HR: 15:25h
AN: T33D-08    [Abstracts]
TI: Exhumation and Plateau Growth in Northeastern Tibet
AU: * Clark, M K
EM: mclark@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences Mailcode 100-23, Pasadena, CA 91125 United States
AU: Farley, K
EM: farley@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences Mailcode 100-23, Pasadena, CA 91125 United States
AU: Zheng, D
EM: zhengdewen2002@yahoo.com.cn
AF: Institute of Geology, China Seismological Bureau, Beijing, 100029 China
AB: The lateral expansion of high topography in northern and eastern Tibet is important to both our understanding of the mechanics of plateau growth and tectonic/climate interactions related to the aridification of central Asia and the intensification of the east Asian and Indian monsoons. In particular, the initiation of basin sedimentation in northern Tibet has been used as a proxy for the development of high mean elevations in northern Tibet. However, poorly constrained thermal histories in adjacent fault bounded ranges and lack of provenance data from basin deposits make linkages between orogenesis, exhumation/sedimentation and climate tentative. Preliminary apatite (U-Th)/He ages from northeastern Tibet are Middle Miocene (7-12 Ma) at low structural positions in the West Qinling and Lajieshan ranges and early Cenozoic to Mesozoic (40-200 Ma) at progressively higher structural positions and within the Yellow River gorge. These data suggest initiation of fault activity by middle Miocene time, but that overall exhumation is limited to a few kilometers and occurs proximal to principle structures. Geomorphic surfaces and Oligocene helium ages (30 Ma) at low structural positions on the Eastern Kunlun Range (Qaidam Fault) suggest minimal amounts of shortening on steep faults along the Qaidam Basin plateau margin despite having a steep topographic escarpment and a major step in the Moho at depth. The Nd isotopic record from Linxia Basin sediments suggests that early mudstone deposition (since 29 Ma) was dominated by loess and that significant unroofing of plutonic and metasedimentary source rocks in the adjacent West Qinling range did not begin until $\sim$14 Ma, coeval with \delta$^{18}$O data suggesting increased aridity at this time (Garzione et al., this session). These data suggest that loess deposition and flexural loading began 15 Ma prior to the major erosional episode that exhumed the W. Qinling range. Differences in ages between initial basin sedimentation and major exhumation of adjacent fault bounded ranges suggested by Garzione et al., and limited erosional exhumation of these structures indicated by our helium data, suggests that the timing of initial basin formation in northeastern Tibet need not be a primary indicator of high topography. Low surface strain and a relatively arid climate could effectively limit any feedback mechanisms between erosion and orogenic growth- a principle difference between northern and southern Tibet despite similarities in their extreme mean elevation and regional topographic gradients.
DE: 9310 Antarctica
DE: 8102 Continental contractional orogenic belts
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting