EARLY MESOZOIC LEFT-LATERAL MOVEMENT ALONG THE LONGMEN SHAN FAULT BELT AND ITS TECTONIC IMPLICATIONS
WANG Er qi 1,MENG Qing ren 1, CHEN Zhi liang 2, CHEN Liang zhong 3 (1. Laboratory of Lithosphere Tectonic Evolution,Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029,China;2.Chengdu Institute of Geology and Mineral
Bounded by the Tibetan plateau on the west and lying in NESW direction, the Longmen Shan fault belt divides the SongpanGanzi fold belt to the west and the Sichuan basin to the east. The former mainly consists of flysch rocks of Triassic age, which mark the eastern margin of the paleoTethys. The latter is situated in the western part of the South China block, filled with a thick pack of purplecolored terrestrial sediments of Mesozoic age. The Longmen Shan fault belt comprises three principal structures: the WenchuanMaowen fault on the west, the BeichuanYingxiu fault in the middle and the eastern boundary of the fault belt on the east. During the IndoSinian period (Triassic to early Jurassic), the fault belt underwent eastverging thrusting and folding deformation along the BeichuanYingxiu fault and the eastern boundary, which emplaced Precambrian and Paleozoic rocks over Mesozoic rocks of the Sichuan basin. The crustal shortening across the Longmen Shan fault belt is generally considered to have resulted from regional NWSE crustal contraction. However, all fold axes and thrust faults within the SongpanGanzi fold belt are perpendicular instead of parallel to the Longmen Shan fault belt, indicating that its shortening resulted from the NESW crustal contraction instead of NWSE crustal contraction. Coeval with the NWSE shortening, the Longmen Shan fault belt was also experienced leftlateral shear in early Mesozoic time, mainly along the WenchuanMaowen fault. The WenchuanMaowen fault is merged by numerous NWtrending arcuate folds and thrust faults within the SongpanGanzi flysch belt. The southwest end of the fault adjoins on the west of the GanziLitang fault, which marks a suture zone of late Triassic age. Based on the deformation style and tectonic configuration, we infer that the WenchuanMaowen fault was formed as a large transfer fault for accommodating the westward movement of the paleoTethys and the SongpanGanzi flysch belt relative to the South China block. The westward movement of the SongpanGanzi fold belt and associated the NESW shortening may have resulted from the NS collision of the South China block with North China block, which resulted in crustal materials undergoing lateral movement. Prior to the collision, the SongpanGanzi flysch belt may have located farther east within the Qinling orogenic belt. In addition to the SongpanGanzi flysch belt, some other crustal fragments lying to the northwest of the Longmen Shan fault belt such as the Bikou block may have also experienced westward escape from the Qinling orogenic belt.
【Fund】： 国家重大基础研究发展规划青藏高原研究资助!项目(19980 40 80 0 )
【CateGory Index】： P54
【CateGory Index】： P54