印度陆块新生代两次仰冲事件及其构造驱动机制:论印度洋、特提斯和欧亚板块相互作用

Cenozoic two-stage obduction of the Indian subcontinent: On the interaction between the Indian Ocean,Tethyan and Eurasian plates

印度陆块与欧亚大陆的碰撞是印度洋扩张和特提斯洋闭合综合作用的结果。本文通过综合分析和研究提出这3个板块的相互作用致使印度陆块发生过2次向北的仰冲:早期(古新世末-始新世初,~57Ma)仰冲受其超高速运动(>140mm/yr)的驱动,与特提斯之间产生的速度差致使两者间的边界发生破裂,密度小的印度陆块沿印度洋东经90°海岭和马尔代夫岛链向北仰冲到特提斯洋壳之上,两者的叠加导致印度陆块北缘——特提斯喜马拉雅地壳增厚(~70km)并且沉积了一套造山磨拉石——柳曲砾岩;晚期(渐新世-中新世之交,~25Ma)仰冲发生在碰撞后,由于高喜马拉雅结晶岩系沿主中央冲断带和藏南拆离断裂发生的垂向挤出,位于上盘的特提斯喜马拉雅沉积盖层同时发生重力垮塌,沿大喜马拉雅反冲断裂仰冲到冈底斯岩浆岩带之上并且造成后者的隆升和前陆下陷,其北缘充填了一套造山磨拉石沉积——大竹卡砾岩。这两次构造事件均受印度陆块的快速运动驱动。此外,在印度陆块超高速运动的挤压下,特提斯洋可能在早白垩世之后就停止了扩张,而老的洋壳不是俯冲消减了就是被仰冲的印度陆块掩盖了,这解释了为什么雅鲁藏布江缝合带只存早白垩世蛇绿岩。印度洋内东经90°海岭和马尔代夫岛链构成印度陆块的南东和南西边界,前者呈右行走滑,后者呈左行走滑,两者勾画出印度陆块向北漂移的轨迹。

The collision between the Indian and Eurasian continents is the result of the combined interaction action between the spreading of the Indian plate and the closure of the Tethys. Either the oceanic subduction of the Tethys or the subsequent continental subduction of the Indian landmass were all directed to the north beneath the Eurasian continent,as shown by the occurrence of the widespread magmatite along the Gangdese belt and the creation of the Tibetan Plateau. However,in fact,there don＇t exist any northdipping structures on the surface of the Yalungzangbo suture zone,but mostly the south-dipping structures,namely the Great Counter Thrust System of the Himalaya,indicating that the Indian landmass obducted to the north with respect to the Asia continent. There exist two sets of the molasse deposits along the Yalungzangbo suture zone,namely Liuqu and Dazhuka,dated as Eocene and Early Miocene in age. Based on synthetic analysis and study,this paper herein proposes that they were all the products of the northward obduction of the Indian landmass. The former was produced by the obduction of the Indian crust over the Eurasian continent,which led the occurrence of flextural bending of the southern margin of the Asian continent,resulting in the uplift of the Gangdese belt. While the latter results from the obduction of the Indian landmass over the Tethyan oceanic crust,as a result of rapid spreading（ 140 mm/yr） of the Indian plate. The subduced oceanic crust of the Tethys may have still preserved beneath the northern margin of the Indian landmass,as shown by the abnormally thick crustal thickness（ - 70 km） of the Tethys Himalayan belt. The northward movement of the Indian landmass is accommodated by the strike-slip movement along the Ninety-east Range and Maldives Islands.