HR: 0800h
AN: H31A-1280    [Abstracts]
TI: Late Oligocene initiation of shortening in the Southwestern Chinese Tian Shan: Implications for Neogene shortening rate variations
AU: * Sobel, E R
EM: sobel@rz.uni-potsdam.de
AF: Universitaet Potsdam, Institut f. Geowissenschaften, Potsdam, 14415 Germany
AU: Chen, J
EM: geochron@ies.ac.cn
AF: China Earthquake Administration, State Key Laboratory of Earthquake Dynamics, Institute of Geology, Beijing, 100029 China
AU: Heermance, R
EM: richard@crustal.ucsb.edu
AF: University of California, Dept. of Geological Sciences, Santa Barbara, CA 93106 United States
AB: The Cenozoic Tian Shan is the preeminent example of an intracontinental orogen. However, there remains a significant controversy over when deformation related to the India-Asia collision commenced and therefore how shortening within the mountains has been partitioned over time. One approach has been to look at the modern shortening rate as measured by geodetic studies and estimates of the total shortening across the range and extrapolate backwards. This approach suggests that the onset of range construction was ca. 10 Ma (Abrakhmatov et al., 1996). An alternate method is to determine the age of the onset of exhumation using thermochronology. We present 19 new apatite fission-track (AFT) results from the southwestern Chinese portion of the belt; this region represents the first area exhumed along a transect at ca. 76°E. Exhumation commenced at the Oligocene-Miocene boundary along the Maidan and Muziduke thrusts, which bound the southern side of the Kokshaal range. Subsequently, deformation propagated south to the Kashi basin-bounding thrust (KBT), which was exhumed by no earlier than 18.9 +/-3.3 Ma. Three detrital AFT samples from strata deposited ca. 20 km farther south during the past 3 My contain FTGA peaks that young monotonically upwards from 20.9 +7.0/-5.3 Ma to 15.9 +5.4/-4.0 Ma with a fairly constant lag time of 16 to 18 Ma. These ages, combined with structural and sedimentary petrographic data, suggest that both the hanging- and the footwall of the KBT experienced a renewed episode of exhumation during the latest Cenozoic. The discrepancy between the Late Oligocene - Miocene initiation of significant exhumation shown herein and the 10 Ma initiation estimate from geodesy suggests that the Tian Shan has undergone a complex Late Cenozoic shortening history. Assuming that the present shortening rate could account for the total amount of Cenozoic shortening in 10 Ma and realizing that shortening initiated at least 15 Myr earlier, we conclude that the shortening rate must have increased over the last 10 Myr, although the exact timing is uncertain.
DE: 1140 Thermochronology
DE: 1815 Erosion
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005