Gravity modelling of extensional basins in Southeast Asia

Comparison of free-air gravity over sedimentary basins in Southeast Asia with sediment accumulations in the basins indicates no correlation between gravity and sediment thickness_ This is due to differences in crustal structure under basins in extensional versus convergent regimes. In convergent regimes, thickened crust creates negative gravity anomalies. In extensional regimes, the crust is thinned creating both positive and negative gravity anomalies. We examine the problem of gravity modelling in extensional regimes, using the Malay basin as an example. We find: 1. while gravity modelling is a nonunique process, the use of a priori knowledge (i.e., prior information derived from other sources) in gravity models greatly reduces the range of possible models. In the Malay basin, a priori knowledge indicates the basin should have a -100 mGal anomaly. However, only a -20 mGal anomaly is seen over the basin. This suggests there is a high density intrusion at the base of the crust, producing an offsetting positive anomaly. 2. the gravity field is only a minor constraint in gravity modelling over extensional basins. This is due to the almost complete cancellation of the positive anomaly associated with the high density intrusion at the base of the crust and the negative anomaly associated with the sedimentary basin in the upper crust. 3. gravity modelling can produce useful geologic information if deep crustal structure is accounted for before attempting to model shallow crustal features. This can best be accomplished with the use of a priori knowledge in the modelling process.