Abstract

Major faults in Sundaland trend NNW to NW, WNW, N and E. Some of the NNW to NW and N-striking faults across Mesozoic areas of the Malay Peninsula were active until mid-Eocene time. Small, fault-bounded Tertiary basins onshore may be pull-apart basins associated with such faults. Mainly from seismic data, NNW to NW, N and E-striking faults have been recognized in the pre-Tertiary basement of the Malay and Penyu basins off the east coast of the peninsula. These faults were reactivated before the Late Oligocene and during the Middle to Late Miocene. N-striking faults in pre-Tertiary areas are common throughout Sundaland. In the field, these faults are found to be the oldest (possibly Jurassic) regional fractures. The regional NNW-NW and WNW fractures are believed to have originated as strike-slip faults when the peninsula was subjected to late Mesozoic deformation. The onshore E-W faults were probably extensional fractures that developed as secondary structures associated with sinistral slip motions along NNW-NW faults. Upper Cretaceous dolerite dykes fill some of the E-W fractures. NW-striking basement faults of the Malay basin continue onshore SE Asia as the Three Pagodas fault zone. Initially these were sinistral basement wrench faults creating secondary E-W extensional fractures. In the Middle to Late Miocene the regional stress field changed, resulting in reversal of slip movement along major wrench faults and structural inversion of the sedimentary basins. This inversion is manifested as E-W anticlines located over half-graben. In the Penyu basin similarly striking half-graben probably developed in the same fashion. There, the NW-striking Rumbia fault divides the basin into two parts. Half-graben in the western part remained orientated E-W, but those in the eastern part became rotated clockwise by continued left-lateral slip along the Rumbia fault. After the Miocene the two basins continued to subside, developing an almost undisturbed blanket of post-Miocene sediments. Locally, residual stress caused some of the structures to grow.