Abstract

Studies of fault patterns at five subsurface horizons in the North Sumatra Basin reveal that the structural development of the basin is also well controlled by strike slip faulting.

During the late Cretaceous-early Tertiary, an oblique convergence between the northeastern portion of the Indian Ocean plate and the southeast Asian continental plate took place with a spreading rate of 18 cm/yr. Several major N-S trending strike slip faults mainly with dextral movements were formed in the present back-arc region and arranged in en echelon pattern.

Since then and until Middle Miocene, the basin is characterized by normal faulted terrain. It is probably closely related to horizontal movements along those strike slip faults which were formed in a right-stepping position, producing zones of tension and depression in which continental clastics and marine sediments were deposited. This episode of tectonism approximately corresponds to a change in the spreading direction of the Indian Ocean from N-S during the early paleogene to NE-SW with the spreading rate of ± 5 cm/yr.

Convergence becoming highly oblique and from the Late Miocene, further horizontal movements along those N-S trending strike slip faults-now in a left stepping position-produced compressive deformation and uplift throughout the basin with deposition of regressive sediments. The change from extensional to compressional stresses is also related to the active spreading which occurred in the Andaman Sea, and corresponds to an increase in the spreading rate of the Indian Ocean from 5 to 7 cm/tr, which occurred at ± 9-10 Ma.

Compressional structures continuously affected the sedimentary cover in Plio-Pleistocene time due to strike slip faulting along the present right lateral Sumatran Fault system.