Cenozoic Strike-slip History Of The Central Altyn Tagh Fault

The ENE-striking Altyn Tagh Fault (ATF) extends along the northern edge of the growingQinghai-Tibetan plateau. As one of major active tectonic features produced during the India-Asiacollision, the initial age of its left-lateral strike-slip and the total offset accumulated along thisfault are key parameters in assessment of the eastward extrusion vs distributed shortening modelsof the Qinghai-Tibetan plateau during the large-scale convergence between the India and Asiaafter their collision. Its active features, left-lateral offset and its markers in surface, the slip rateand its distribution along the fault have been well documented since early 80's, while, little wasknown on the uplifting history of the Altyn Tagh mountains and the strike-slip process of the faultin Cenozoic.This paper mainly presents new evidence from the sedimentary process of the Cenozoicbasins along the central Altyn Tagh fault and the uplifting history of the Altyn Tagh mountains toreconstruct the strike-slip history of the fault during Cenozoic, and to discuss the dynamicrelation between the Cenozoic strike-slip faulting of the ATF and the uplift and deformation ofthe Qinghai-Tibetan plateau.The main advancements achieved are as following after more than three-year hardwork:1) Studies on the sedimentary process and tectonic deformation of Cenozoic basinsdeveloped along the central Altyn Tagh fault, through field investigation and geological mapping,showe that the filling process, formation and deformation of these basins were highly related withthe strike-slipping of the ATF.In north Suo'erkuli basin, the Early Pleistocene Qigequan Formation is firstly distinguishedand a planation surface developed on the top of the Qigequan Formation is newly recognized. Thesedimentary sequence is divided into two units. The lower unit is composed of Oligocene LowerGancaigou Formation to Miocene Upper Ganaigou Formation and Lower Youshashan Formation,with the sedimentary facies changing from alluvial fan in the edge and lacustrine in the center ofthe basin in the bottom, up to lacustrine in the middle, and alluvial and pluvial fan in the top. Theupper one, Qigequan unit, is mainly alluvial fan deposits. The reconstruction of the relationshipbetween sedimentation and strike-slipping of the ATF has been attempted, suggesting that thisbasin was a strike-slip-related basin.In South Suo'erkuli basin, detailed field measurement of the characteristics of sedimentsshows most deposits in this basin belong to fluvial deposits with northwest side as its resourance.An obviously angular unconformity between the Pliocene Shizigou Formation and the EarlyPleistocene Qigequan Formation has been founded in the field. A relative flat erosional planationsurface is well preserved on the top of the Qigequan Formation. Syn-sedimentary faults, normalfaults parallel to the main trace of the ATF, and scrab-linenation in some of the fault surfaceindicate regional extensional setting during the formation of this basin. The geomography and field investigation show that the Suo'erkuli basin was one ofintra-mountain depression, originated from a tectonic erosional river valley. Both Yitunbulakebasin and Kumutashi basin share similarities not only in Cenozoic sediments inside, but also thenarrow and elongate extending geometry, and unconformbaly overlay on Pre-Cambrian basemen,suggesting sediments in these two basins were deposited in an initially uniform strike-slip basin.Sediments in Suo'erkuli valley, the distribution of active Altyn Tagh fault and its branches,and the long and narrow extending geometry, indicate that this valley was a newly formed,probably since Late Pleistocene, special pull-apart basin, which was also related with somenormal partitions in strike-slipping of the fault.2) The Cenozoic sedimentary sequence is established based on the composition of thesediments, sedimentary facies changes, and unconformity relations in those basins, and is dividedinto three units as following: Oligocene to Middle Miocene, Late Miocene to Early Quaternary,and Late Pleistocene to present. The paleo-topographic reconstruction shows the sedimentation inthese basins was tightly related with the fault, indicating that the Altyn Tgah fault has experiencedthree stages and five periods strike-slipping during Cenozoic.New geological evidence from the distribution of Cenozoic sedimentary basins and theformation of the Suo'erkuli basin provide constraints on the displacement of the fault, whichindicating 80～100 km left-lateral offset of the fault has been accumulated in Late Cenozoic,with the average strike-slip rate as 10-12mm/y.3) 25 samples from gneisses and granites in the Altyn Tagh mountains are dated by fissiontrack of apatite. The result yields ages varying from 61Ma to 7Ma, indicating the heterogeneticuplifting of Altyn Tagh mountains. The fission track age of samples from the NEE-trending partof the Altyn Tagh mountains varies from 36.5Ma to 13.6Ma, suggesting that the uplift of theNEE-trending Altyn Tagh mountains began from Oligocene till to Miocene, with lower uplift rateearlier (from Oligocene to Miocene) and probably a quickly uplifting occurred later. Supposedly,the uplift of the NEE-trending mountain was only related with the strike-slip of the Altyn Taghfault, the author put forward an initiation age of the left-lateral strike-slip of the fault inOligocene. Regional data also show that a large-scale regional uplift of mountains in northernedge of the Qinghai-Tibetan plateau occurred during Oligocene to Miocene.While, fission track dating of the apatites from granites near the fault yields agesconcentrated on the 7-9Ma, with about 2Ma difference. Regional data also show that 8Ma±byfission track dating of apatite is widely spread along the fault, suggesting a period of rapidstrike-slipping of the fault took placed during this time.4) Stable isotope of pedogenic minerals and calcite cement, as a most powerful andquantitative parameter of the paleo-climate change, is very useful to document the uplift-historyof mountains. Analyses results of calcite cement from the Cenozoic sediments in theJianggalesayi area, north-western edge of the Altyn Tagh mountains, show that rapid changes ofboth the value ofδ¹⁸O andδ¹³C occurred in early Oligocene and early Miocene, suggesting theuplift of mountains initiated from early Oligocene, and a rapid uplift occurred in early Miocene. According to the formula from Chamberlain (2000), and the decrease of oxygen in calcitecement in early Miocene, it is calculated that the uplift of mountain at that time was less than1500meters, suggesting the modern sharp difference in topography, more than 3000 meters,between the northern plateau and the Tarim basin predominately formed later than Miocene.Correspondingly, the study on sedimentary both in field and under microscope showed thata quickly up-coarsing of the sediments grain size, the ratio of conglomerate layers in stratigraphy,and the unstable composition of pebbles in clast of sandstone, and the degree of rounded andsorted of pebbles in sandstones and conglomerates was recorded in early Miocene, andsedimentary velocity increased rapidly during Pliocene and Early Quaternary.5) Sedimentary process and deformation history of Cenozoic basins along the central AltynTagh fault, fission track dating of apatite, and the value ofδ¹⁸O andδ¹³O of calcite cement fromCenozoic sediments in northwestern edge of the Altyn Tagh mountains as the parameter of theclimate change, suggested that the left-lateral strike-slip of the Altyn Tagh fault initiated at EarlyOligocene, about 36Ma, and an important rapid strike-slipping of the Altyn Tagh fault occurred atabout 8Ma.The Cenozoic strike-slip history of the Altyn Tagh fault can be divided into three stages. Thefirst stage, corresponding to Oligocene to Middle Miocene (36-8Ma), the Altyn Tagh fault mostlybehaved as a transtensional boundary. Regional deformation occurred at about 8Ma, associatedwith the position change of the main trace of the ATF. The second stage, from Late Miocene tothe end of Pliocene, the ATF displayed transtensional feature in Late Miocene and Early Pliocene,and with some more normal partition in strike-slip faulting during Late Pliocene. A change infault behavior occurred in early Pleistocene. The decreasing of the normal partition in strike-slipfaulting was associated with regional NW-NWW-trending compression deformation, whichprobably lasted until to the end of Pleistocene, and the ATF behaved as a transpressionalboundary. From Late Pleistocene to present, during the third stage, the ATF mainly worked as alarge-scale strike-slipping fault with some normal partitions in its central segment.Based on the sedimentary sequence, together with division of the Cenozoic strike-slippingof the Altyn Tagh fault and the total left-lateral offset constrained by the comparison of thestrike-slip basins (the Yitunbulake basin with the Kumutashi basin) and the outlet of theSuoerkuli basin with the paleo-drainage in south Suoerkuli area, the reconstruction of strike-slipfaulting history in the central Altyn Tagh fault during Late Cenozoic has been attempted.6) Most regional geological evidence shows that the Cenozoic strike-slip faulting of the ATFwas tightly related with the deformation and uplift of the Qinghai-Tibetan plateau. For example,8Ma probably was also one of most important deformation and uplift stages of the wholeQinghai-Tibetan plateau. The global climate change tightly related with large scale mountainuplift also happened at this time. Thus, the author suggests that all Cenozoic deformation alongthe ATF could be interpreted in the geodynamical setting of uplift and deformation of theQinghai-Tibetan plateau resulted from the collision between India and Asia and the large-scaleconvergence between them after the collision.