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TECTONICS, SCISMISITY AND DYNAMICS OF LONGMENSHAN MOUNTAINS AND ITS ADJACENT REGIONS

Deng Qidong; Chen Shefa; Zhao Xiaolin  
The Longmenshan region inches the Longmenshan thrust belt,the western Sichuan fore-land basin , the Longquanshan tectonic belt, and the Minshan uplift. The NE-trending Long-menshan thrust belt is composed of 4 major thusts and related napes, which form an extensive-compressive belt. The thrusting, mainly in a forward propagation style, started in the middle tolater part of Late Triassic and propagated from northwest to southeast. The Quaternary activityof the central and southern parts of the Longmenshan belt is stronger than the northern part.The western Sichuan foreland basin began to develop in the Late Triassic. The basin graduallyretreated toward southwest since the late stage of Late Triassic, and the Quaternary deposition islimited on the Chengdu plain. The Longquanshan tectonic belt forms the eastern boundary of theforeland basin. It is a fault-propagation fold controlled by a master fault on its western margin.The NS-trending Minshan uplift northwest of the Longmenshan thrust belt is active since 2 Maago. The uplift is controlled by the Minjing and Huya faults, both with late Quaternary activi-ties, on its western and eastern margins, respectively. The left-lateral slip rate of the Minjiangfault is about 1 mm/a. The Longmenshan thrust belt coincides with a steep gravity and magnetic gradient zone.The velocity structure and crustal thickness are very different on its northwestern and southeast-ern sides. There is a low velocity and high conductivity layer at a depth of 20 km on the west ofthe Longmenshan belt. The layer may be a detachment. Therefore, the Longmenshan belt maybe formed by a series of listric thrusts, and the wedges cut by the thrusts are thicker on the westand thinner on the east. The southern and central Longmenshan thrust belt and the Minshan uplift apparently con-trol the seismicity in the region. This NE-to NS-trending seismic belt may be the active east-central margin of the Tibatan Plateau. Focal rnechanism solutions suggest that the principal com-pressive stress axis in the region trends NWW.The formation of the Longmenshan belt may be caused by the NW-SE directed compressionon the Songpan-Garze fold belt on the west. From Eocene to present, collision of the Indian andEurasian plates caused southeastward movement of the western Sichuan block. This movementproduced a continuous compression on the Longmenshan belt. The movement of the westernSichuan block produced reverse faulting with left-lateral component on the boundary faults of theNS-trending Minshan uplift. The Quaternary activity in the northern part of Longmenshan beltis relatively weak due to the existence of the Minshan uplift.
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