HR: 1340h
AN: T23C-0570    [Abstracts]
TI: Magnetic polarity stratigraphy of Fenghuoshan and Wandouhu basins: an indirect tool for constraining the Tertiary uplift history of northern Tibet
AU: * Yi, H
EM: yhs@cdut.edu.cn
AF: Chengdu University of Technology, Institute of Sedimentary Geology, Chengdu, 610059 China
AU: Zhao, X
EM: xzhao@pmc.ucsc.edu
AF: University of California, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Liu, Z
EM: lzhifei@online.sh.cn
AF: Tongji University, Laboratory of Marine Geology, Shanghai, 200092 China
AU: Wang, C
EM: chshwang@cugb.edu.cn
AF: China Univesity of Geosciences, 29 Xueyuan Road, Beijing, 100083 China
AU: Deng, B
EM: db@cdut.edu.cn
AF: Chengdu University of Technology, Institute of Sedimentary Geology, Chengdu, 610059 China
AU: Lin, J
EM: linjinh673@yahoo.com.cn
AF: Chengdu University of Technology, Institute of Sedimentary Geology, Chengdu, 610059 China
AB: Rock uplift can be revealed by stratigraphic and chronologic studies of erosional sediments and their accumulation rates. Magnetic polarity stratigraphy (magnetostratigraphy) is a powerful tool for precise temporal correlation and accurate dating of sediments. Magnetostratigraphy is also subject to limitations. For example, it is first necessary to demonstrate that the remanent magnetization of the sediment is a stable primary magnetization acquired at the time of deposition. Second, the pattern of magnetic reversals recorded in the particular sedimentary sequence depends on both reversal frequency and sedimentation rate. In this poster, we report our work on rocks from the Fenghuoshan and Wandouhu basins, northern Tibet, to provide a proxy example based on stratigraphic approach for paleoelevation study. We collected individual oriented paleomagnetic samples (spaced at stratigraphic intervals) from seven measured sections of the basins to determine a chronostratigraphic framework for the sediments. These sections exhibit well-exposed outcrops of red mudstone, sandstone, and conglomerate. Progressive thermal demagnetization to 700°C revealed a very high unblocking temperature component (HTC) between 625 and 700°C in almost all red sandstone samples. The HTC has both normal and reversed polarities and is interpreted as the characteristic remanent magnetization (ChRM) on the basis of linear trajectories of demagnetization towards the origin and similar directions from sample to sample. The fold test results on the ChRM are positive, leading us to interpret the ChRM as primary. Several fossils and radiometric dates were obtained from parts of these two basins. We have used these few age constraints and the spacing of the observed polarity intervals to attempt to anchor our polarity column to the geomagnetic polarity timescale. Sedimentation rates derived from the magnetostratigraphy suggest that a sharp increase in deposition is recorded near 40 Ma, corresponding to the appearance of the conglomerate in the stratigraphic section. Because this increase in sedimentation rate coincides with the onset of titling of the redbeds and shift in paleo-current directions and with the appearance of the conglomerate (especially a coarsening-upward sequence of fan-delta facies) at ~41 Ma, our working hypothesis is that this speed up in sedimentation indicates significant uplift of northern Tibet during the middle Eocene.
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8107 Continental neotectonics (8002)
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005