Structure and Cenozoic evolution of the rifted northern margin of the South China Sea.

摘要

A study of three geophysical transects across the Cenozoic rifted northern margin of the South China Sea was conducted in order to define east-west variations in the rifting style of the margin and to determine the extension mechanisms responsible for forming the present margin.; The amount of crustal thinning is one assumed measure of extension; therefore, crustal thickness profiles have been constructed for the three margin transects using Expanding Spread Profile (ESP) velocity-depth determinations in conjunction with gravity modeling. The following east-west variations in extension are apparent: approximately twice as much material was involved in rifting on the western portion vs. the eastern portion of the margin; approximately 275 km of horizontal extension of the continental crust occurred in the west as compared to 150 km in the east; occurrences of extremely localized crustal thinning are more prevalent in the west than in the east.; Multichannel seismic reflection profiles reveal the presence of highly reflective, northward-dipping normal faults (with dips of {dollar}sim{dollar}25{dollar}spcirc{dollar}-40{dollar}spcirc{dollar}), cutting through the entire crust at several locations on the margin. Local crustal thinning of 5-15 km is associated with these faults, which are proposed locations of simple shear extension of the crust.; The applicability of existing pure shear and simple shear extension models to the rifting of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow against observations. Unless heat flow is postulated as rather high prior to rifting, neither model predicts the high heat flow observed at present across the extended margin. Such initially high heat flow could have been produced either by a thinner than average steady-state lithospheric thickness ({dollar}sim{dollar}60 km) or by a significant amount of crustal radiogenic heat production (1.5-2.1 {dollar}mu{dollar}W/m{dollar}sp3{dollar}). Based on modeling results, a pure shear model is preferred over a simple shear model for explaining the large scale lithospheric extension of the south China margin. However, the presence of through-going crustal faults indicates that simple shear has also contributed to the extension of the crust.