Study On Crust-Scale Deformation In Tectonic Transition Zone,in Qilian Shan,the Northern Margin Of The Qinghai-Tibet Plateau

The collision between Asian continent and India continent constructed the Qinghai-Tibetan Plateau which owns the highest topography and thickest continental crust all over the world.Mechanism related to the uplifting and thickening of the Tibetan Plateau remains as a charming issue confused the earth scientists.A varity of models have been proposed to understand the process of plateau uplifting and thickening,such as,(1)double crust model,(2)continued shortening model,(3)rigid block extrusion and subduction,(4)lower crust channel flow and so on.The geodynamic setting of these models is suggested as unidirectional compression induced by the northward subduction of the India continent beneath the Himalayan.During the 80s?90s,the Yadong-Ge'ermu-Ejina Geotransect was conducted and the result implied that the Tibetan Plateau was in a bidirectionally compressional geodynamic setting induced by the northward subduction of the Indian continent and southward underthrusting of the Asian continent.More and more recently discovered evidences indicate the southward underthrusting of the Asian lithospheric mantle.This maybe means that researches in the northern margin of Tibetan Plateau(Qilian Shan,"Shan means Mountain")contribute to understanding the uplift and expansion of plateau.Restricted by the precision of detection means,the high-resolution crust structure of northern margin of Tibetan Plateau(Qilian Shan)has not been revealed.Thus,here in our thesis,based on two high-resolution deep seismic reflection profiles in the northern Tibet margin,combined with basic geological data and previous geophysical data,we conducted detailed interpretation of these two seismic profiles,and achieved some key results as following:(1)Analysis of newly obtained seismic reflection profile in east Qilian Shan suggests that the main left-slip faults,i.e.Maxianshan,Haiyuan and Tianjing faults through the upper and middle crust.The faults likely terminate at a low-angle detachment shear zone in the lower crust.Haiyuan and Tianjing faults have two strands within the deep crust.(2)Interpretation of the both two seismic sections implys that the crust deformation was decoupled,and the inner crust decollement with different depth from west to east Qilian Shan is the decoupled boundary.(3)The inner crust decollement of west Qilian Shan coincides with the top face of low velocity layer,shallowing from south with depth at?27 km to north with depth at?18 km.Crust deformation above the decollement is featured by south-dipping,north thrusting faults which downward ended at the decollement,implying the northward out-growth of the plateau.While Crust below the decollement experienced complicated shorteng deformation marked by the complex Moho geometry.Decollement of the east Qilian Shan is located at a depth of?40 km,extended from south to north with little fluctuation.Crust above it is mainly characterized by left lateral strike slip faults compared with mainly shortening deformation in crust below the decollement.(4)Look forward from tip(3),our analysis of seismic profiles suggests that the whole crust of tectonic transitional zone in northern plateau margin underwent strong shortening deformation.Our evidence obviously conflicts with previous popular models related to uplifting and thinkening of northern plateau margin,i.e.,(a)thin-skinned tectonics model with deformation occurred just at upper crust,(b)middle-lower crust thickening model with thickening only at middle-lower crust.(5)Bed-length balancing of the seismic profile in west Qilian Shan indicates that the upper crust shortening strain may reach at least 38.0%,and the lower crust shortening strain may reach 43.9%during the Cenozoic time.This two shortening strain budgets of upper crust and lower crust suggest that the upper and lower crust of the west Qilian Shan underwent deformation in same magnitude.Based on the shortening strain,we also estimate the initial crust thickness with 31.5?34.9 km pre-Ceonozic time.The shortening magnitude is sufficient to explain the current elevation of the region without an appeal for additional contributing factors such as channel flow and/or a thermal event in the upper mantle.The east Qilian Shan is out of balanced cross section rules,so we could not estimate the correct magnitude like the west Qilian Shan.