Deformational Mechanism Of Crustal Ductile Channel Flow: Sandbox Modeling Of Shallow Crustal Thickening In The Eastern Tibetan Plateau

The crustal thickness of the Tibetan plateau is about70km, which is oneinternational hot scientific issue in geosciences regarding on its crustal thickening anduplift mechanism. Faulting and overthrusting were used to explain its process of thecrustal thickening and uplift firstly. However, this opinion is incompletely inaccordance with the detachment characteristics at depth reflected by the deep seismicsounding, magnetotelluric imaging data and some other geophysical data.Subsequently, middle-lower crustal ductile channel flow (CDCF) models were putforward and discussed widely. These models or hypotheses hold the opinion thatcentral Tibetan plateau materials at depth flow to the plateau margins, which had beendemonstrated by geologic and geophysical data on a geological timescale. Due to theaccumulation, inflation of the CDCF in the middle-lower crust, competent crustallayers above the CDCF were thickened and ruptured by thrusting, triggering thedevastating12May Wenchuan earthquake and its aftershocks in the eastern margin ofthe plateau probably. But, results of geophysical data can only demonstrate theexistence of CDCF layers in the Tibetan plateau. How these CDCF layers flow atdepth and cause the crustal thickening and uplift are unable to get answers from theexisting observations. To take the eastern Tibetan plateau as an example, we havesimulated the two-stage rapid shallow crustal thickening and uplift process of thestudy area based on sandbox modeling experiments, taking advantage of the geodetic,magnetotelluric imaging and deep seismic sounding data.Comprehensive structural analyses indicate that the eastern Tibetan plateau hadexperienced two rapid uplift and exhumation. The earlier stage (S1) crustal thickeningand uplift occurred in the Oligocene, about30~25Ma ago; the latter stage (S2) beganin the Late Miocene, about15~10Ma ago, which continued to present.Sandbox modeling results suggest that:(1) The contribution percentage of directthickening amount to the total shallow crustal thickening that caused by faulting andoverthrusting is about22.7~30.3%at the first stage;(2) The contribution percentagesof CDCF layer to the total shallow crustal thickening at the second stage in Songpan-Ganzi plateau, Longmen Shan hinterland belt and foreland belt are <1.0%,2.9~5.9%and <1.0%, respectively. As for faulting and overthrusting, the contributionpercentages are>99.0%,94.1~97.1%and>99.0%, respectively;(3) Thrust nappestructure resulting in the crustal thickening behaves piggyback propagation above theCDCF layer and a thrust duplex is produced at the bottom of the sandbox model;(4)The direct contribution of CDCF layer to the shallow crustal thickening and uplift ofeastern Tibetan plateau is very little. However, CDCF layer may cause the existenceof the large-magnitude ductile detachment fault by the form of ductile shear flow,playing a key controlling role in the development of multiple overthrusting structures,which has resulted in the tectonic decoupling between the middle and upper crusts.