HR: 0800h
AN: T31D-0669    [Abstracts]
TI: Vorticity Studies in Mabja Dome, southern Tibet: Late Eocene-Middle Miocene Ductile Flow in the Middle Crust
AU: * Langille, J
EM: langille@geology.cwu.edu
AF: Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926,
AU: Lee, J
EM: jeff@geology.cwu.edu
AF: Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926,
AU: Seward, G
EM: seward@geol.ucsb.edu
AF: Department of Earth Science, University of California, Webb Hall, Santa Barbara, CA 93106,
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: Department of Earth Science, University of California, Webb Hall, Santa Barbara, CA 93106,
AB: Channel flow in the middle crust of southern Tibet is predicted to be top-north simple shear at the top of the channel grading into pure shear toward the center and then into top-south simple shear at the bottom. Channel flow is driven by a low-viscosity middle crust, a pressure gradient between Tibet and India, and surface denudation along the southern flank of the high Himalaya. Mabja Dome, southern Tibet, exposes high-grade (chloritoid-zone to incipient migmatites) middle crustal rocks proposed to have originated from this mid-crustal channel. To test this hypothesis we completed kinematic, vorticity and quartz LPO measurements across a 28 km-long transect from chloritoid-grade rocks (metamorphic depths of 1.5-4.5 kb) to incipient migmatites (metamorphic depths of > 8 kb). Outcrop and thin section kinematic indicators show a downward progression from top-north and top-south shear in chloritoid-zone rocks, dominantly top-south shear in garnet-zone rocks, and solely top-south shear in kyanite-zone and deeper rocks. Rigid porphyroblasts such as chloritoid, garnet, and tourmaline, and quartz grain-shape foliation were used to calculate mean kinematic vorticity numbers (Wm). Wm in schists and orthogneisses varies from 0.71-0.82 (c. 50-38% pure shear) in chloritoid and garnet-zone rocks to 0.52-0.69 (c. 64-51% pure shear) in kyanite-zone and deeper rocks. Quartzites yield Wm that increase downward from 0.90 to 0.99 (c. 29-1% pure shear). A combination of mineral assemblages, quartz microstructures, and preliminary EBSD-generated quartz crystal preferred orientations indicate that deformation temperatures during flow were the same as recorded metamorphic temperatures: ~475°C in chloritoid-zone rocks to ~625° C in kyanite-zone rocks to ~700°C in sillimanite-zone rocks. These data indicate that the vorticity record in these rocks formed during peak metamorphism, implying that these fabrics formed in the middle crust during the late Eocene to middle Miocene. In contrast to channel-flow models, the Mabja Dome shows a more complex flow regime involving (a) an increase in the pure shear component with depth in schists and orthogneisses that may be the consequence of an increasing lithostatic load; (b) a higher simple shear component in quartzites, the mechanically weakest layers; (c) mixed top-north and top-south shear at the highest structural levels that may reflect temporal/spatial variations in flow direction as a result of changes in viscosity.
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting