APPLICATION OF PLATE RECONSTRUCTIONS AND 2D GRAVITY MODELING TO QUANTIFY CRUSTAL STRETCHING DURING CONTINENTAL BREAK-UP: A SOUTH CHINA SEA CASE STUDY

摘要

Plate models reconstructing Southeast Asia through the Cenozoic can be divided into two groups: Indochina extrusion models and slab-pull models. In extrusion models, the opening of the South China Sea is accommodated by strike-slip motion along the Red River Fault, while slab-pull models employ a southward subducting proto-China Sea and minor motion on the Red River Fault. We compare crustal stretching values calculated from 2D gravity models for offshore Vietnam and south-eastern China with values implied by representative plate models. These constraints, when combined with other geological evidence reveal that the opening of the South China Sea requires the presence of a southward subducting proto-South China Sea.Purely strike-slip extrusion models do not result in the E-W extension required to form sedimentary basins offshore of central Vietnam, as their entirely pull-apart origin is unsupported. One proposed solution is to implement significant extension (about 130 km) across the Red River Fault in onshore northern Vietnam (e.g. Briais et al., 1993), in addition to about 550 km of strike-slip motion on the same fault. However, this solution causes approximately 400 km E-W extension offshore Vietnam since 30 Ma, while our 2D models reveal that only 180 km of E-W extension associated with the opening of the South China Sea. In order to match the calculated 180 km E-W extension we have incorporated into our plate model: Red River strike-slip displacement of 170 km (calculated at the coast of the Beibu Gulf); 50% less Red River extension than the amount proposed by Briais et al.1993;and a southward-subducting proto-South China Sea.