HR: 12:05h
AN: V32A-08    [Abstracts]
TI: Crustal and Upper Mantle Structure in the Eastern Northern China Craton from Seismic Array Studies and Implications for the Mesozoic-Cenozoic Lithospheric Reactivation
AU: * Chen, L
EM: lchen@mail.iggcas.ac.cn
AF: Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, No.19 Beituchengxilu, Deshengmenwai Qijiahuozi, Chaoyang District, Beijing, 100029, China
AU: Zhu, R
EM: rxzhu@mail.iggcas.ac.cn
AF: Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, No.19 Beituchengxilu, Deshengmenwai Qijiahuozi, Chaoyang District, Beijing, 100029, China
AU: Ai, Y
EM: ysai@mail.iggcas.ac.cn
AF: Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, No.19 Beituchengxilu, Deshengmenwai Qijiahuozi, Chaoyang District, Beijing, 100029, China
AU: Chen, Y
EM: johnyc@pku.edu.cn
AF: Department of Geophysics, School of Earth and Space Sciences, Peking University, No.5 Yiheyuan Street, Haidian District, Beijing, 100871, China
AB: A detailed knowledge of the crustal and upper mantle structure beneath the North China Craton (NCC) is important for understanding the processes and mechanisms of the Mesozoic-Cenozoic lithospheric reactivation in the region. We report results from several dense temporary seismic arrays deployed in the region since 2000. Our imaging results show substantial lateral heterogeneities in crust and upper mantle structures among different tectonic settings in the eastern NCC and adjacent areas, indicating a complex crustal and upper mantle deformation process in the region. The crust and lithosphere of the region are generally thin and displays significant structural variations both laterally and vertically. In particular, the dramatically thinned lower crust, the 60~80-km thin lithosphere, and the general NW-SE anisotropy direction of shear waves in the east Bohai Bay Basin are all considered as specific features associated with the dominant NW-SE tectonic extension in the Mesozoic and Cenozoic. The well developed crust-mantle transition zone in addition to the relatively thick crust and lithosphere and the NE-SW fast shear wave polarizations under the west Taihang Mountain Range is presumably attributed to the extension related magmatic underplating at the base of the crust since the late Mesozoic. In contrast, the Yan Mountain Belt at the northeastern margin of the NCC is characterized by a sharp Moho, a slow but normal-in-thickness lower crust and a 90~100 km thin lithosphere. These features suggest that different lithospheric processes (e.g., multiple phases of contraction and probably crust involved delamination) might have played a significant role at the northeastern margin of the NCC besides the widespread tectonic extension that was more dominant within the cratonic interior. Most importantly, a sharp step of the lithospheric thickness from ~90 km to >130 km was imaged within a short distance of <100 km at the triple junction of the basin and mountains where an abrupt change of upper mantle seismic anisotropy was found. Such observations may indicate the preservation of the tectonic imprint of earlier contractional deformations in the present-day lithosphere and provide evidence for diverse mantle deformations in response to the Mesozoic-Cenozoic lithospheric thinning in the eastern NCC. Unlike the thinned crust and lithosphere that may reflect a warm shallow upper mantle beneath the region, the mantle transition zone was imaged to be on average 10~15 km thicker than the global average, implying a cold mantle environment presumably associated with the flat¡§Clying Pacific slab at the bottom of the upper mantle under the region. This discrepancy indicate that the shallow and deep upper mantle of the eastern NCC probably have not reached a thermal equilibrium, and the Cenozoic lithospheric process and magmatism in the region might have originated within the shallow upper mantle, possibly no deeper than the middle of the transition zone.
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting