HR: 17:30h
AN: T34C-08    [Abstracts]
TI: Did the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya?
AU: * Leech, M L
EM: leech@sfsu.edu
AF: San Francisco State University, Department of Geosciences, 1600 Holloway Avenue, San Francisco, CA 94132, United States
AB: There is a marked change in the volume and age of granitoids from west to east across the Himalaya; that change occurs at the southeastern termination of the Karakoram fault where it merges with the Indus-Yarlung suture zone, near the Gurla Mandhata gneiss dome. These granitoids are derived from a ductile mid-crustal channel formed when anatectic melts from the mid-crust beneath the Tibetan plateau were driven south by erosion at the Himalayan topographic front. The "channel flow" model predicts upwellings of these granites within the Tethyan Himalaya Sequence as part of a chain of gneiss domes and exposure of the channel at its southern termination in the Greater Himalaya Sequence (GHS) as evidenced by widespread migmatites and leucogranite bodies at the top of the GHS in the footwall of the South Tibetan Detachment. Leo Pargil is the westernmost gneiss dome in a chain of domes formed within the Tethyan Himalaya Sequence that extends eastward 1600 km through the North Himalayan gneiss domes (that includes the better known Kangmar dome). New U-Pb SHRIMP dating of zircons show leucogranite bodies from the Leo Pargil gneiss dome are Early Miocene (22-20 Ma) corresponding to some of the older granites dated from across the Himalaya. Transects through the GHS along the Beas and Sutlej River valleys in the western Himalaya near Leo Pargil reveal rare to minimal amounts of migmatites and leucogranite, whereas there are abundant migmatites and large leucogranite bodies in a transect along the Friendship Highway from Lhasa to Kathmandu in the area of the North Himalayan gneiss domes. The timing of the initiation of slip on the Karakoram fault is dated from syn- kinematic rocks in the shear zone at Tangste/Pangong Tso at c. 25-21 Ma. If the Karakoram fault acted as a barrier the flow of granitoid melts in a mid-crustal channel, one would expect more abundant and younger granites east of the Karakoram fault as the channel continued to flow south after c. 20 Ma; the youngest granites reported from the eastern Himalaya are 7 Ma from the Renbu dome and there are also abundant leucogranite bodies from 20-12 Ma. This model also predicts that granitoids would leak up the Karakoram fault and this is demonstrated by 20-16 Ma leucogranites along the Karakoram fault in the Tangste/Pangong Tso area. If the Karakoram fault is indeed a barrier to ductile flow and melt migration, then future tectonic models for the Himalaya should recognize the differences between the western and eastern segments of the orogen, rather than emphasizing along-strike uniformity.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1115 Radioisotope geochronology
DE: 5104 Fracture and flow
DE: 8104 Continental margins: convergent
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting