HR: 16:00h
AN: V42G-01 INVITED     [PDF]
TI: Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet
AU: * Wallis, S R
EM: swallis@eps.nagoya-u.ac.jp
AF: Nagoya University, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-8602 Japan
AU: Tsujimori, T
EM: tatsukix@rins.ous.ac.jp
AF: Okayama University of Science, Research Institute of Natural Sciences, Okayama University of Science, Okayama, 700-0005 Japan
AU: Aoya, M
EM: f030402r@mbox.nagoya-u.ac.jp
AF: Nagoya University, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-8602 Japan
AU: Kawakami, T
EM: tetsuo@cc.okayama-u.ac.jp
AF: Okayama University, Department of Earth Sciences, Faculty of Education, Okayama University, Okayama, 700-8530 Japan
AU: Terada, K
EM: terada@letitbe.geol.sci.hiroshima-u.ac.jp
AF: Hiroshima University, Department of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, 739-8526 Japan
AU: Suzuki, K
EM: suzuki@nendai.nagoya-u.ac.jp
AF: Nagoya University, Nagoya Univeristy Center for Chronological Research, Nagoya University, Nagoya, 464-8602 Japan
AU: Hyodo, H
EM: hhyodo@rins.ous.ac.jp
AF: Okayama University of Science, Research Institute of Natural Sciences, Okayama University of Science, Okayama, 700-0005 Japan
AB: The Tibetan Plateau is characterized by both its great height (much of it above 5,000 m) and flatness. It has been proposed that the flat topography is due to ductile flow of highly mobile lower crustal rocks. This idea has lead to a radical new idea in continental tectonics that horizontal outflow of mobile lower-crustal rock from topographic highs is one of the principal mechanisms by which plateaus grow laterally. It has been proposed that the large-scale topography of the whole of the SE Asian continent has been strongly influenced by this process. The best case for such a mechanism is made in the eastern border of Tibet in the Longmenshan orogen. The Longmenshan region lies on the western fringe of the S-China craton and has been involved in two stages of orogenesis: one event at 200 Ma associated with the collision between the N- and S-China cratons and a second, the Cenozoic Himalayan orogeny related to the collision between India and Asia. However, a number of studies have emphasized that the upper crustal deformation in the Longmenshan is almost entirely Mesozoic and, therefore, unrelated to the India-Asia collision, which began around 54 Ma. This discrepancy can be explained if the crustal thickening in the Longmenshan was caused by solid-state inflow of mobile lower-crustal metamorphic rocks from a topographic high. New zircon U-Pb and mica $^{40}$Ar/$^{39}$Ar dating combined with structural studies confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. The deepest exposed parts of the orogen have undergone Barrovian facies metamorphism with estimated peak conditions around 8 kbar and $725\deg$C (Huang et al, 2003) and associated deformation generally thought to have occurred around 200 Ma. However, new apatite U-Pb and monazite electron microprobe dating in the sillimanite zone give ages around 65 Ma. Metamorphic minerals occur both surrounding and as inclusions of the dated monazite suggesting 65 Ma is the age of metamorphism. The main phase of syn-metamorphic deformation is overprinted by a phase of folding associated with N-S fold axes and steep axial planes with geometries that indicate an E-W shortening of 10 - 50%. The evidence for originally thick crust existing around the time of the India-Asia collision and the presence of moderate amounts of Cenozoic E-W shortening can account for most if not all of the present thickness of the continental crust in the Longmenshan area. These results do not, therefore, support models of crustal thickening in the region caused by solid-state lateral flow of mid-crustal metamorphic rocks. Reference: Huang, M.-H., Buick, I. S. \& Hou, W. 2003. Tectonometamorphic evolution of the Eastern Tibet plateau: evidence from the central Sogpan-Garze orogenic belt, western China. Journal of Petrology, 44 255-278
DE: 8102 Continental contractional orogenic belts
DE: 9320 Asia
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting