Mesozoic-Cenozoic Tectonic Events And Sedimentary Responses In Liupan Shan Area

The Liupan Shan area is situated in the northeast of present Tibetan Plateau. Studies of Mesozoic-Cenozoic evolution in this area will provide constraints to test the models which have been proposed to explain the formation, uplift and mechanism of the plateau. The studies of active faults, especially the timing, would be with importance for the earthquake evaluation. Meanwhile, as the boundary between the arid northwest China and humid southeast China, this area provides detailed information to the palaeo-climate evolution of China.Based on the studies of sedimentology, fission-track thermochronology, structural deformation and palaeoclimate since late Cretaceous, combined with other results, the following implications have been achieved.1 Based on the gravel counting in Xiaohonggou section southeast of Xiang Shan, gravels in upper segment of Sikouzi Formation, lower and upper segments of Hongliugou Formation, Quaternary and present riverbed, appear to be mainly sandstone and quartz sandstone, which is similar to the lithology in Xiang Shan, indicating they came from Xiang Shan area. The gravels are mainly pebbles and cobbles, sub-rounded and sub-angular, and well sorted to moderately well sorted. The grain-size distribution seems to be mainly fine skewed and leptokurtic normal distributing. These characters show all the gravels have experienced similar transport processes. It is interpreted that the gravels was formed by the alluvial fans moderate distance far from the Xiang Shan. The ages of the gravels have been constrained by magnetostratigraphical work conducted in adjacent Hejiakouzi section. This indicates that there has been a relatively high relief in the Xiang Shan, northeast of Tibetan Plateau, when the Sikouzi Formation deposited (Eocene). During Oligocene, when the Qingshuiying Formation deposited, the existing high relief had been eroded. The second high relief of Xiang Shan occurred during early to middle Miocene, when the Hongliugou Formation deposited. During the deposition of Ganhegou Formation (later Miocene to Pliocene), Xiang Shan area experienced another tectonically stable period. The latest uplift event happened in Pleistocene. The high relief in Xiang Shan during Eocene might be caused by the impact of tectonic events during the later Cretaceous to early Cenozoic. The second high relief during early to middle Miocene can be related to the impact of India-Asian collision. These imply that this area has experienced periods of uplift.2 Apatite fission-track analysis and related thermal history modeling results reveal that Liupan Shan area has widely undergone two periods of cooling/uplift, as late Cretaceous to early Cenozoic and since Miocene. The uplift rate since Miocene is much larger than that during late Cretaceous to early Cenozoic, implying more intense tectonic activity since Miocene. The cooling/uplift event in early Cenozoic can be traced to late Cretaceous, implying that it can not be considered as the initial response to the India-Asian collision, but the continuing impact of the late Yanshanian Movement.3 The sequences at the bottom of the Cenozoic strata in Sikouzi section are fluvial. Lithology and palaeo-current directions from cross-bedding both indicate that there was a relatively highland at the Madong Shan-Liupan Shan area at the early Cenozoic, providing sediments for the Sikouzi section. These evidences also imply the continuing impact of the late Yanshanian Movement.Within the fifth lithologic package, sedimentary architecture displayed dramatic variation at around 9.5 Ma. Synchronously, the palaeo-current direction changed, from previous E→W to later W→E. The accumulation rate increased to more than double at the meanwhile. All of these sedimentary events have been interpreted as the implications to the commencing uplift of the Madong Shan-Liupan Shan area. After the initial uplift at around 9.5 Ma, this area underwent rapid uplift during around 7.2-8.2 Ma as suggested by the apatite fission-track data.4 Regarding to the red sands at the lower part of the Sikouzi section, both the sedimentary structures and surface texture suggest that they deposited under the desert dune circumstance. The interpretation of the palaeo-climate background depends on their timing. If they deposited in the beginning of Miocene, they probably result from the aridity triggered by the uplift of the Tibetan Plateau. If deposited during Oligocene or even older, they might be the results of the aridity controlled by the planetary climatic belts. Considering the unconformity between the third and fourth association, the author is inclined to the later interpretation.5 The sediment records, combined with pollen and magnetostratigraphic results, reveal a palaeoclimate event during Mid-Miocene (around 16～14 Ma). Generally humid with sporadic humid-arid fluctuations prevailed in this area before 16～14 Ma, while after 16～14 Ma, this area was dominated by semi-arid to arid climate. This palaeoclimate event has been interpreted under a local context, not a global context as some other geologists did. During this period, the Tibetan Plateau underwent intense uplift and growth, which intensified both the aridity in northwest China and the summer monsoon in southeast China. However, the aridity overcame the humidity and predominate this area.6 Geological survey and luminescence dating have been conducted to study the active faults. In the Sikouzi section, the folding and planation happened older than 26.4-38.7 ka. The thrust fault, which thrusted the lower Cretaceous and Cenozoic strata over the Quaternary sequences, has a latest active timing younger than 14.7 ka. In Xiaohonggou section, the thrust fault, which thrusted the Hongliugou Formation over the Quaternary sequences, has an active timing during 108.3-39.2 ka. In Chejiawan section, the thrust fault, which thrusted the Cenozoic strata over the Quaternary sequences, was still active even after 4.0 ka.7 The results of this dissertation support multiple-period uplift model. It seems there has not been a synchronous surface uplift at around 8 Ma, which rejects the convective removal model. With regards to the oblique stepwise rise and growth model, our results indicate that the large strike-slip thrust faults play important role in the plateau growth. However, the existing tectonic conditions should be taken into consideration when studying the response of the Indian-Asian collision. In the present northeast of the plateau, it seems the initial response to the collision happened at Miocene.Moreover, our results also support the proposal that the uplift and growth of the Tibetan Plateau play a vital role in the China palaeo-climate, including the forming and intensification of the Asian monsoon and desertification. VII...