Effect Of Indian Slab Detachment On The Post-collisional Tectonics In The Tibetan Plateau

The Himalayan-Tibetan orogen,the largest and youngest continent-continent collision zone on earth,is an ideal place to study the relationship between the deep lithospheric deformation and crustal-level tectonic activities.In recent years,a number of geological and geophysical evidence indicates that the subducting Indian lithosphere has undergone detachment from west to east during ?25–10 Ma.Indian slab detachment could cause significant changes in the stress in the subduction zone,and cause the orogenic thermo-mechanical rebalance,which was considered as an effective mechanism responsible for a series of geological phenomena.However,there are few studies on how the stress released by slab detachment affects tectonic deformation,which makes the effect of slab detachment in the post-collisional tectonic evolution of the Tibetan Plateau unclear.As the most typical post-collisional tectonics in the Tibetan Plateau,the structural zones in the northern margin of the Tibetan Plateau and the eastwest extensional structures inside the Tibetan Plateau function through the entire Indian slab detachment process in time and space,providing an excellent window for studying this issue.In this study,we focues on the development process of the above two types of post-collisional tectonics through an integrated study of 2D thermo-mechanical modeling and low-temperature thermochronology.Based on these researches,the applicability of the slab detachment is evaluated,and the geodynamic mechanism for post-collisional evolution of the Tibetan Plateau is further explored.The following conclusions are drawn:(1)2D thermo-mechanical modeling results suggest that the detachment of the Indian slab plays a decisive role in the uplift of the northern margin of the Tibetan Plateau.Although the uplift of these structural zones depends to a certain extent on the nature of the terranes before the India-Eurasian collision,it is more controlled by the slab detachment process.In the northwestern margin of the plateau,structural zones are characterized by the diachronous rise process.Slab detachment caused the boundary stress to transfer northward,and the stress was preferentially concentrated in the hotter Tian Shan lithosphere,which leads to an earlier uplift of the Tian Shan than the West Kunlun.In the northeastern margin of the plateau,structural zones are characterized by the two-phase rise process.When the Qilian Shan has a sufficiently high Moho temperature,the boundary stress can be quickly transferred northward,leading to an instantaneous uplift of the northern margin of the plateau,but the late-phase uplift with a larger amount of uplift is controlled by the slab detachment.Therefore,no matter how high the Moho temperature of the structural zone is,only after the slab detachment can the convergent force of the Indian-Eurasian collision be released on a large scale.This will cause the stress in the overlying slab to increase rapidly and transfer northward,which in turn leads to the uplift of the northern margin of the plateau.(2)The east-west extension of the Tibetan Plateau is mainly accommodated by a series of north-trending rifts.Among them,the activity history of the Cona rift in the easternmost side of the plateau is crucial to reveal the the spatiotemporal pattern of all rifts.The low-temperature thermochronological results suggest that the Cona rift initiated at ?3.0–2.3 Ma,and is the youngest rift system in the Himalayan-Tibetan orogen.Results from this study and previous research reveal a monotonic eastward development of rifting,which is consistent with the detachment process of the Indian slab from west to east.Therefore,it can rule out the possibility of the rift systems with regional coeval onset and a bidirectional center-ward younging trend.On this basis,combined with post-collisional magmatism and geophysical observations,it is inferred that the geodynamic mechanism of the east-west extension is controlled by the gravitational potential energy gradient driven by the Indian slab detachment and the resulting eastward lithospheric flow.(3)Analysis of the relationship between the structural zones in the northern margin of the Tibetan Plateau and the east-west extensional structures inside the Tibetan Plateau and the Indian slab detachment,this study constructs a geodynamic model for post-collisional tectonic evolution of the Tibetan Plateau,as the following:(a)the subducting Indian slab started to detach laterally from the west since ?25 Ma.On one hand,the boundary stress in the overlying slab was transferred northward and preferentially concentrated in the hotter Tian Shan lithosphere,causing an earlier uplift of the Tian Shan(?26–24 Ma)than the West Kunlun(?23–20 Ma)in the northwestern margin of the plateau.On the other hand,topographic rise above the slab segment released by detachment built up an eastward propagation of gravitational potential energy gradient,driving eastward flow of lithosphere and inducing the development of east-west extensional structures(i.e.,the initiation of the Leo Pargil rift at ?23 Ma);(b)as the slab detachment propagated eastward,the East Kunlun-Qilian Shan began to uplift on a large scale(?20–10 Ma)in the northeastern margin of the plateau,and the east-west extensional structures were developed eastward;(c)the Indian slab was completely detached at ?10 Ma,leading to the continuous activity of the above postcollisional structures.