Preliminary Study On Meso-Cenozoic Strike-slip Uplifting History Of The Altyn Mountain And Its Constraints To The Formation Of The Western Qaidam Basin, China

The Altyn fault belt is one of key regions as resolving the dynamics ofthe Qinghai-Xizang Plateau, therefore, it becomes a focus in the recentyears. The Altyn strike-slip fault has a cumulative offset of 350-400 km, itgive rise to a large scale uplifting of the Altyn Mountain during itsstrike-slipping, therefore, it have been the prominent provenance of theTarim and Qaidam basin since Cenozoic. The research of the Altynmountain uplifting also becomes a focus, but most of works only focus onone certain stage of the uplifting, it is still argued that whether the AltynMountain primarily was a long-term uplift which separates the Qaidam andTarim basins or it uplifted after the Early Pleistocene. In order to reveal theuplifting history of the Altyn Mountain since Mesozoic, It is discussed thatthe strike-slip uplifting of the Altyn Mountain constrains the sedimentaryand tectonic evolution of the western Qaidam basin in Meso-Cenozoic bythe summarizing reported Meso-Cenozoic thermochronological data andcombined with stratigraphy, sedimentology, lithofacies paleogeographyand seismic data.The Altyn Mountain didn't exist based on the comparison of theJurassic-Cretaceous isopach map and the features of sedimentarylithofacies of the southeastern Tarim basin and western Qaidam basin.The migration toward east of the depocenter in the western Qaidam basinsuggests that the Altyn Mountain started to rise in Cretaceous. Thereoccurred a strike-slip uplifting event in Cretaceous in the Altyn Mountainbased on the syn-tectonic growing mineral 40Ar/39Ar ages within the faultbelt, but the magnitude of the strike-slip and mountain uplifting is small,and the Tarim basin was still connected with the Qaidam basin. Thedistribution of the Luluohe-Xiaganchai Formation is beyond the currentboundary of the Qaidam basin according to the isopach maps, outcropsand bore hole data, the paleo-relief is relatively higher in the east andrelatively lower in the west, The most parts of the Altyn mountain waslower.The fission track ages of 40-30Ma indicates an uplifting event in thelast Eocene-initial Oligocene in the Altyn Mountain, which is consistentwith the increasing of synchronous sedimentary velocity in the westernQaidam basin and the initial uplifting of the Qinghai-Xizang Plateau. Itresulted in the rising of the Akatengneng -Jinhong hill and the primaryreversed S-type structure in the western Qaidam basin. The sedimentgrain size changes to be fine rapidly away from the mountains toward tothe basin center. The Qaidam basin was connected to the Tarim basin inPaleogene. The paleo-Qaidam basin underwent a stable stage during thePaleocene – Early Eocene. The strike-slip event along the Altyn fault andthe mountain uplifting was related to the initial collision of the India andEurasia continentsThe cooling age of c.a. 20Ma is recorded in the northern mountains ofthe Qinghai-Xizang Plateau. The sedimentary velocity in the both sidebasin of the Altyn Mountain increases rapidly during this period, whichindicates the rapid denudation of the Altyn Mountain. The Mang'ai sagstarted to migrate to the east, it suggests the uplifting of the Akatengneng -Jinhong mountains and the expandation toward the center of the westernQaidam basin. The magnitude of this uplifting is stronger than that in theLate Eocene-Early Oligocene, and most areas of the western Qaidambasin have risen to a certain altitude. This event probably was related tothe India continent subduction toward the Eurasia continent along the MainCentral Fault (MCT).The cooling ages of c.a.10Ma are recorded widely in the AltynMountain, which indicate a rapid uplifting event that strongly influenced theQaidam basin. An angle unconformity occurs between ShangyoushashanFormation and Xiayoushashan Formation in the front of the Altyn Mountain,which becomes a parallel unconformity in the center of the Qaidam basinaccording to the seismic data. The event was resulted from the subductionof the Tarim Plate to the north of the Qinghai-Xizang Plateau. Thesubduction also gave rise to some large scale thrust faults along the AltynMountain.A strike-slip uplifting event took place in the Late Pliocene-EarlyPleistocene, accompanying with the small vertical uplift of the mountains. Alower angle unconformity occurred between the Shizigou Formation andthe Qigequan Formation around the Qaidam basin margin, but the contactbecome conformity or parallel unconformity between the tow formations inthe basin center. The horizontal Early Pleistocene sediments were stillpreserved on the top of the 4000m high Jinghong Mountain, it imply theheight of the Jinghong Mountain was lower than the lake level inPleistocene, there probably are some water chennal connected to theQaidam basin.The strongest rapid uplifting of the Altyn Mountain happened in thelate Early Pleistocene-early Middle Pleistocene. This event gave rise to aextensive uplifting in the Altyn Mountain, sedimentary velocity changedsharply in the western Qaidam basin. This tectonic event resulted in notonly mountain uplifting but also a large scale sinistral strike-slip anddraught fold. This study confirms that the event began between 1.5Ma and0.3Ma according to 36Cl dating in the Goukou section of the westernQaidam basin.