Transtensional Tectonism and Its Effects on the Development of Submarine Fan in Changchang Sag, Qiongdongnan Basin, Northern South China Sea

The Changchang Sag, situated at the north continental slope of the South China Sea, is a main sub-unit in the Central Depression Belt of Qiongdongnan Basin. In this Sag, the upper Oligocene Lingshui and lower Miocene Sanya Formations had developed in the evolution stage of faulted basin. Seismic stratigraphic sequences and fault structures revealed that the Eocene-lower Oligocene Changchang sag was controlled by large-scale NE-strike faults, while the upper Oligocene-lower Miocene Changchang sag was dominanted by NW-strike faults, especially the western Changchang sag had been controlled by two groups of broomlike fault belts, which developed in the southwest and northeast of this Sag respectively. These broomlike faults, together with en echelon faults and diapirs, indicated the transtensional activities, which had resulted in clockwise rotation of subsidence center during the upper Oligocene-lower Miocene. Moreover, two large submarine fans during the first member of upper Miocene Sanya Formation were identified by seismic facies and seismic attributes in the southwest and northeast of western Sag respectively. Further, contrastive analysis of sedimentary characteristics and palaeogeomorphology between the southwestern and northeastern submarine fans indicates that they belong to the same submarine system, and were loaded from a large EW-trending turbidite channel in shallow sea setting. But the southwest fan is significantly larger than the northeastern fan, which is controlled by the development scale of the broomlike faults and the different strike between broomlike faults and turbidite channel. During the Eocene-lower Oligocene, the NE-trending faults were developed by a NW-SE stretching stress. During the upper Oligocene-lower Miocene, however, the stretching stress was clockwise rotated because of the NE-trending seafloor spreading and its non-uniformly spreading rate in Southwest sub-sea basin of South China Sea. So the broomlike faults were response to clockwise rotation of stretching stress on South China Sea seafloor spreading.