECLOGITE BELT IN WESTERN CHINA—ANOTHER HP-UHP METAMORPHIC BELT TRUNCATED BY LARGE SCALE STRIKE-SLIP FAULT IN CHINA

The Altun and North Qaidam Mountains at the northern margin of Qinghai\|Tibet plateau are separated by the Altyn Tagh sinistral strike\|slip fault, which is one of the largest strike\|slip fault systems in the world and was considered as the key element in the escape tectonics model for Euraisa\|India continent\|continent collision.Recently,the eclogites within quratzifeldspathic gneisses or pelitic gneisses characterized by amphibolite\|facies paragenesis were discovered in the Altun and the North Qaidam Mountains(Fig.1). They occur as lens or boundins within the Altun Group and Dakendaban Group respectively which previously were considered as metamorphic basement of Tarim block and Qaidam block. Our studies indicate that the eclogites outcrop in both the Altun and North Qaidam Mountains show similar occurrences, associated country rocks, rock and mineral assemblages, p\|T\% estimates, geochemistryand protolith feature and ages of peak metamorphism (see table) . The garnet\|omphacite\|phengite geothermobarometer gave equilibrium condition of \%p\%=2 8～3 0GPa and t =820～850℃ for the Altun eclogite and p =2 8GPa and \%t\%=730℃ for North Qaidam eclogite respectively(Fig..2). These p\|T conditions are in the coesite stability field. Moreover, Po lycrystalline quartz pseudomorphs after coesite have been identified in the Dulan area, North Qaidam Mountains (Song et al, in review). Therefore, these features suggest that both eclogites of Altun and North Qaidam Mountains probably are a same HP\|UHP metamorphic belt formed from the same of Early Paleozoic age deep subduction and collision, and subsequently displaced by the Altyn Tagh fault.The case is similar to the Dabie\|Sulu HP\|UHP metamorphic zone which was truncated by the Tanlu sinistral strike\|slip fault and splitted it into two distincts, the Dabie region and Sulu region. These correlations support an about 350～400km displacement of the Altyn Tagh sinistral strike\|slip fault (Fig.1).

Neotectonics of the Eastern Margin of the Tibetan Plateau: New Geological Evidence for the Change from Early Pleistocene Transpression to Late Pleistocene-Holocene Strike-slip Faulting

We present in this paper some new evidence for the change during the Quaternary in kinematics of faults cutting the eastern margin of the Tibetan Plateau. It shows that significant shortening deformation occurred during the Early Pleistocene, evidenced by eastward thrusting of Mesozoic carbonates on the Pliocene lacustrine deposits along the Minjiang upstream fault zone and by development of the transpressional ridges of basement rocks along the Anninghe river valley. The Middle Pleistocene seems to be a relaxant stage with local development of the intra-mountain basins particularly prominent along the Minjiang Upstream and along the southern segment of the Anninghe River Valley. This relaxation may have been duo to a local collapse of the thickened crust attained during the late Neogene to early Pleistocene across this marginal zone. Fault kinematics has been changed since the late Pleistocene, and was predominated by reverse sinistral strike-slip along the Minshan Uplift, reverse dextral strike-slip on the Longmenshan fault zone and pure sinistral strike-slip on the Anninghe fault. This change in fault kinematics during the Quaternary allows a better understanding of the mechanism by which the marginal ranges of the plateau has been built through episodic activities.

Characteristics of strike-slip inversion structures of the Karatau fault and their petroleum geological significances in the South Turgay Basin,Kazakhstan

The Karatau fault is one of the important strike-slip faults in central Asia,and the South Turgay Basin is located towards its northern end.Detailed seismic interpretation indicated that the strikeslip tectonism of the Karatau fault weakened gradually from west to east in the South Turgay Basin.Typical flower structures developed on the section,and strike-slip faults showed an echelon pattern on planar view.The Karatau strike-slip fault affected the South Turgay Basin in two periods：（1） The South Turgay strike-slip pull-apart rift basin formed as a result of regional extensive stress in the Early-Middle Jurassic,characterized by the juxtaposition of horsts and grabens.The formation of horsts provided favorable reservoir spaces for later hydrocarbon accumulation,and different filling stages of grabens controlled different reservoir-forming factors in grabens.（2） Two stages of tectonic inversion occurred in the Late Jurassic and Late Cretaceous and played a crucial role in the final shape of the structure in the South Turgay Basin.The oil and gas migrated to form reservoirs and mainly concentrated in the horsts,graben slopes and in both sides of the strike-slip fault zone.In the case of the degree of accumulation of petroleum,the factor explaining why horsts are better than grabens is the strike-slip pull-apart of the South Turgay Basin,and the structure inversion of the South Turgay Basin explains why the west graben is better than the east one.Overall,the Karatau strike-slip fault played a very important role in the formation of the South Turgay Basin and its hydrocarbon accumulations.

NORMAL-SLIP ALONG THE NORTHERN ALTYN TAGH FAULT, NORTH TIBET

The east\|west striking Northern Altyn Tagh Fault, about 240km long between Bashkaogong (90°E, 39°25′N) and Lapeiquan (92°15′E, 39°25′N), was previously mapped as a north\|dipping thrust, juxtaposing late Archean\|Mesoproterozoic gneisses in the hanging wall over Paleozoic volcanics, plutons, turbidite, and melange complexes in the footwall. In order to estimate the total magnitude of slip along the Cenozoic Altyn Tagh fault, we conducted geologic mapping along four traverses across the Jinyan Shan where the fault lies. Our field observations suggest that the fault is south\|dipping, with dip angles varying from <25° in the east to about 40° in the west. The eastern fault zone exhibits mylonitic fabrics, whereas the western fault zone is characterized by cataclastic deformation. Kinematic indicators in the ductily deformed mylonitic shear zone consistently show a top\|to\|the\|south sense of shear, suggesting that the Northern Altyn Tagh fault is a south\|dipping normal fault, not a north\|dipping thrust.. The ductile shear zone is typically 30～40m thick, consisting of highly sheared metasediments (pelite and marble), granites, and granitic veins.The latter are systematically cut by small\|scale, south\|dipping ductile normal faults with displacements between 10s of cm to several meters, forming spectacular asymmetric boudinages in the sheared meta\|pelite matrix.The minimum displacement along the detachment is about 20km, as measured by the north\|south width of the exposed footwall gneisses. We renamed the Northern Altyn Tagh Fault in the Jinyan Shan region as the Lapeiquan detachment fault to avoid confusion with other east\|west trending Cenozoic faults to the west along the northern edge of the Altyn Tagh range (e.g., the Cenozoic Jianglisai fault near Qiemo), collectively known as the Northern Altyn Tagh fault system (see Cowgill et al., Geology,in press). The lower age bound of the Lapeiquan fault is Ordovician, as the fault cuts Ordovician volcanics and plutons in its hanging wall. As the Ordovician volcanic rocks are folded together with Carboniferous marbles and Jurassic sedimentary strata, it is likely that normal faulting along the Lapeiquan detachment postdates the Jurassic. The Lapeiquan detachment fault is covered by Quaternary sediments of the Tarim basin in the west, and is apparently truncated by the Cenozoic left\|slip Altyn Tagh fault to the east as indicated by regional geologic maps. If true, this relationship implies that the Lapeiquan fault predates the Cenozoic Altyn Tagh fault. The apparent truncational relationship between the Lapeiquan fault and the Altyn Tagh fault posses an important question: where is the counterpart of the Lapeiquan fault south of the Altyn Tagh fault? Preliminary mapping in the Yema Nan Shan south of the Altyn Tagh fault reveals a fragment of a low\|angle mylonitic shear zone, which is interpreted as a detachment fault because it puts lower\|grade meta\|pelite over higher\|grade mylonitic quartzite. The correlation of detachment faults in the Yema Nan Shan and the Lapeiquan area would imply an amount of about 280～300km left slip along the Altyn Tagh fault. Alternatively, movement along the Lapeiquan detachment fault could have been synchronous with the development of the Cenozoic Altyn Tagh fault. This interpretation requires no counterpart of the Lapeiquan fault south of the Altyn Tagh fault. Instead, it implies that a major topographic collapse event occurred in the Cenozoic along the northern edge of the Tibetan plateau during movement along the Altyn Tagh fault. On\|going thermochronologic analysis will provide constraints on the age of the detachment fault and a test for the two distinctive hypotheses.

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.

Characteristics of the Late Quaternary right-lateral strike-slip movement of Bolokenu-Aqikekuduk fault in northern Tianshan Mountains,NW China

The characteristics of the Bolokenu-Aqikekuduk（Bo-A） fault,a right-lateral strike-slip fault that runs for more than 700 km long and obliquely cuts North Tianshan Mountains,are evaluated here based on remote sensing data,and through an analysis of the results from field investigations as well as climate-geomorphic events. The fault is composed of a western segment with a NW strike and an eastern segment with a NWW strike.The western segment is nearly 250 km long,extending northwestward into Kazakhstan with a right-lateral strike-slip rate of 5 mm/a.This domain consists of 4-5 rupture sections,with 3-4 deformation belts,caused by ancient or historical earthquakes,and suggesting the potential for the occurrence of further strong earthquakes（with M≈7.5） in future. The eastern segment of the fault shows a right-lateral strike-slip rate of 1-1.4 mm/a,with the development of 3-4 deformation belts caused by ancient or historical earthquakes,and with a potential for future strong earthquake with M≈7.0. A typical strain partitioning style in the compression area has developed between the intermontane BoA fault and the piedmont thrust structures of Northern Tianshan Mountains,under the effect of oblique compression,as indicated by the piedmont thrust structure and the strike-slip fault in the mountains.

The First Quantitative Slip-Rate Estimated Along the Ashikule Fault at the Western Segment of the Altyn Tagh Fault System

As one of the longest strike-slip fault in Asia,the Altyn Tagh Fault（ATF）defines the northern boundary of the Tibetan Plateau and plays a significant role inaccommodating the deformation resulting from the IndiaAsia convergence.

Present-day tectonic activity along the central section of the AltynTagh fault derived from time series InSAR

The Altyn Tagh Fault(ATF) is a large-scale complex tectonic system. In this study, the present-day crustal deformation of the central section of the ATF(90.8E-91.58E) was obtained using 14 images on a descending track acquired between 2007 and 2010 from Advanced Land Observing Satellite(ALOS). To improve the accuracy of the interferograms, ALOS World 3 De30 m(AW3 D-30)Digital Surface Model(DSM) from the Japan Aerospace Exploration Agency was used in Small Baseline Subset(SBAS) Interferometric Synthetic Aperture Radar(InSAR) processing. The Line of Sight(LOS) deformation map show that there is an obvious zoning feature. With the main ATF as the boundary, the north block is concentrated around -35~ -60 mm, and the south block is concentrated around -9 ~ 11 mm. Based on the InSAR velocity map, we inverted for the strike-slip rate and locking depth of the fault using the twodimensional strike-slip fault buried-dislocation model. The inversion results for the strike-slip rate at three selected cross-section locations perpendicular to the ATF were 6.1 mm/a, 5.3 mm/a and 7.9 mm/a from west to east;the corresponding locking depths were 9.5 km, 6.8 km and 12.3 km from west to east.The location and trend of the fault obtained by inversion are coincides with the Xorkol seismic belt.

LARGEST ALTYN TAGH LITHOSPHERIC SHEAR FAULT IN CENTRAL ASIA

The Altyn Tagh fault (ATF) extending in NEE—SWW direction lying at the northwestern boundary of Qinghai—Tibet plateau is the largest strike\|slip fault in Central Asia. On the basis of recent geologic mapping and detailed study of lithotectonic characteristics for the paleotectonic units at the two sides of the Altyn Tagh Fault ( Altun Mt. to west and the Qilian Mt. to east ) we propose that the paleotectonic units in the Altun Mt. can be correlated with those in the Qilian Mt. assuming 400km left\|lateral displacement for the Altyn Tagh fault. Natural seismic data across the Altun Mt. indicate that the Altyn Tagh fault is a lithospheric shear fault and the lithospheric shearing is probably related to southward intracontinental oblique subduction of the Tarim terrane beneath the Altun Mt.1\ Comparison of the major paleotectonic units at the two sides of the Altyn Tagh fault\;(1) The Alxa\|Dunhuang Massif:The Alxa massif lying at the southern margin of the Sino\|Korean craton consists mainly of an Early Proterozoic basement including high\|grade and middle\|grade metamorphic rocks, which were intruded by granite at 1719Ma. The Paleozoic passive margin sediments is well developed. In the Altun Mt., the Early Proterozoic and late Archean basement of the Duhuang massif includes high\|grade and middle\|grade metamorphic rocks dating 2789Ma (Sm\|Nd method) and 2405Ma (U\|Pb method).

AGE OF THE ALTYN TAGH STRIKE—SLIP FAULT

A 100～500m\|wide mylonite zone in the Altun Group of Lower Proterozoic age was discovered along the Altyn Tagh strike\|slip fault. The zone is mainly composed of amphibolitic and granitic mylonites. The planar joints of the rocks strike in NE70 and dip steeply (nearly vertical), coincident with the striking of Altyn Tagh fault zone, and their stretched lineations are nearly horizontal. Shear strains are well developed and show sinistral sheared. The amphibolitic and granitic mylonites are most probably the products of deep\|seated melting caused by sinistral strike\|slip shearing as suggested by the evidence below: (1) The migmatization is intensely developed and spatially controlled by the shear zone, and the rock has a set of NNE perspective foliation which is in accord with the direction of the Altyn Tagh strike\|slip fault zone. (2) The recrystallized hornblende aggregate is distributed in band with obvious nebulous texture, indicating the characteristics of anatexis, and the hornblendes are oriented and form the nearly horizontal stretching lineation; some hornblendes have titanite inclusions, and magmatic long\|columnar zircons are in directional arrangement.(3) The banded felsic material is developed, and the plagioclase is characteristic of high\|temperature plastic deformation and shows sinistral shear stain. (4) In mylonites, all the axis C fabric of quartz shows the feature of sinistral shearing and the majority is generally middle to low temperature fabric, but there also exists high temperature fabric, which suggests that high temperature shearing occur in the early stage of strike\|slip deformation and it is characterized by middle to low temperature shearing at the beginning of anatexis or in the late stage of the deformation. (5) On the XZ plane of mylonite and mylonitized rocks, there exists the consistent sinistral shear stain, which suggests the products of the same strike\|slip shearing.Zircons were separated from three samples of mylonitized granitic rocks for age dating. Two groups of zircon were distinguished in morphology: one is elongate prismatic grains, and the other shows slight rounding. Some zoned structure of selected grains were examined by cathodoluminescence. Dating was completed in the SHRIMP laboratory of Stanford University. Fifteen analyses were made on 14 zircon grains. Sample S99\|25 show two obvious two age groups, one is 527～549Ma, and the other is 466～472Ma. Sample S99\|9 contains two age groups either, one is 475～507Ma and the other is 279Ma. Sample S99\|6 shows three groups: ① 528Ma; ② 365Ma and③ 238～243Ma, here the 365Ma is explained as mixture age between the other two age groups according to its exact location in the grain. In summary, from the three samples we found at least three age groups: 507～548Ma; 466～472Ma; 238～243Ma.The ages of 507Ma to 548Ma and 466Ma to 472Ma represent the deformation and metamorphism of Early Paleozoic age, which is most likely correspond to the close of the Qilian Sea and continental subduction and collision reported in recent papers (Yang Jingsui et al., 1998; Zhang Jianxin et al., 1999; Xu Zhiqin et al., 1999). The 238～243Ma most likely represent the formational age of the Altyn Tagh strike\|slip fault, which is consistent with the formational age (200～240Ma) of the large\|scale sinistral strike\|slip fault zone of the South Margin of East Kunlun (Li Haibing et al., 1996), and both can be attributed to the oblique subduction and collision of the Bayan Har terrane with the East Kunlun terrane during Indosinian period.

SEDIMENTARY PROCESS OF THE CENOZOIC BASIN AND ITS RESPONSE TO THE SLIP-HISTORY OF THE ALTYN TAGH FAULT, NW CHINA

The NEE\|striking Altyn Tagh Fault (ATF) has been well known as one major point to know the growth history of the Tibetan plateau. Lots of investigations done since 1970’s were mostly focus on active features, particularly on determining slip, slip rate and their distribution along the fault. However, Cenozoic slip\|history of this fault remains poorly understood, and the age of initiation and total offset are controversial. Several Cenozoic sedimentary basins develop in Suo’erkulinan to Mangya regions (Fig.1). Their sedimentary processes are closely related with the ATF. The studies of the Neogene sedimentary sequences and the reconstruction of the paleo\|geography are essential to establish the displacement history of the fault during Late Cenozoic.Located at the southern side of the ATF, the Suo’erkulinan basin consists of more than 600\|meter\|thick Pliocene Shizigou Formation below and about 120\|meter\|thick Early to Middle Pleistocene Qigequan Formation above according to the 1∶200000 geological map by Xinjiang Province. An obvious erosional surface can be seen on the top of the lower sequence. Sediments in the Shizigou Formation are characterized by 400\|meter\|thick yellow to red cobble\|sized conglomerates in the bottom, up\|grading to sandstones and grey\|green mudstones. This indicated that the sedimentary facies changed from alluvial fan to fluvial fan and sediments became more and more mature. The upper sequence, the Qigequan Formation, corresponds to an alluvial facies series composed of yellow to white cobble\|sized conglomerates intercalated with lenticular sandstones. Paleo\|current indicators showed that the Shizhigou conglomeratic series were sourced from northwest. Well\|developed syn\|sedimentary faults, normal faults mostly inherited from syn\|sedimentary faults, and some striation lineations on the surface indicated transtensional tectonic environment of the strike\|slip faulting.

THE FORMATION AND EVOLUTION OF ALTYN TAGH FAULT SYSTEM AND ITS RELATIONSHIP TO THE GROWTH OF TIBETAN PLATEAU

One of the focus views of the uplifting of Tibetan Plateau is the growth history of the plateau. This is an unresolved question because of the poor study in north margin, where the ATF (Altyn Tagh Fault) is acting an important role in the growth and deformation of the plateau. The fault links two huge contractional belts, e.g. Qilian Nan Shan and West Kunlun, and merges a series of thrusting\|folding arcs in southeast. Mapping of piercing points, such as unconformities between Cenozoic, Mesozoic and Paleozoic strata, and magmatic arcs, shows left slips of ca. 240km and ca. 550km along the middle and western segments of the ATF. About 140～450km of crustal shortening, approximately the same magnitude as the west segment of the ATF, is deduced from balanced sections in West Kunlun foreland thrusting belt. This implies that left\|slip displacement along the west segment of the ATF was absorbed by the contraction in West Kunlun. The ATF system merged bunches of WNW arcuated fold\|fault belts in Qaidam basin, implying anti\|clockwise rotation. Tertiary and some Lower to Middle Pleistocene strata involved in fold\|fault belts, and dip in ESE due to the uplifting of Altyn Tagh. The newest strata involved in the deformation is more and more younger from south to north, that is, from Lower Pliocene to Middle Pleistocene, showing the uplifting trends from south to north in the SE side of the fault.