GNEISS DOMES IN WESTERN TIBET: WHAT IS THEIR ROLE IN ACCOMMODATING INTRACONTINENTAL DEFORMATION?

Two contrasting views of deformation in Asia dominate the debate about how continents deform: (1) The deformation is primarily localized on major faults separating crustal blocks or (2) deformation is distributed throughout the continental lithosphere. I will be testing the application of these two models to the western Himalaya. In our research area, the major tectonic elements include the Karakoram Fault System, the Gangdese Thrust and the Ayishan detachment. The Ayishan detachment exhumes metamorphic rocks and is structurally located between the Gangdese Thrust and the Karakoram fault. Two models for the Ayishan detachment are proposed which explain the function of the Ayishan detachment in accommodating deformation associated with India-Asia collision, one is a transtension model which supports the extrusion hypothesis (model 1) and the other one is passive-roof fault model which supports the distributed crustal thickening hypothesis (model 2). For the transtension model, the Ayishan detachment is composed of discrete segments at releasing bends along the Karakoram fault and is contemporaneous with it. The branch lines between the Karakoram fault and the Ayishan detachment are located at the two ends of the bends and the Ayishan detachment shows oblique motion. In this scenario, the Ayishan detachment accommodates right-lateral shear between the Tibetan plateau and Himalaya. For the passive-roof fault model, the Ayishan detachment merges southward with the Gangdese thrust forming a branch line. In this model, the Ayishan detachment accommodates crustal thickening within the Himalaya. In addition, the Ayishan detachment is contemporaneous with the Gangdese thrust and older than the Karakoram fault. In addition, thermobarometry techniques will be used to determine metamorphic P and T conditions for the metasedimentary rocks in the lower plate of the Ayishan detachment. This data will be used to estimate the depth extent of the Ayishan detachment. The transtension model predicts the Ayishan detachment to root deeply (> 20km). The passive-roof model predicts the Ayishan detachment to root structurally high (<20km).