HR: 0800h
AN: T31D-0678    [Abstracts]
TI: Deformation History of the Ayishan, Western Tibet
AU: * Zhang, R
EM: rzhang8@mail.uh.edu
AF: University of Houston, Department of Geosciences, Houston, TX 77204-5007, United States
AU: Murphy, M A
EM: Michael.Murphy1@mail.uh.edu
AF: University of Houston, Department of Geosciences, Houston, TX 77204-5007, United States
AB: Geologic mapping and structural analysis of the Ayi shan in western Tibet was conducted in order to better understand the deformation history along the Karakoram fault. Three regional-scale faults were mapped along the length of the Ayi shan for a distance of approximately 200 km. From oldest to youngest they are; the Great Counter thrust, Ayishan detachment, and the Karakoram fault. The Ayishan detachment lies between the Karakoram fault on its east and the Great Counter thrust on its west. The detachment mantles two large doubly plunging gneiss domes that are elongated towards the NW-SE. Our results show that the detachment consists of low-angle top-to-SE extensional faults that overlie a thick (~1500 m) top-to-the SE shear zone which juxtaposes Cretaceous granitoids in its hanging wall against mylonitic rocks in its footwall. Both the faults and underlying shear zone are broadly folded about an axis parallel to the strike of the range. Shear zone fabrics display ductilely deformed feldspar porphyroclasts indicating their formation at mid-crustal depths. The shear zone passes structurally downwards into weakly foliated granitoids. Mineral stretching lineations in the footwall rocks indicate the mean slip direction along the Ayishan detachment is S60&°&E. We correlate the hanging wall granitoids with the Gangdese batholith based on its composition and texture. In the northern Ayi shan the footwall of the detachment consists of mylonitic gneisses, biotite schist and migmatite in the core. Leucogranite sills are pervasive in the footwall above the migmatite. In the central Ayi shan, the footwall of the detachment correlates to that exposed in the northern range with the exception that migmatite is not exposed and leucogranite sills are less common. On the east side of the Ayi shan the detachment merges and shares similar kinematics with the Karakoram fault. Moreover, the detachment forms the master fault of the Neogene Gar valley releasing bend basin. On the west side of the range the detachment dips to the southwest structurally beneath the Great Counter thrust. Field relations suggest that locally the Great Counter thrust and Ayishan detachment merge towards the west, placing rocks we correlate with the Gangdese batholith beneath the Tethyan sedimentary sequence as well as ophiolitic rocks within the Indus-Yalu suture zone. In order to explain these field relations we suggest the Ayishan detachment had an early phase of slip that together with the Great Counter thrust facilitated southwest emplacement of Gangdese batholithic rocks beneath the Tethyan sedimentary sequence. In this scenario the Ayishan detachment operates as a passive roof thrust. This model predicts the presence of a SW- directed thrust, possibly the Gangdese thrust, at structurally deeper levels. Subsequent development of the Karakoram fault in the Middle to Late Miocene reactivated the detachment and facilitated exhumation of the footwall rocks. This two-phase deformation history implies significantly more crustal thickening along the Indus- Yalu suture zone than has been previously estimated and indicates that rocks with an Asian affinity lie, at least locally, southwest of and beneath the surface trace of the suture zone.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8038 Regional crustal structure
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting