Late Cenozoic Crustal Shortening In The Longmen Shan Fold-and-thrust Belt And Tectonics Stress Field Changes

The Tibetan Plateau was uplifted and thickened by the collision between the Indian and Eurasian Plates at approximately 50 Ma. The Longmen Shan exhibits greater topographic relief than anywhere else on the plateau, but geodetic and geologic surveys measured<3 mm/yr shortening across the range front. Competing models have been proposed to explain the production process of the remarkable topography. However, due to lack a coeval foreland basin, the crustal shortening of the eastern Tibetan Plateau remains uncertain. Therefore, testing these models has been difficult. The devastating Mw 7.9 Wenchuan earthquake in Longmen Shan in the eastern Tibetan Plateau on May 12,2008 resulted in enormous economic losses and casualties. Its occurrence also provides an excellent opportunity to resolve debates of uplift mechanisms for the Tibetan Plateau. In this PhD thesis,1) two major crustal shortening events have been discussed during the Mesozoic and Cenozoic, respectively,2) crustal shortening and a change in the field direction of the tectonics stress may be occurred in late Cenozoic,3) 3-D structural models have been applied to earthquake hazards of range front thrust in Longmen Shan, and 4) uplift mechanisms have been discussed based on 3-D structural model.Uplift mechanisms of Longmen Shan fold-and-thrust belt during the late Cenozoic have been debated for a long time. Field investigation and seismic interpretation imply that two major crustal shortening events have occurred along the belt since the Mesozoic, the compressional event during the late Triassic and the deformation during the late Cenozoic. The late Triassic deformation is mainly occurred in middle and north segments of the belt, boundary is located in Dujiangyan city. Moreover, the total shortening measured by balanced geologic cross-sections of Longmen Shan and western of Sichuan basin, reduce from 27 km in north segment to 17 km in middle segment. However, the late Cenozoic compressional event is likely controlled the entire Longmen Shan. The total shortening measured by balanced geologic cross-sections in south segment of Longmen Shan and western of Sichuan basin is 30 km, whereas is only 10 km in the middle and north segments.The co-seismic slip sense of the 2008 Wenchuan earthquake (Mw 7.9) has demonstrated the present east-west (E-W) crustal shortening and oblique thrusting across Longmen Shan, which are inconsistent with southeast-directed thrusting that occurred during the late Triassic. Although the two major periods of compressional deformations in Longmen Shan have long been recognized, the fault slip rate of the late Cenozoic deformation and the initial E-W crustal shortening remain poorly investigated. The E-W crustal shortening is investigated by using 3-D seismic reflection data, interpreting satellite image, and conducting a field investigation in the Dayi Fault System (DYFS) to determine tectonic stress field changes during the late Cenozoic. Two-period tectonic deformations during the late Cenozoic are found in the DYFS, which correspond to the NE- and NS-trending structures, respectively. The activities of the DYFS may reflect a change in the field direction of the tectonic stress—from NW-SE during the Oligocene to early Pliocene to E-W during the late Pliocene to Holocene. The 120 km NS-trending structures in the southern Longmen Shan range front as well as the Wenchuan earthquake co-seismic ruptures are assumed to reflect the active, E-W crustal shortening in Longmen Shan. Based on quantitative fault-related folding theory, an average fault slip rate of 0.25 mm/yr is obtained from the late Pliocene to present for the range front thrust of Longmen Shan since the late Pliocene.There is a seismic gap extending about 75 km from the aftershock cluster of the Lushan earthquake to the main main-shock of the Wenchuan earthquake. Moreover, the calculations suggest that the 2008 Wenchuan and 2013 Lushan earthquakes increased the Coulomb failure stress to the segment of the range front thrust of Longmen Shan of the seismic gap, it would promote earthquake hazard. Wenchuan and Lushan earthquakes have been occurred in Longmen Shan, which is consistent with the empirical relationship among moment magnitude, rupture, and subsurface rupture length. On the basis of empirical relationship between rupture area and moment magnitude, ruptures on the area of modeled range front thrust (750 km2) could yield an earthquake of Mw 6.8. On the basis of empirical relationship between subsurface rupture length and moment magnitude, ruptures on the length of the seismic gap could yield an earthquake of Mw 7.4. We infer that the range front thrust in the seismic gap may pose a heightened risk of an earthquake of Mw 7.0.The total shortening on the southern segment of Longmen Shan in the eastern Tibetan Plateau is 30 km. Moreover, structural relief and topography are strongly correlated in the range front thrust of Longmen Shan and western Sichuan basin, combined published data related to crustal shortening and topography are largely coupled reflect upper crustal shortening deformation may be the primary mechanism for uplift and topography on the eastern Tibetan Plateau.