Middle crustal ductile deformation patterns in southern Tibet: Insights from vorticity studies in Mabja Dome

Abstract

Mabja Dome, southern Tibet, exposes mid-crustal rocks proposed to have originated from a southward flowing mid-crustal channel. Kinematic, mean kinematic vorticity (W m), and metamorphic petrography analyses on these mid-crustal rocks were performed to test this hypothesis. Kinematic indicators show a transition with structural depth from top-north and top-south shear to solely top-south shear. Along the northernmost transects, W m in schists and orthogneisses range from 0.52-0.84 (63-36% pure shear). W m for quartzites ranges from 0.9-0.99 (27-1% pure shear). Deformation temperatures increase from ~450 °C in the chloritoid-zone to ~700 °C in the sillimanitezone and were recorded between ~35-16 Ma. These patterns exhibit a complex flow regime characterized by: (1) opposing shear sense driven by heterogeneous viscosity and/or channel thickness, (2) broad top-south shear along the Main Central Thrust, (3) simple shear partitioned into weaker quartzite horizons, and (4) an increase in lithostatic load with depth. iv ACKNOWLEDGMENTS I received assistance from many helpful people and organizations while working on this project. Funding was provided by generous grants from the Geological Society of America, CWU Department of Graduate Studies, and Sigma Xi. Additional funding for travel to conferences in order to present this work was provided by the CWU Department of Geological Sciences, the CWU Department of Graduate Studies, and Jeffrey Lee. I wish to thank my committee members Wendy Bohrson and Charlie Rubin for their input and support of my research. Of course, I must thank my advisor Jeffrey Lee for his exceptional guidance and support while working on this project. Thanks to Bradley Hacker and Gareth Seward for their collaboration and the use of their facilities at the

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