Controls Of The Altyn Fault And Mountain Uplift On The Structure And Sedimentation Of Western Qaidam Basin In Cenozoic

The Altyn fault zone, more than 1900km long, extending toward NEE, is a large-scale sinistral strike-slip fault zone in middle Asia continent. The fault zone not only cuts across the western Kunlun range to the south and Qilian Mountain to the north, but also displaced Tarim and Qaidam basins. It plays a very important role in the tectonic evolution of western China and even of the middle Asia region. This paper mainly focuses on the control of Cenozoic structural characteristics and evolution of Altyn fault zone on the Westren Qaidam basin, combining with macrostructure, microstructure, petrofabric statistical analysis, the strike-slip model of the fault, granulometric statistical analysis and the structural study of the reversed "S"type structure, to discuss the tectonic evolution of Altyn fault zone. Based on the a plenty of field data and the investigations of former researchers: (l)We carried out microstructure analysis study on these samples which were collected in Mangai-Dangjin Shan Pass. On the basis of the petrological features, the petrofabric orientation measurement of quartz optic axis has been done, the petrofabric diagrams have been ploted with reasonable explanation; (2) The systemic mineral identification, granulometric analysis and detrital composition statistics have been done for the sediment samples that collected alonethe Hongsanhan section. Combining with sedimentology, paleontology and isotope geochemistry the strike-slip features of the Altyn fault and mountain upliftinghave been discussed; (3) We have studied the reversed "S"type fold structures in Cenozoic in Qaidam basin.Some typical reversed "S" type belts in the Western Qaidam basin, such as Ahati , Ganchaigou, Hongsanhan, were selected and studied in detail, on the basis of which the structural formation of the reversed "S" type strucutes in Cenozoic in Qaidam basin, kinematics and geotectonic significance of the Altyn fault have been discussed. The conclusions are as follows: 1. By the studies on the macrostructural feature, metamorphism and deformation of the samples along Akesai-Dangjinshan pass section, we found that in the middle shear zone the rocks deformation degree is higher than that in both sides away from the middle zone. The closer to the middle shear zone the rock is located, the more intensive ductile shear deformation the rock is suffered, away from the middle zone the deformation changes into ductile-brittle deformation gradually, and into brittle deformation finally. 2. The microstructure and petrofabric analyses suggest that the Altyn fault is a sinistral strike-slip fault. The middle shear zone exhibits mainly simple shear deformation, the farther away from the middle shear zone, the deformationchanges from simple shear deformation, to simple shear-pure sheardeformation gradually, and to pure shear deformation finally. 3. By studying the control on Cenozoic structural formation and the evolution of the Altyn fault zone in western Qaidam basin, we concluded that: (l)The reversed "S" type fold structures of the whole basin have the same structural pattern from the south to the north, this kind of structure pattern is universal in the Qaidam basin, and the formation mechanism is also the same, the basin was not only suffered a SN-direction compression, but also, meanwhile, suffered the west-directed dragging by the Altyn strike-slip fault so that the hings of the fold belts turn to the west and converge to the Altyn fault to form the reversed "S" type fold structures. (2) The most fold structures in the Western Qaidam basin are fault-propagation folds. The folds are isocline folds with overturn to the S along the Youshashan-Ahati zone in the south, but with overturn to the N along the Hongsanhan No. 1 zone. The seismic data show that there is often a thrust fault developed in the deep of the overturn limb and cuts across the overturn limb. (3) Almost all Early Pleistocene strata were involved in folding, it suggests that a large-scale structural movement should occurred at the end of Ealty Pleistocene. 5. The depositional environment of Qaidam basin (Hongsanhan No.l area) has changed with uplift of Qinghai-Tibet Plateau and the Altun Mountains since Tertiary. By granulometric analysis of sandstone samples of the different periods of Cenozoic along the sections of Hongsanhan No.l high 3, we conclude as follows: (l)At the period of Eocene (lower Ganchaigou Formation) depositional characteristics of Hongsanhan sedimentary area indicate that Altyn Mountains didn't experience a rapid uplift relative to Qaidam basin, probably at the stage of slow or steady uplift. (2) At the period of Oligocene (upper Ganchaigou Formation) Altyn Mountains began to uplift rapidly relative to Qaidam basin, and the climate of the sedimentary area became drought. (3) From Miocene to Pliocene(lower,upper Youshashan Formation and Shizigou Formation), the whole Hongsanhan area was uplifted, Altun Mountains didn't have a largetopographical height difference relative to depositional basin area.