HR: 14:30h
AN: T33F-04    [Abstracts]
TI: Pulsed Episodes of Shortening Within the Tian Shan Foreland: Implications for Deformation Rate Interpretations Throughout Central Asia
AU: * Heermance, R V
EM: rheermance@usgs.gov
AF: United States Geologic Survey, 520 N. Park Ave, Tucson, AZ 85719, United States
AU: Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106, United States
AU: Chen, J
EM: chenjie@eq-igl.ac.cn
AF: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, 100029, China
AB: Unsteadiness in short-term deformation, such as earthquake clustering or development of new faults in lieu of deformation on older structures, poses problems when trying to develop an appropriate context (e.g. regional tectonic rates and active structures) for assessing seismic risk. Here we show that a similar unsteadiness can be found on an orogen scale at million-year time scales, and suggest spatial heterogeneity of deformation rates are a fundamental part of mountain-building in central Asia. The Tian Shan are an intracontinental mountain range that formed north of the Himalayan orogen but in response to the ongoing Indo-Asian collision. Although continuous since the early Miocene, deformation has occurred in an erratic sequence that suggests deformation rates are not constant within the foreland. In order to reconstruct a detailed history of foreland deformation bounding the southern Tian Shan in western China, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission-track dating, and changes in sediment-accumulation rates from the >6500 km² Kashi basin. Three cross-sections spaced along a 55-km-wide, E-W section of the range-front document variable shortening from 11 to 31 km since the initiation of uplift, of which 7-12 km occurred since ~4 Ma. Both overall shortening and total shortening rates throughout the Miocene decrease towards the east, but match the expected differences in magnitude due to 0.7°/M.y clockwise rotation of the Tarim basin around a pole at 93°E, 37°N (Thatcher, W., 2007, JGR 112: B01401). Temporal constraints on individual structures, however, document at least four distinct stages of deformation. Initial uplift (stage 1) of hinterland structures began at 20-25 Ma. Stage 2 occurred at ~16.3 Ma when the basement-involved deformation front stepped south to the Kashi Basin thrust that bounds the foredeep strata. Stage 3 occurred from ~14 to 4 Ma, as the deformation front migrated south episodically above a shallow detachment. Finally, stage 4 began at ~4 Ma with growth of the Keketamu anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. Although average shortening rates since 16.3 Ma were 0.8-1.3 mm/year, rates were faster from 16.3-13.5 Ma (1-3 mm/yr), after which they decreased to 0.5 mm/yr between 13.5 and 4 Ma before accelerating to 2-2.5 mm/yr after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially ~3-7 mm/yr for the first 10 Ma of basin development, slowed to ~1.5 mm/yr between 13.5 and 4 Ma, and increased to >10 mm/yr since 4 Ma. A mismatch exists between our geologically determined data and present-day geodetic rates: the latter are 3-4 times greater. Furthermore, changes in shortening rates over time suggest that the loci of deformation shifted in and out of the foreland, and imply either changes in tectonic rates or shifting of the locus of tectonic activity throughout the orogen during the Miocene.
DE: 1209 Tectonic deformation (6924)
DE: 1520 Magnetostratigraphy
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: 2007 Fall Meeting