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

Kinematic, kinematic vorticity (Wm), and deformation-temperature analyses were performed to test the hypothesis that mid-crustal rocks exposed in Mabja Dome, southern Tibet, were penetratively deformed within a southward-flowing mid-crustal channel during the late Eocene/early Oligocene to early Miocene. Outcrop and thin-section kinematic indicators show a downward transition from mixed top-N and top-S shear in chloritoid- and garnet-zone rocks, through dominantly top-S shear in garnet- and kyanite-zone rocks, to solely top-S shear in staurolite-zone and deeper rocks. Along mineral elongation lineation-parallel transects, Wm in schists and orthogneisses decreases with structural depth from ∼0.80 (∼40% pure shear) to ∼0.55 (∼63% pure shear). Deformation temperature increases from ∼450 °C in the chloritoid-zone to >700 °C in the sillimanite-zone, coincident with peak metamorphic temperatures, indicating that Wm was recorded during peak metamorphism. These mid-crustal rocks thus exhibit deformational patterns characterized by: (1) locally opposing shear sense suggesting bulk pure shear at moderate structural depths; (2) a broad top-S shear zone above the Main Central Thrust; and (3) increasing pure shear with structural depth, suggesting an increase in lithostatic load. Our results from mid-crustal rocks exposed in the core of Mabja Dome yield patterns of ductile deformation in southern Tibet that define non-ideal channel flow.