Consequences of Elevated Pre-Rift Lithosphere Geotherm on the Rifting and Breakup of the South China Sea

Before Oligocene continental breakup at ~30 Ma, the South China Sea (SCS) lithosphere had an elevated geotherm following Cretaceous northward subduction of Pacific or Proto-SCS oceanic lithosphere under the continental South China block resulting in an Andean style orogeny and volcanic arc. We examine the consequences of this elevated geotherm on SCS crustal thickness determined from gravity inversion and determine the amount of lithosphere extension required for continental breakup and sea-floor spreading initiation.

Subsidence analysis of the northern SCS rifted margin shows up to 2 km subsidence of the base Oligocene unconformity to the present day that cannot be explained by observed extensional faulting and that we attribute to thermal subsidence from a very large pre-breakup lithosphere thermal perturbation. Parameterising the magnitude of this thermal perturbation by a McKenzie β factor requires a very large β factor > 4.

SCS crustal thickness predicted from gravity inversion incorporating an elevated pre-Oligocene lithosphere geotherm (GI model P3) is compared with that produced using an equilibrium initial lithosphere (GI model K1b). For very thinned continental crust and oceanic crust, GI models K1b and P3 give similar Moho depths that calibrate well against seismic reflection Moho depth. GI model K1b produces Moho depths consistently too deep (~ 5 km) for the northern SCS margin. In contrast GI model P3 with an elevated pre-rift geotherm produces Moho depths that calibrate well against seismic observations.

We examine profiles crossing the SCS to determine how much extension is required to stretch and thin continental lithosphere to generate continental breakup and initiate sea-floor spreading? Cumulative extension is calculated by integrating lithosphere thinning factor (1-1/β) determined by gravity inversion using GI model P3. Measured lithosphere extension prior to continental breakup and sea-floor spreading initiation in the SCS ranges between 303 km in the east and 558 km in the west predicted by GI model P3. In contrast measured lithosphere extension prior to rupture and separation of continental crust on the Iberia-Newfoundland conjugate rifted margins is 180 +/-20 km. Substantially more extension of continental crust (>200%) occurs before continental crustal breakup in the SCS compared with that between the Iberia and Newfoundland Atlantic margins

Our gravity inversion predicts a very wide region of continental crust with thicknesses between 25 and 10 km in the SCS, very much wider than for Atlantic type margins, due to a weak inherited SCS lithosphere rheology. The hot lithosphere geotherm prior to rifting and breakup gives a weak lithosphere rheology favouring extensional boudinage of the continental crust rather than crustal rupture and separation. Hot SCS lithosphere deformation contrasts with colder Atlantic Ocean type margins (e.g. Iberia-Newfoundland) where colder and stronger lithosphere rheology generates necking and focussing of lithosphere stretching and thinning.