The seismicity in the middle section of the Altyn Tagh Fault system revealed by a dense nodal seismic array

The left-lateral Altyn Tagh Fault(ATF) system is the northern boundary of the Qinghai-Xizang Plateau, separating the Tarim Basin and the Qaidam Basin. The middle section of ATF has not recorded any large earthquakes since1598 AD, so the potential seismic hazard is unclear. We develope an earthquake catalog using continuous waveform data recorded by the Tarim-Altyn-Qaidam dense nodal seismic array from September 17 to November23, 2021 in the middle section of ATF. With the machine learning-based picker, phase association, location, match and locate workflow, we detecte 233 earthquakes with M_L-1–3, far more than 6 earthquakes in the routine catalog. Combining with focal mechanism solutions and the local fault structure, we find that seismic events are clustered along the ATF with strike-slip focal mechanisms and on the southern secondary faults with thrusting focal mechanisms. This overall seismic activity in the middle section of the ATF might be due to the northeastward transpressional motion of the Qinghai-Xizang Plateau block at the western margin of the Qaidam Basin.

Geochemistry and Zircon U-Pb Dating of Changshagou Adakite from the South Altyn UHPM Terrane: Evidence of the Partial Melting of the Lower Crust

Changshagou adakite, an outcrop in the middle segment of the South Altyn Tagh ultra-high pressure metamorphism （UHPM） terrane, contains medium-K cal-alkaline and weakly peraluminous compositions （SiO2 = 66.79% to 68.65%, Al2O3 = 17.48% to 18.31%, K20 ＋ Na20 = 6.32% to 6.88%, K2O/Na2O = 0.25 to 0.33, A/CNK = 1.01 to 1.06）. This outcrop is also enriched with large ion lithophile elements but with depleted high-field strength elements （HFSE） showing clearly negative Nb, Ta, and Ti anomalies. REE distribution patterns show a positive anomaly of Eu （6Eu = 1.15 to 1.31） and weakly enriched with LREE compared with HREE （LREE/HREE = 1.02 to 4.20）. Experimental results and several characteristics, including relatively low Nb/Ta ratios （6.03 to 8.45） and high Sr, Sr/Y, （La/Yb）N and low Y and Yb, which indicate the presence of residual garnet and the absence of plagioclase in the source region, show that adakite may form at a pressure ranging from 1.2 GPa to 1.5 GPa and at a temperature of approximately 900~C. Low Cr, Ni, and Mg# values, trace element patterns, and SiO2- Mg# and SiO2-MgO diagrams indicate that rocks are formed by the partial melting of a thickened lower continental crust. LA-ICP-MS in situ U-Pb dating yields two group ages： 503.1±1.7 Ma （core） and 453.1±3.0 Ma （rim）. The Th/U ratios of the core and the rim are 0.11 to 0.40 and 0.03 to 0.07, respectively. Considering the zircon CL image characteristics, Th/U ratios, and previous studies on regional UHPM rocks, adakite formed at 503.1 ± 1.7 Ma and underwent a tectothermal event as a result of the break-off of the Altyn deep subducted continental crust at 453.1 ± 3.0 Ma.

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 Tashisayi nephrite deposit from South Altyn Tagh, Xinjiang,northwest China

The Tashisayi nephrite deposit is located in South Altyn Tagh.in Qiemo County,Xinjiang Province,northwest China.It is a recent discovery in the vast,well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin,producing more than half of the nephrite from the whole belt in 2017.Field investigations revealed that it is a dolomitic marble-related(D-type)nephrite deposit,but little is known about its age of formation and relationships between the granites and marble.Here we report field investigations,petrography of the neph rite,as well as petrography,geochemistry,geochronology of the zoisite-quartz altered intrusive rock and adjacent granites.An A-type granite is identified with a SHRIMP U-Pb zircon age of 926±7 Ma,suggesting it was emplaced in an extensional tectonic environment at that time.The altered intrusive rock has a cluster of U-Pb zircon age of 433±10 Ma.with similar trace element features to the A-type granite,suggesting it was formed in an extensional regime at this later time.Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids.It is inferred that Ca^2+was released from the dolomitic marble by metasomatism forming Ca-rich fluids,which caused alteration of both the intrusive rocks(6.00-8.22 wt.%CaO)and granite(1.76-3.68 wt.%CaO)near the nephrite ore bodies.It is also inferred that Fe^2+from the granite migrated towards the dolomite marble.The fluids gave rise to the formation of Ca-minerals.such as zoisite,in the nephrite and altered intrusive rock,and epidote in the granite.Based on the contact relationships.similarity in hydrothermal processes,and consumption of Ca^2+,the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks,i.e.~433 Ma.The geochronological similarity(~926 Ma.433 Ma)of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past,which was offset by the Altyn Tagh fault(ATF).The similar formation ages of the nephrites from Altyn Tagh(433 Ma)and the previously studied areas of West Kunlun(378-441 Ma)and East Kunlun(416 Ma)indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanving post-collisional.extensional environment.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

Geochemistry, Zircon U–Pb Dating and Hf Isotopic Characteristics of Neoproterozoic Granitoids in the Yaganbuyang Area, Altyn Tagh, NW China

The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt, in which numerous Neoproterozoic granitoids crop out. Granitoids are mainly located in the Paxialayidang-Yaganbuyang area and can provide indispensable information on the dynamics of Rodinia supercontinent aggregation during the Neoproterozoic. Therefore, the study of granitoids can help us understand the formation and evolutionary history of the Altyn Tagh orogenic belt. In this work, we investigated the Yaganbuyang granitic pluton through petrography, geochemistry, zircon U-Pb chronology, and Hf isotope approaches. We obtained the following conclusions： （1） Yaganbuyang granitoids mainly consist of two-mica granite and granodiorite. Geochemical data suggested that these granitoids are peraluminous calc-alkaline or high-K calc-alkaline granite types. Zircon U-Pb data yielded ages of 939~7.1 Ma for granodiorite and ~954 Ma for granitoids, respectively. （2） The ~Hf（t） values of two--mica granite and granodiorite are in the range of-3.93 to ＋5.30 and -8.64 to ＋5.19, respectively. The Hf model ages （TDM2） of two-mica granite and granodiorite range from 1.59-.05 Ga and 1.62-2.35 Ga, respectively, indicating that the parental magma of these materials is derived from ancient crust with a portion of juvenUe crust. （3） Granitoids formed in a collisional orogen setting, which may be a response to Rodinia supercontinent convergence during the Neoproterozoic.

Lithospheric Electrical Structure across the Eastern Segment of the Altyn Tagh Fault on the Northern Margin of the Tibetan Plateau

Project INDEPTH （InterNational DEep Profiling of Tibet and the Himalaya） is an interdisciplinary program designed to develop a better understanding of deep structures and mechanics of the Tibetan Plateau. As a component of magnetoteUuric （MT） work in the 4th phase of the project, MT data were collected along a profile that crosses the eastern segment of the Altyn Tagh fault on the northern margin of the plateau. Time series data processing used robust algorithms to give high quality responses. Dimensionality analysis showed that 2D approach is only valid for the northern section of the profile. Consequently, 2D inversions were only conducted for the northern section, and 3D inversions were conducted on MT data from the whole profile. From the 2D inversion model, the eastern segment of the Altyn Tagh fault only appears as a crustal structure, which suggests accommodation of strike slip motion along the Altyn Tagh fault by thrusting within the Qilian block. A large-scale off-proffie conductor within the mid-lower crust of the Qilian block was revealed from the 3D inversion model, which is probably correlated with the North Qaidam thrust belt. Furthermore, the unconnected conductors from the 3D inversion model indicate that deformations in the study area are generally localized.

THREE-DIMENSIONAL DEFORMATION ALONG THE ALTYN TAGH FAULT ZONE AND UPLIFT OF THE ALTYN MOUNTAIN, NORTHERN TIBET

he convergence between India and Eurasia is partly accommodated by motion of a few large blocks along strike\|slip faults. About 1800km long Altyn Tagh fault strikes in N60～80°E and allows the northeastward displacement of the Tibet plateau relative to the Tarim. The Altyn Tagh fault zone is a typical transpressional fault zone, characterized by blocks rotation and crustal shortening and vertical extrusion of blocks within the Altyn Tagh strike\|slip system. Differences of three\|dimensional deformation and configuration of the active structures are recognized at different segment of the Altyn Tagh fault zone.1\ Structural configuration of the Altyn Tagh fault zone\;In the Altyn Tagh strike\|slip fault zone, the assemblage pattern of the (active) faults is in the form of parallel plumes, especially in the eastern and the western segments of the Altyn Tagh fault zone. These plumes structures in the eastern segment are assembled by string\|like left lateral strike\|slip fault and broom\|like thrusting faults, and in the western segment by arc\|like left lateral strike\|slip faults along with thrusting faults and normal faults. In the middle segment of the Altyn Tagh fault zone, the structures are characterized by the string\|like left lateral strike\|slip faults in the center and reverse thrusting faults on the two sides.

CRETACEOUS (?) VOLCANIC ROCKS ON THE NORTH END OF THE ALTYN TAGH FAULT:CHARACTERISTICS AND TECTONIC IMPLICATIONS

The age and evolutional history of the Altyn Tagh fault and its role in the formation and uplift of the Qinghai—Tibetan plateau have been focused for years. Many geologists believe that the formation of the fault is a result of the collision between India and Asia. Some people thought that it should have formed earlier than Cenozoic but have no critical evidence. Here we report a preliminary result from our recent investigation on the volcanic rocks at the north end of the Altyn Tagh fault.1 The volcanic rocks on the north end of the Altyn Tagh fault The volcanic rocks are located on the north end of the Altyn Tagh fault, northern Qinghai—Tibetan plateau. The investigated volcano occurs in the Jiuxi basin, a Cretaceous and Tertiary depositional basin. It is about 300m×100m in size and form about a 100m high cliff above the folded Cretaceous strata. It likes relic neck of a volcano rather than a kind of widely distributed lava flow commonly seen in the northern Tibet. The country rocks are Cretaceous sandstone, silt and fine\|grained conglomerate. The cliff formed most likely due to the differing erosion between the hard volcanic rocks and soft rocks.

GRANITOIDS,VOLCANIC ROCKS AND CHERTS FROM NORTH ALTYN TAGH,NW CHINA: IMPLICATION FOR THE TECTONIC ENVIRONMENT DISCRIMINATION

Granitoids,volcanic rocks and cherts,mainly of early Paleozoic,in northern Altyn Tagh(Fig.1) are analyzed here for the purpose of the determination of the tectonic environments and their development since Early Paleozoic.(1) Granitoids\ The collection of 128 main\|element petrochemical data from North Altyn Tagh area shows that most of the granitoids here are granites and granodiorites of calc\|alkaline series.They consist mainly of metaluminous and peraluminous in Shand’s index,and only one of them,which belongs to Mesozoic,is peralkaline.Most of the granitoids plot in the IAG+CAG+CCG fields in the Maniar and Piccoli’s diagrams (1989) for tectonic discrimination of granitoids.IAG (Island arc granitoids)and CAG (Continental arc granitoids) can be distinguished for Early and Late Paleozoic granitoids,and maybe some CCG (Continental collision granitoids) for Early Paleozoic.Granitoids of Mesozoic and Cenozoic inherited the characteristics of those of Paleozoic.Destructive active plate margin (pre\|plate collision)and Anatectic magmatism(syn\|orogenic,S\|type granites)are distinguished (Fig.2)for Paleozoic granitoids using de la Roche R 1\| R 2 multicationic diagram (Batchelor and Bowden,1985).Mesozoic and Cenozoic granitoids inherited the characteristics of Anatectic magmatism (syn\|orogenic)of these early ones,and post\|orogenic(A\|type)granites occurred in Mesozoic.The mechanism for magma formation is mainly partial melting.