Abstract:

The Dongpu Diwa (depression) is located at the southern end of the Bohai Bay Diwa series in Huanghe-Huaihe Diwa system, North China Diwa region.It developed on the basement formed in platform tectonic layer of the Paleo-zoic or structural basement of the Mesozoic, and had the structural form ofdouble fault graben. One of the most striking characteristics is the development of rhombic fault blocks on the basement. NNE striking Lanliao, Changyuan,Huanghe faults and nearly EW stretching Wenmingzhai-Fanxian, Huzhuangji-Maogang, Gaopingji-Xichengji, Changyuan-Gegangji, Naoli-Sanchunji faultscut the basement of the Dongpu Diwa into eight blocks. The sediments that arearranged into zones in E-W direction and blocks in S-N direction are also con-trolled by these faults. The isobath of 34 km in the Moho depth draw clearlythe outline of the Dongpu Diwa, this indicates the relation of mirror image with NNE trending swell of the mantle. Its tectonic evolution underwent two stages. The early period (the Late Eocene-Oligocene) was tensil-faulted stage,forming fault-depressed diwa; the late one (the Miocene-Pleistocene) was com-presso-depressed stage,forming depressed diwa. The formation mechanism of the Dongpu Diwa is that the tectonic evolution began with the conjuction of a de-crease of the crustal thickness and diapiric uprise of the mamtle caused by re-gional tensil stress in the early stage. The main feature was that the volume of crustal extension vary directly as the thickness of sediments; subsequently,the crust depressed due to balanced adjustment assisted by SE-NW compression in the late stage. Its principle character was the evolution of separated sagsto an unified and extensive depressed basin.