Print this pageEarly Cenozoic Transtension of the Bohai Basin, Northern China
ALLEN, MARK B., STEPHEN J. VINCENT, CHRISTINE BROUET-MENZIES, DAVID I. M. MacDONALD, and ZHAO XUN
The Bohai Basin is one of a family of early Cenozoic rift basins that lie along the eastern margin of Asia from Russia to Vietnam. Initial 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 fourth (lowest) member of the Shahejie Formation in a series of elongate half grabens. These half-grabens have master faults with a NNE-SSW orientation. Secondary faults commonly have a northeast trend, indicating dextral transtension. Deposition was focused in the west and south of the present basin. These rocks are mainly alluvial fan and fluvial red beds. The architecture of the basin underwent an important change at c. 43-45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. The Bozhong Depression in the central part of the basin formed at this time, when continued activity of the earlier Tertiary rifts to the northeast and southwest created an extensional overlap between them. During this phase, the basin as a whole resembled a typical pull-apart basin, with the Bozhong Depression as its major depocentre, but individual sub-basins in the northeast and southwest were controlled by transtensional fault systems.