Middle Crustal Ductile Deformation Patterns in Southern Tibet: Insights from Vorticity Studies in Mabja Dome

Jackie Langille
June 2008

Abstract

Mabja Dome, southern Tibet, exposes mid-crustal rocks proposed to have originated from a southward flowing mid-crustal channel. Kinematic, mean kinematic vorticity (Wm), 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, Wm in schists and orthogneisses range from 0.52–0.84 (63–36% pure shear). Wm 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 sillimanite- zone 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.

Full Thesis (PDF format)

Update: Jackie was awarded the University's Distinguished MS Thesis award for 2008-2009. Her thesis was also accepted for publication in the Journal of Structural Geology [download PDF file].