The Geometry And Kinematics Of The Central Segment Of The Longmenshan Mountains: Implications For The Uplifting Mechanism In The Southeastern Margin Of Qinghai-Tibet Plateau

The Longmenshan Mountains (LMS) are the boundary of Southeast Margin of Qinghai-Tibet Plateau and Sichuan Basin, which central segment has the most complicate geologic characteristics with Ms.8.0 earthquake occurred in 2008. However, the uplifting mechanism in the Southeastern Margin of Qinghai-Tibet Plateau inspiring a vigorous debate. Therefore, its necessary to reveal the shallow geological structures of Longmenshan Mountains and construct some models of deep structures. In this paper, the fine interpretations of seismic profiles in the central Longmenshan Mountains have been accomplished with the application of tectonics and fault-related folds geometry principles. We try to understand the geometric and kinematic structures of the Central Longmenshan Mountains by integrating the geology map, seismic reflection profiles, remote sensing, DEM and well drilling data. It may provide some constraints on the dynamical mechanism of uplifting in the Southeast Margin of Qinghai-Tibet Plateau.1. Our studies find that there are multi-detachment layers with different structural deformation in the Central Longmenshan Mountains front belts. The upper detachment layers are the Upper Triassic Xujiahe formation, which develop the imbricate structures and duplex; another is the Lower Triassic Jialingjiang-Leikoupo formation, which develop the fault-bend folds, pop-up structures and structural wedges; The middle detachment layers are the Lower Cambrian formation and develop duplex; The Lower detachments which below the Pre-Cambrian basements at the depth of 15～20 km are developing the duplex and structural wedges. The structural wedges are one of the most important structural styles in this area. The Cenozoic deformation are the basements thrusting and shortening, which formed the structural wedges and make the Upper terrane uplifting quickly and developed the monocline in the thrust front belt.2. The Pengguan thrust belts have three major n faults and another branch faults in the central LMS. The co-seismic ruptured fault in wenchuan Ms8.0 earthquake is the furthest front one. The 3-D modeling of Pengguan faults show the fault surfaces are unhomogeneous. Pengguan fault with the dip about 20°～30°and with the depth about 10 km. It’s first developing in the Late Jurassic period and keeping on activity in the Cenozoic. 3. According to the surface geology, unconformities, growth statra, axial surface analysis etc, we identify three major periods of contractional deformation in the Late Triassic, the Late Jurassic-Early Cretaceous and the Cenozoic. The Mesozoic deformation are the shallow sedimentary cover with large-scale shortening and the shortening rates are more than 40% in Bailu areas and about 57% in Hanwang areas. The structures deformations in Bailu areas are more intense than Hanwang area in the Cenozoic. Because of the different characteristics of deformation, the front of central Longmenshan Mountains could individed into two sub-segments, one is the Bailu area and the other is Hanwang area.4. There are some regional detachment faults transferring its slipping from Longmenshan Mountains to the Sichuan Basin. The Upper detachments develop the Longquanshan structures etc. The Lower detachments fault transfer its slipping from Longmenshan Mountains thrust belt to the hinterland of Sichuan Basin, and finally to the piedmont zone of southwest Huayingshan (HYS) with slipping about 4 km. The Suining Ms5.0 Earthquake which occurred in 2010 in response to the basement detachment fault re-activation. Therefore, It’s necessary to re-assess the stability of Sichuan Basin and earthquake hazard.