青藏高原东缘龙门-锦屏造山带的崛起——大型拆离断层和挤出机制

Uplift of the Longmen-Jinping orogenic belt along the eastern margin of the Qinghai-Tibet Plateau:Large-scale detachment faulting and extrusion mechanism.

龙门-锦屏山的东缘发育一系列逆冲断裂和飞来峰构造,逆冲作用使山体向东叠置在四川盆地之上。新的野外调查、显微构造分析和糜棱岩石英组构的EBSD测量表明,在龙门-锦屏山的前震旦纪变质杂岩体西缘(即青藏高原东缘)发育一条近NS向的大型韧性拆离断裂,被20Ma以来形成的NW—SE向鲜水河韧性走滑剪切带[1]左行错位80km。青藏高原东缘韧性拆离断裂中黑云母40Ar-39Ar测年获得112～120Ma的年龄,表明龙门-锦屏山的崛起可能与白垩纪开始的垂向挤出机制密切关联。结合四川前陆盆地的沉积及演化特征,认为晚三叠世时期羌塘/东昆仑/扬子陆块的碰撞形成松潘-甘孜造山带,晚三叠世—侏罗纪在其东南缘形成四川前陆盆地沉积;早白垩世龙门-锦屏山开始抬升,晚白垩世快速崛起,在四川前陆盆地沉积之上叠置白垩纪—第四纪再生前陆盆地的沉积。龙门-锦屏山的崛起与白垩纪以来扬子板块岩石圈对于松潘-甘孜地体的陆内俯冲作用有关,使位于中下地壳的变质基底岩石在挤出机制下隆起。

A series of thrust faults and ldippen occur along the eastern margin of the Longmen-Jinping mountains, and the mountains were thrust eastward over the Sichuan basin. Recent field survey, microstructural analysis and EBSD measurements of quartz LPOs in mylonite indicate the presence of a nearly N-S-trending large-scale detachment system along the western margin of the pre-Sinian metamorphic complexes of the Longmen-Jinping mountains. This detachment system has been offset sinistrally about 80 km by the NW-SE-trending Xianshuihe ductile strike-slip fault during the last 20 Ma. The uplift of the Longmen-Jinping mountains probably occurred as a result of vertical extrusion along this detachment system. New ^39Ar/^40Ar dating of biotite fi：om the detachment yields ages of 112-120 Ma, which suggests that the uplift began in the Cretaceous. The above-mentioned study, combined with the sedimentary and evolutionary features of the Sichuan foreland basin, indicates that： the collision of the Qiangtang, East Kunlun and Yangtze blocks during the Late Triassic resulted in the formation of the Songpan-Garze orogenic belt and the deposits of the Sichuan foreland basin occurred on its southeastern margin during the Late Triassic-Jurassic; the uplift of the Longmen-Jinping mountains began to uplift in the Early Cretaceous and rose rapidly in the Late Cretaceous; and the deposits of a Cretaceous - Quaternary regenerated foreland basin were superimposed upon the deposits of the Sichuan foreland basin. The rise of the Longmen-Jinping mountains is related to the intercontinental subduction of the Yangtze plate beneath the Songpan-Garze block in the Cretaceous and later extrusion and rise of the metamorphic basement rocks in the lower-middle crust.