EARLY MESOZOIC LEFT-LATERAL MOVEMENT ALONG THE LONGMEN SHAN FAULT BELT AND ITS TECTONIC IMPLICATIONS
EARLY MESOZOIC LEFT-LATERAL MOVEMENT ALONG THE LONGMEN SHAN FAULT BELT AND ITS TECTONIC IMPLICATIONS
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作者：
W. Er
龙门山断裂带西接青藏高原，北西西向，将松潘-甘孜褶皱带和四川盆地一分为二。前者主要由三叠纪复理石岩组成，标志着古特提斯的东缘。后者位于华南陆块西部，充填着一套厚层的中生代紫红色陆相沉积物。龙门山断裂带由三个主要构造组成，即西部的汶川-茂汶断裂、中部的北川-映秀断裂和东部的断裂带东边界。印支期（三叠纪-早侏罗世），断裂带沿着北川-映秀断裂及东界发生向东的逆冲褶皱变形，使四川盆地内的前寒武纪和古生代岩石侵位于中生代岩石之上。龙门山断裂带的地壳缩短一般认为是区域性NWSE向地壳收缩的结果。然而，松潘-甘孜褶皱带内的褶皱轴和逆冲断层均垂直于龙门山断裂带，而不是平行于龙门山断裂带，表明其缩短是由NESW向地壳收缩而不是NWSE向地壳收缩造成的。龙门山断裂带在中生代早期也经历了以沿着汶川-茂汶断裂为主的左旋剪切作用。文川-茂文断裂由松潘-甘孜复理石带内众多NW向弧形褶皱和逆冲断裂合并而成。断裂西南端与甘孜-理塘断裂西侧相接，标志着晚三叠世缝合带的形成。根据其变形样式和构造形态，推测汶川-茂汶断裂是为适应古特提斯和松潘-甘孜复理石带相对于华南陆块的向西移而形成的大型转换断裂。松潘-甘孜褶皱带的向西移及与之相关的北西西向缩短，可能是华南地块与华北地块南北向碰撞导致地壳物质横向运动的结果。在碰撞之前，松潘-甘孜复理石带可能位于秦岭造山带更远的东部。除松潘-甘孜复理石带外，龙门山断裂带西北部的碧口地块等地壳碎块也可能经历了向西的逃逸过程。
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.