科希斯坦-拉达克弧及其周缘地块的古地磁学研究进展

Paleomagnetic constraints on paleogeography for the Kohistan-Ladakh arc and its surrounding terranes

对于科希斯坦-拉达克弧在印度-亚洲碰撞前的构造属性,学界存在两种不同的认识:陆缘弧和洋内弧.厘定其构造属性,将直接关联新特提斯洋古地理重建方案,对进一步解析印度-亚洲碰撞时间、位置和方式,以及恢复亚洲大陆构造变形历史等具有重要科学意义.本文在总结白垩纪至古新世科希斯坦-拉达克弧及其周缘地块古地磁数据的基础上,主要从古地磁学的视角,重建科希斯坦-拉达克弧及周缘地块的时空构造格局,厘定科希斯坦-拉达克弧的构造属性.在晚白垩世,喀喇昆仑地块与拉萨地块西缘的古纬度一致,两者基本稳定位于亚洲大陆南缘;科希斯坦-拉达克弧在90 Ma位于南半球赤道附近,而在64 Ma已经向北漂移到北半球低纬度地区;特提斯喜马拉雅在75 Ma左右作为印度被动大陆边缘,其中部和东部分别位于16.7°和19.4°S.在64 Ma科希斯坦-拉达克弧与拉萨地块西缘之间仍然被南北向宽度约为900 km的洋盆所分隔,意味着科希斯坦-拉达克弧为洋内弧.在重建印度-亚洲碰撞系统时,综合考虑特提斯喜马拉雅及科希斯坦-拉达克弧的古地磁数据,构建了印度-亚洲三阶段碰撞模型.该模型不仅协调了一系列地质证据,还为新特提斯洋古地理重建及亚洲陆内变形提供重要约束.例如,什约克缝合带与雅鲁藏布江缝合带曾经相连并可对比,据此推断,喀喇昆仑断裂至少具有400 km的位移量.

Understanding the nature of the Kohistan-Ladakh arc before the India-Asia collision is crucial for reconstructing the Tethyan paleogeography,deciphering geodynamic process of the India-Asia collision,and restoring Asia’s history of intracontinental deformation.However,there are two opposing views dealing with nature of the Kohistan-Ladakh arc:The continental marginal arc and intra-oceanic island arc.Based on the accumulated published paleomagnetic data for the Kohistan-Ladakh arc and its surrounding terranes during the Cretaceous-Paleocene,we reconstruct paleogeography of the Kohistan-Ladakh arc and its surrounding terranes,and explore the nature of the Kohistan-Ladakh arc.The Karakoram terrane was situated on the southern edge of Asia at a paleolatitude consistent with that of the western Lhasa terrane in the late Cretaceous.The Kohistan-Ladakh arc was situated in the equatorial Neo-Tethys Ocean(southern hemisphere)in the early Late Cretaceous(ca.90 Ma)before drifting northward to reach the low-latitude northern hemisphere in the early Paleocene(ca.64 Ma).The central and eastern parts of the Tethyan Himalaya,as the northern margin of India prior to the collision,were located at paleolatitudes of 16.7°and 19.4°S at ca.75 Ma,respectively.The Kohistan-Ladakh arc was separated from the western Lhasa terrane by an ocean basin with a north-south extent of~900 km at ca.64 Ma.The above analyses suggest that the Kohistan-Ladakh arc demonstrated intra-oceanic characteristics prior to the India-Asia collision.Therefore,the Kohistan-Ladakh arc should be considered in the India-Asian collision system.Recently published paleomagnetic data obtained from the Tethyan Himalaya terrane,data obtained from the Kohistan-Ladakh arc,and the apparent polar wander path of India inspired us to propose a triple-stage India-Asia collision model.The first collision occurred at ca.64 Ma between the Tibetan Himalaya terrane and the Kohistan-Ladakh arc.The second collision occurred at ca.61 Ma between the Tibetan Himalaya terrane(including the accreted Trans-Tethyan subduction zone)and Asia.The final collision occurred during 53–47 Ma between India and the Tibetan Himalaya terrane,leading to diachronous suturing of the North India Sea from west to east.The proposed collision model reconciles multiple lines of geologic,geophysical,geochemical,and paleontologic evidence.Especially,we highlight that the new scenario well matches the history of the India-Asia convergence rates,which were documented by the relative plate motions between India-Africa and IndiaAntarctica.In addition,this new model provides important constraints for intracontinental deformation in Asia.For example,the second collision between the Tibetan Himalaya terrane and Asia occurred along the Shyok-Yarlung Tsangpo suture zone,rather than the Indus-Yarlung Tsangpo suture zone.The Shyok and Yarlung Zangbo suture zones have ever been connected,implying a total offset greater than 400 km for the Karakoram fault.