LOWER CRUSTAL FLOW AS A CAUSE OF SUPER-DEEP BASIN FORMATION ON RIFTED CONTINENTAL MARGINS

Many continental margins are marked by the presence of super-deep (>15 km) sub-basins within the overall structure but their origin is often obscure and enigmatic as they often involve phases of rapid post-rift subsidence not predicted by standard basin rifting models. The sedimentary basins around the South China Sea show dramatic examples of such phenomena. The deep sub-basins do not only passively receive sediment but their tectonic evolution is also influenced by the loading in the basin centers, as well as the associated erosion and rock uplift in the onshore source regions. This is particularly the case along the southern margin of China where a number of super-deep basins have been identified within the continental margins, most notably the Baiyun Sag and Song Hong-Yinggehai Basins. These basins are unusual because they appear to have experienced significant tectonic subsidence of the basement well after the end of active extension. The correspondence between rapid sediment deposition and tectonic subsidence suggests that the crystalline crust has been actively thinned as a result of the sediment loading, not just driven downwards by the mass of the sediment cover. Combined uplift onshore and loading offshore in the presence of a weak ductile middle crust has resulted in middle crustal flow and a much thinner than normal middle and lower crustal section under these sedimentary basins. In this new and unusual form of climate-tectonic interaction surface processes onshore have resulted in the thinning of the crystalline basement in the offshore regions. Similar subsidence anomalies would be anticipated in other areas of extension in the presence of hot and weak crust such as an arc type settings. Other examples would include the Gulf of California and the Woodlark Basin of the SW Pacific.