Active Fault-related Folding In The South Longmen Shan Fold-and-thrust Belt And Earthquake Hazard Risk Analysis In The Sichuan Basin

The Longmen Shan fold-and-thrust belt, located between the high Tibetan Plateau and the lowland of the Sichuan basin, which experienced at least two major periods of crustal shortening in the Late Triassic and Cenozoic, and forms the typical nappe structure system in China. The formation and maintenance of Longmen Shan have a closely related to the uplift of Tibetan Plateau and present earthquake hazard. In this PhD thesis, I present the study on the segmentation on Longmen Shan fold-and-thrust belt, active fault-related folding and paleoseismic evidences in the southern Longmen Shan range front, as well as3D seismic velocity models for Sichuan region.Based on different surface ruptures characteristic and focal mechanisms, the Wenchuan surface ruptures can be divided into two segments:the southern segment (Beichuan-Yingxiu and Pengguan surface ruptures) and northern segment (Beichuan-Qinchuan surface rupture). Field observations indicate that the slip on south segment is mainly caused by thrusting, with lesser amounts of dextral oblique-slip, whereas the north segment is characterized largely by dextral strike-slip accompanied by a thrusting component.3-D models show that the co-seismic fault has a complex structural geometry, with slip in the south partitioned between the steeply-dipping Beichuan-Yingxiu fault and the shallower Pengguan fault, which appear to merge at17km depth. In contrast, slip in the north is limited to the Beichuan-Yingxiu fault. Thus we conduct that the segmentation of co-seismic surface ruptures of Wenchuan earthquake is resulted from the pre-existing segmentation of Longmen Shan belt in the Late Triassic time.Geological evidences for active faulting and folding in the southern Longmen Shan remains poorly documented. We define the structural geometry, fault kinematics, and seismic hazard of the Qiongxi thrust fault system (QTF) along the southern Longmen Shan range front by integrating seismic reflection data and geomorphic observations. The QTF is a50-km-long, N-S-trending set of faults and associated folds that exhibit geomorphic evidence of Quaternary surface deformation. Geomorphic observations and seismic reflection data reveal that these faults dip steeply to the east and merge at depth with a blind, west-dipping thrust ramp. Based on uplift of stratigraphic horizons across the fault zone, we define a late Pliocene to early Pleistocene fault slip rate of0.2-0.3mm/yr, and a middle Pleistocene to present rate of0.4-1.2mm/yr on the west-dipping thrust ramp. This ramp soles to a basal detachment in the Triassic section at a depth of4.5-5.5km. To the west, this detachment steps down onto a blind, northwest-dipping thrust termed the Range Front Thrust. A rupture of the QTF in combination with the Range Front Thrust could generate a Mw7.8earthquake with average displacement of5.7m.Seismic reflection profiles and field investigations reveal that the50-km-long Qiongxi thrust fault of the southern Longmen Shan fold-and-thrust belt is currently active. The QTF consists of three west-dipping ramp segments overlain by fault-bend folds rooted in a regional detachment that transfers shortening from the mountain belt into the Sichuan basin. Trench investigations, coupled with radiocarbon results, show that two surface-rupturing earthquakes occurred on the QTF between3960±40BP and2860±40BP, with the recent one in the past400years. In addition, seismic reflection profile and topographic analysis indicate the presence of a subtle topographic, produced by kink-band migration folding above a fault bend at about5km depth.We present three-dimensional fault model and seismic velocity model of the crust and upper mantle for the eastern margin of the Tibetan Plateau, developed using the concept of Community Fault Model and Community Velocity Model. The model describes the velocity structure of the Sichuan basin and surrounding fold-and-thrust belt systems, as well as the Kunlun and Xianshuihe-Anninghe strike-slip fault systems in the eastern Tibetan Plateau. This three-dimensional P and S wave velocity model is derived from direct velocity measurements, such as sonic logs, vertical seismic profiles, and stacking velocity from seismic reflection data that are used to describe basin structures. The model consists of3D surfaces representing major geologic units and faults, and is parameterized with velocity-depth curves calibrated using sonic logs from wells. The model incorporates data from1166oil wells, isopach maps, surface geological maps, and digital elevation models. By calculating the attenuation relationship, we reveal that the range front blind thrust fault poses important seismic hazard to the population density area of the Cheng Plain in Sichuan region.