HR: 1340h
AN: T53A-0472    [Abstracts]
TI: Late Cenozoic Conglomerate Progradation in the Southwestern Chinese Tian Shan: Tectonic, Climate or Erosion Control?
AU: * Chen, J
EM: chenjie@eq-igl.ac.cn
AF: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Seismological Bureau, P.O. Box 9803, Beijing, 100029 China
AU: Heermance, R V
EM: richard@crustal.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106 United States
AU: Burbank, D
EM: burbank@crustal.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106 United States
AB: The southwestern Chinese Tian Shan of Central Asia is an actively deforming part of the Indian-Asian collision with its associated foreland basin along the northern margin of the Tarim Platform. The Xiyu Formation, an upper Cenozoic massive conglomerate, is widely distributed along the margins of the Tian Shan. The commonly assigned age of the Xiyu Formation is based on lithostratigraphic correlations from the adjoining region (e.g. conglomerate = Xiyu Formation = Pleistocene) and the relationships of such facies to changes in hinterland tectonism, climate, or erosion rates are often unclear. As a result, geologists have assumed that the conglomerate exposed along the margins of the Tian Shan represents synchronous deposition and indicates a late Plio-Pleistocene initiation of deformation (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). However, our data have shown the large-scale stratigraphy of southwestern Chinese Tian Shan foreland basin is punctuated by major episodes of gravel progradation and any claim for synchronous deposition of the upper Cenozoic conglomerate is erroneous. Four units of upper Cenozoic conglomerates could be identified after detailed field mapping. The northernmost conglomerate lies in the piggyback basin atop the Tuotergongbaizi thrust fault and has experienced only slight tilting and folding. The lowest unit, the basal conglomerate (1-25 m-thick), consists of pearl-like, well-rounded pebbles (dominated by quartz and cherts which are reworked from underlying Cretaceous rocks) and lies unconformably above Cretaceous alluvial strata, giving a definitive marker for initiation of basin deposition. The third conglomerate is a thick (2-3 km?), but highly confined, middle-to-upper Miocene fault-proximal syntectonic facies. These strata lie on the southern side of the present basin-bounding Tuotergongbaizi fault and comprise poorly sorted, debris-flow facies near the fault, which implies the development of significant erosional topography along the Tuotergongbaizi fault prior to 5.2 Ma. About 10 km south of the Tuotergongbaizi fault, the fourth coarse unit consists of a well-sorted, thick (2-3 km ?) fluvial conglomerate and forms a wedge that progrades both ~20-30 km southward and ~100 km eastward where the conglomerate pinches out into the distal silt and sand deposits in the Tarim Basin. Our preliminary magnetostratigraphic studies show that, these coarse strata have great temporal and spatial variability. Although this thick conglomerate is as old as 5.2 Ma in the north, age of the basal conglomerate varies by 1-4 My across distances of 15-30 km. The facies geometry is controlled by the interference between major southward-flowing transverse rivers and a major eastward-flowing axial river: the Kezile Su. This time-transgressive conglomerate wedge was initially formed during active faulting. Continued southward-migrating deformation caused the conglomerate to prograde over the more distal basin deposits. Although initially syntectonic in nature, conglomerate progradation may be responding to both tectonics and climate change since onset of global glaciation.
DE: 8005 Folds and folding
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting