HR: 11:50h
AN: T22C-07    [Abstracts]
TI: New Age-Constraints on Syn-Tectonic Stratigraphy and Basin Evolution in the Southwestern Chinese Tian Shan Foreland
AU: * Heermance, R V
EM: richard@crustal.ucsb.edu
AF: Department of Earth Sciences, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106-9636 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
AU: Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: Department of Earth Sciences, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106-9636 United States
AU: Sobel, E R
EM: sobel@rz.uni-potsdam.de
AF: Institut fuer Geowissenschaften, Universitaet Potsdam, Postfach 601553, Potsdam, 14415 Germany
AB: The Kashi foreland basin in NW China contains >8 km of syn-tectonic sediments related to uplift and deformation along the southern margin of the Tian Shan during late Tertiary time. Foreland deposits consist of fine-grained, gypsiferous, lacustine beds (Wuqia Group) near the base that grade up-section into fluvial sandstone and siltstone (Atushi Fm.), with the fluvial Xiyu Conglomerates capping the basin sequence. The distinctive Xiyu Conglomerates form a wedge that thins and interfingers with silt and sand ubiquitously towards the south (distal basin) and locally east-west. The Xiyu Formation is widely distributed around the margin of the Tian Shan, and has been inferred to represent Plio-Pleistocene initiation of deformation of the Tian Shan (e.g. Huang et al., 1980), or a change in either climate (e.g. Burchfiel et al., 1999; Molnar et al., 1994) or a climate-affected erosion rate (Zhang et al, 2001). In contrast, our data show that the Xiyu conglomerates are a time-transgressive formation that has continuously prograded south since at least ~16 Ma, and likely does not represent a distinct climatic or tectonic event from one time period. Eight magnetostratigraphic sections totaling 10,000 m of vertical section through the entire basin sequence, and correlated with the Geomagnetic Polarity Timescale of Cande and Kent (1995), indicate continuous deposition between 0 and ~18 Ma. Paleocurrent data from throughout the basin are consistent with longitudinal drainages moving south as transverse (north-to-south), braided, fluvial systems encroach on the foreland. Clast size, sorting, and composition are consistent with initial unroofing of Paleozoic source rocks to the north, and subsequent re-working and erosion of this conglomerate during progradation into the foreland in concert with a migrating deformation front. Deposition rates increase from <200m/Myr in the early to middle Miocene to ~800 m/My during the Pliocene. These sedimentary data, combined with an 18+/-3 Ma fission track age from the hanging wall of the basin-bounding thrust fault (KBT), indicate that deposition in the Kashi basin began at ~18 Ma. The basin age of 18 Ma is younger than fission-track ages (ca. 25 Ma) from the major range bounding faults ~ 20 km north of the KBT. Thus, the stratigraphy and thermochronolgy support continuous and generally accelerating deposition in the Kashi foreland basin since 18 Ma related to uplift on the KBT, and a syn-tectonic origin for the time-transgressive Xiyu conglomerate. Prior to 18 Ma, the flexural foreland may have lain farther to the north.
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8169 Sedimentary basin processes
DE: 9605 Neogene
SC: Tectonophysics [T]
MN: Fall Meeting 2005