EARLY CENOZOIC 2-PHASE EXTENSION AND LATE CENOZOIC THERMAL SUBSIDENCEAND INVERSION OF THE BOHAI BASIN, NORTHERN CHINA

The Bohai Basin is one of a family of early Cenozoic extensional basin s that lie along the eastern margin of Asia from Russia to Vietnam. In itial extension was probably triggered by subduction roll-back of the oceanic Pacific Plate from the Asian continent. There were two phases in the Bohai Basin's rift history. The earlier, Paleocene-early Eocene phase resulted in the deposition of the Kongdian Formation and the fo urth (lowest) member of the Shahejie Formation in a series of elongate half grabens. These half grabens have master faults with a NNE-SSW or ientation. Secondary normal faults are typically clockwise oblique to the master faults, indicating a component of dextral transtension. Dep osition was focused in the west and south of the present basin. These rocks are mainly alluvial fan and fluvial red beds. The architecture o f the basin underwent an important change at ca. 43-45 Ma (middle Eoce ne), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half g rabens as the Kongdian Formation, but the Bozhong Depression in the ce ntral part of the basin originated at this time, and became the major depocentre. The Bozhong Depression superficially resembles a pull-apar t basin. it formed when continued transtension of the earlier Tertiary fault systems to the east and west created an extensional overlap bet ween them. During this phase, the basin as a whole had a geometry with elements typical of both pull-apart and transtensional basins. Region al extension in many east Asian basins ended at the end of the Oligoce ne, probably because of the onset of transpression within eastern Asia , caused by the collision of Australia with the Philippine Sea Plate. Dextral transpression caused minor inversion of some of the earlier no rmal faults in Bohai, but as a whole the basin began to subside in a p ost-rift phase of thermal subsidence that has lasted until the present day. (C) 1998 Elsevier Science Ltd. All rights reserved.