EXTENSION IN THE GRABEN SYSTEMS AROUND THE ORDOS (CHINA), AND ITS CONTRIBUTION TO THE EXTRUSION TECTONICS OF SOUTH CHINA WITH RESPECT TO GOBI-MONGOLIA

The graben basins around the Ordos block are major tectonic features o f late Cenozoic extension in north China. This paper gives a synthetic view of the basic characteristics of the graben basins and of the kin ematics of the graben-bounding faults. This view is based on the avail able geological data, SPOT imagery analysis, field observations, and f ocal mechanisms of earthquakes from this region. These graben basins h ave been grouped into two major systems: the Yinchuan-Hetao graben sys tem along the northern and northwestern margins of the Ordos and the W eihe-Shanxi graben system along the southern and eastern margins of th e Ordos. Crustal deformation along these graben systems involves norma l 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 ext ension. The amount and rate of deformation resulting from this extensi on have been estimated from geological data, using a cross-sectional a rea balance and the extensional factor beta method deduced from the ti lt of faulted blocks. The results of these estimates show that the upp ermost Pliocene-Quaternary extension rate in a NW-SE direction is in t he order of 1.6 mm yr(-1) across the Weihe graben, 0.5 mm yr(-1) acros s the central Shanxi graben, on the southeastern margin of the Ordos b lock, and 3.1 mm yr(-1) across the Linhe graben, on the northwestern m argin of the Ordos block. The extension across;these graben systems is accompanied with counterclockwise rotation of continental blocks in n orth China (Ordos, Taihang Shan, north China plain). A kinematic model is proposed in which the late Cenozoic extension and block rotation i n north China are interpreted as the effect of the eastward terminatio n of the left-lateral slip Haiyuan fault and Kunlun-Taibai fault syste m. The Ordos block rotates counterclockwise as a result of the push of Tibet through the left-lateral strike-slip Haiyuan fault and the Liup an Shan thrust and fold belt. The extension along the Weihe-Shanxi gra ben system may represent the effect of the left-lateral slip on the Ku nlun-Taibai-Qinling fault system. The rough estimates of the extension al deformation around the Ordos block indicate that this contributes t o about 5 +/- 2 mm yr(-1) of left-lateral movement in a N125 degrees 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 degrees E direction), between the Qinling Shan and the Hua Shan (north China). Thus, the eastward motion of south China with res pect to Gobi-Mongolia is partitioned between normal faulting around th e 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 degrees E direction during the Quaternary. (C) 1998 Elsevier Science B.V. All rights rese rved.