龙门山断裂带印支期左旋走滑运动及其大地构造成因

EARLY MESOZOIC LEFT-LATERAL MOVEMENT ALONG THE LONGMEN SHAN FAULT BELT AND ITS TECTONIC IMPLICATIONS

位于青藏高原东缘的龙门山构造呈北东—南西向将松潘—甘孜褶皱带和华南地块分割开。前者主要是由一套巨厚的三叠纪复理石沉积组成 ,分布在古特提斯海的东缘。后者由前寒武纪基底和上覆的古生代和中生代沉积盖层组成。位于汶川—茂汶断裂以东的前龙门山存在一系列倾向北西的逆掩断层 ,它们将许多由元古宙和古生代岩层组成的断片向南东置于四川盆地的中生代红层之上 ,构成典型的薄皮构造。许多研究由此断定松潘—甘孜褶皱带和四川盆地之间在中生代发生过大规模的北西—南东向挤压。然而 ,汶川—茂汶断裂西侧的松潘—甘孜褶皱带内部的挤压构造线大多是垂直于而不是平形于龙门山断裂带 ,这表明当时的挤压应力不是北西—南东向而是北东—南西向。近年来在龙门山构造带内发现 ,在三叠纪时龙门山断裂带在发生推覆的同时还经历过大规模的北东—南西向的左旋走滑运动 ,协调走滑运动的主要构造为汶川—茂汶断裂。走滑运动的成因与松潘—甘孜褶皱带北东—南西向缩短有关。汶川—茂汶断裂的左旋走滑在龙门山的北东端被古特提斯海沿勉略俯冲带的消减和发生在大巴山的古生代 /中生代岩层的褶皱和冲断作用所吸收 ,在龙门山的南西端被古特提斯海沿甘孜—理塘俯冲带的消减和松潘—甘孜三叠纪复理石的褶皱和冲断作用所吸?

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.