The gravity field of Sundaland : acquisition, assessment and interpretation
The 1995 Sandwell & Smith WGI7.2 satellite gravity map has been compared with high quality ship gravity data offshore Sabah. The standard deviation of the differences between gravity profiles interpolated from WGI7.2 and the ship survey was 5.7 mGal. Spectral analysis indicated that WGI7.2 resolves features with half-wavelengths greater than 10 km. Three hundred new gravity stations, including ten base stations tied to IGSN71, were established during a survey of Sabah and were merged with two previous surveys to complete the gravity coverage of the state at reconnaissance level. New gravity maps of Sundaland, compiled from the Sabah and WGI7.2 gravity data sets have been interpreted. ' Subtraction from these data sets of the long wavelength GEM-T3 gravity anomaly field, which is dominated by the effects of the subducted proto-South China Sea and the ongoing subduction of the Australian, Indian and Philippine Sea Plates, achieved a simple 'regional-residual' separation and the isolation of crustal-scale anomalies. 5' x 5' maps of the depth to Moho, crustal thickness and β (crustal extension) factor beneath the Sunda Shelf and the South China Sea have been produced by 1-D geometric manipulation of the marine gravity field using sediment isopach and bathymetry control data. The average difference between the new Moho map and seismic refraction depth-to-Moho estimates at the south China margin was 0.2 ± 2.0 km. The Moho rises from an average depth of 29 km beneath the Sunda Shelf to a depth of about 16 km beneath the South China Sea oceanic basin. It is estimated that approximately 1100 km of oceanic crust of the proto-South China Sea was subducted at the northwest Borneo margin. Gravity modelling independently confirmed the average 35 mm/yr spreading rate prediction for the southwestern subbasin of the South China Sea oceanic basin from magnetic lineation interpretation. Quantitative interpretation indicates that Sabah is underlain by thick continental crust, and gravity modelling of the Maliau Basin predicts a minimum sediment thickness of 8 km. The ophiolite bodies at Telupid and Darvel Bay are separated by a gravity low and there is no evidence that the two are related. The crustal root of the Crocker Ranges extends to a depth of ~49 km below sea level. In order to reach this present day crustal thickness, the turbiditic sediments forming the Crocker Ranges must have been compressed to less than half of their original lateral extent during the Sabah Orogeny.