Cenozoic Sedimentary And Structural Features Of Southwest Qaidam Basin And Its Coupling With Oiman Tagh

The Qiman Tagh and Southwest Qaidam Basin(SWQB)are located at the northern part of Tibetan plateau and bounded by two huge left-slip strike-slip faults,i.e.,the East Kunlun Fault to the south and the Altyn Tagh Fault to the northwest.They define the largest topographic relief inside the plateau,indicating prominently differential uplift during the Cenozoic.Due to its particular location,studying the sedimentary and structural evolution features of the SWQB and its coupling with Qiman Tagh is important for us to understand the mechanism of the northward growth of Tibetan Plateau.In this dissertation,we attempt to explore the Cenozoic evolution history of this key area based on detailed sedimentary and structural records revealed by high-resolution seismic reflection and well-logging data within the SWQB and low temperature thermochronological dating in Qiman Tagh.The conclusions are summarized as follows:(1)SWQB was divided into several subunits by major faults according to their different structural and sedimentary features.Gesi Uplift,located in the west comer of SWQB,was a palaeohigh since the late Mesozoic and seperated Alar Sag to the north and Qiekelike Sag to the south.The latter two sags were finally connected in the Neogene because of progressive overlapping of Early Cenozoic sediments onto the Gesi Uplift.The Alar Sag and Hongshi Sag in the northern part of the SWQB was characterized by gradual thickening of the Lulehe(LLH)formation(-65-52 Ma)toward the Altyn Tagh indicating that the Altyn Tagh didn't rise and that the Qaidanm basin might be partially linked to the Southwestern Tarim Basin at that time.Since the deposition period of Xiaganchaigou(XG)formation(52 Ma),the sedimentary center started to migrate toward the east and a nose-like structure formed in the Alar area.This structure persisted by the end of the deposition of the Xiayoushashan(XY)formation(?15 Ma)when strong fault activity happened and resulted in obvious erosion.Since the deposition of the Shangyoushashan(SY)formation(?15 Ma),this area turned to be a simple southeast-dipping slope.The Yuejin-Wunan area in the central part of SWQB was featured by an east-dipping slope in the Early Cenozoic,but divided into Yuejin Slope?Qiekelike Sag?Zhahuaquan Sag and Wunan Slope due to intensive fault activities since the deposition of the SY formation(?15Ma).Kunbei area in the south of SWQB is quite close to Qiman Tagh and lack the SY formation and above strata under the control of NW-trending faults,which suggesting intensive activities of the Qiman Tagh since the end of SY formation.Wanxi area,located in the southeast of SWQB,was in a stable sedimentary environment until the end of SY formation when it was subjected to intense deformation related to the activity of Qiman Tagh.Yingxiong Range,located to the northeast of SWQB,was an important sedimentary centers during most of the Cenozoic era,but deformed intensively by the end of Shizigou(SZG)formation(ca.3 Ma).Now the Yingxiong Rang has become the largest and the most active structural unit within the basin.(2)We present a detailed structural analysis of geometry and temporal development of the Cenozoic faults in the SWQB based on abundant seismic data.The Cenozoic faults are mostly steep,basement-involved reverse faults trending primarily NW and secondarily nearly N-S.The NW-trending faults also had some left-slip component while the N-S-trending faults had certain dextral-slip component,which are both negligible compared to that of thrusting.Several major faults such as Alar fault,Kunbei fault and Kunbei 1th fault became active since as early as the Paleocene,and were inferred to be pre-existing faulting before the Cenozoic.Temporal and spatial development of faults indicates that faults propagated northward and eastward over time,which probably resulted from the thrusting from the Qiman Tagh to the south and the activities of Altyn Tagh faults to the west.(3)The thermochronological dating results,including apatite fission track(AFT)and apatite(U-Th)/He(AHe),from massifs in the Qiman Tagh show that the Qiman Tagh uplifted since the beginning of the Cenozoic era.The AFT modeling indicates that the Qiman Tagh Mountains were subject to two-stage of rapid exhumation(ca.65-40 Ma and since ca.20 Ma)in the Cenozoic era with slow exhumation in the intervening period.The tectonic activity of Qiman Tagh became fairly weak since mid-Miocene due to the rapid northward propagation of deformation into the Qaidam Basin interior.In addition,the AHe result indicates that the South Qiman Tagh and the North Qiman Tagh have different uplifting processes.The South Qiman Tagh was exhumed as a whole with weak influence of local fault activities in the Cenozioc;while exhumation of the North Qiman Tagh was largely affected by faults.This difference may be attributed to their different pre-Cenozoic structural deformation history.(4)The contact between the Qiman Tagh and SWQB is a north-dipping unconformity to the west,but changed to faulting to the east.We thus proposed a five-stage evolution process for the study area in the Cenozoic based on above mentioned analyse.The first stage is from the Paleocene to the Middle-Eocene.The Qiman Tagh was a paleouplift inherited from Late Mesozoic paleotopography and partially separated the Kumukol and Qaidam Basins at the beginning of the Cenozoic.It started to uplift and be exhumed in the early Cenozoic in response to the India-Eurasia collision-The SWQB expanded with a feature of filling and leveling up,and several major NW-trending faults became active at the time.The second stage is in the late Eocene(-47-35.5 Ma).The SWQB expanded rapidly and water dcepened quickly.The deposits covered part of the North Qiman Tagh,while the South Qiman Tagh was rapidly exhumed,shedding clastics into the SWQB.The third stage is in the latest Eocene-middle Miocene(35.5-15.3 Ma).The Altyn Shan started to uplift at the time,resulting in fault activities in the Alar area close to it and a local angular unconformity between the SY formation and underlying strata.Meanwhile,Qiman Tagh was in a relatively stable period.The fouth stage is since middle Miocene(<-15.3 Ma).The Altyn Tagh fault and East Kunlun fault initiated large-scale sinistral strike-slip,causing a series of tectonic adjustment events in the northern Tibetan Plateau.Qiman Tagh went into another rapid cooling period and thrusted intensively northward onto the SWQB,especially its eastern part,resulting in the fault contact between them at present.The last stage is since ca.8 Ma.Deformation propagated from peripheral area to the Qaidam Basin interior rapidly at the time,with a large number of NW-trending faults and folds forming within the basin.The strongest deformation occurred in the Yingxiong Range,changing it from a stable sedimentary center into a large-scale uplift belt.