Extension in the graben systems around the Ordos (China), and its contribution to the extrusion tectonics of south China with respect to Gobi-Mongolia

Abstract

The graben basins around the Ordos block are major tectonic features of late Cenozoic extension in north China. This paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben-bounding faults. This view is based on the available geological data, SPOT imagery analysis, field observations, and focal mechanisms of earthquakes from this region. These graben basins have been grouped into two major systems: the Yinchuan-Hetao graben system along the northern and northwestern margins of the Ordos and the Weihe-Shanxi graben system along the southern and eastern margins of the Ordos. Crustal deformation along these graben systems involves normal slip on NE-SW-striking graben-bounding faults, normal right-lateral and normal left-lateral slip on NNE-SSW-and WNW-ESE-striking faults, respectively. This fault pattern is consistent with NW-SE-trending extension. The amount and rate of deformation resulting from this extension have been estimated from geological data, using a cross-sectional area balance and the extensional factor β method deduced from the tilt of faulted blocks. The results of these estimates show that the uppermost Pliocene-Quaternary extension rate in a NW-SE direction is in the order of 1.6 mm yr −1 across the Weihe graben, 0.5 mm yr −1 across the central Shanxi graben, on the southeastern margin of the Ordos block, and 3.1 mm yr −1 across the Linhe graben, on the northwestern margin of the Ordos block. The extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north China (Ordos, Taihang Shan, north China plain). A kinematic model is proposed in which the late Cenozoic extension and block rotation in north China are interpreted as the effect of the eastward termination of the left-lateral slip Haiyuan fault and Kunlun-Taibai fault system. The Ordos block rotates counterclockwise as a result of the push of Tibet through the left-lateral strike-slip Haiyuan fault and the Liupan Shan thrust and fold belt. The extension along the Weihe-Shanxi graben system may represent the effect of the left-lateral slip on the Kunlun-Taibai-Qinling fault system. The rough estimates of the extensional deformation around the Ordos block indicate that this contributes to about 5 ± 2 mm yr −1 of left-lateral movement in a N125°E direction between south China and Gobi-Mongolia. This motion adds to the left-lateral slip on the Qinling fault zone (about 7 ± 2 mm yr −1 in a N90°E direction), between the Qinling Shan and the Hua Shan (north China). Thus, the eastward motion of south China with respect to Gobi-Mongolia is partitioned between normal faulting around the Ordos and strike-slip faulting in the Qinling Shan. It is estimated to be in the order of 12 ± 4 mm yr −1 in a N105°E direction during the Quaternary.