Magnetostratigraphy of Cenozoic Sediments From the Kuche Depression, Tarim of NW China and Tectonic Significance

A combined paleomagnetic and magnetostratigraphic study is reported of Cenozoic sediments, comprising of a total of 991 red bed horizons within the upper part of the Kumugeliemu Formation and the lower part of the Kuche Formation of the Kuche Depression in the Tarim Basin, northwest China. Thermal demagnetization is used to separate the magnetic components. The characteristic remanent magnetization with both normal and reversed antipodal directions is then isolated generally between 300 and 690° C and a total of 969 data were obtained for construction of a composite magnetostratigraphic sequence. The composite magnetostratigraphic sequence correlates with the C12n to the C3r, between ∼31.0 and 5.8 Myr, of the geomagnetic polarity timescale of Cande and Kent (1995). Results from correlation suggest that the boundaries of Kuche/Kangcun Formations, Kangcun/Jidike/ Formations, Jiedike/Suweiyi Formations, and Suweiyi/Kumugeliemu Formation are at about 5.9, 13.5, 26.0, and 29.0 Myr respectively. We argue that the Paleogene/Neogene boundary is most likely to be at the bottom of the Jidike Formation in the Kuche Depression and the dramatic transition from the marine/lacustrine to fluvial/alluvial facies in the Suweiyi Formation and the lower Jidike Formation, which may indicate the initiation of the Cenozoic thrusting in the Kuche Depression (Yin et al., 1998), is probably occurred in the late Oligocene, at about 28-25 Myr. The mean characteristic remanent magnetization directions every 5 Myr suggest that the Kuche Depression was subjected to a counterclockwise vertical-axis rotation at a rate of ∼1°/Myr during about 31.0-5.8 Myr, and a clockwise rotation at a rate of ∼2°/Myr during the past ∼5.8 Myr. This study thus suggests that at least two rotations of the Kuche Depression relative to the Earth rotation axis have occurred during the past 31 Myr.