ABSTRACT: Cainozoic Basin-Forming Tectonism in the Southern Fore-Arc Zone of Sumatra

HALL, D. M., B. A. DUFF, M. C. COURBE, B. W. SEUBERT, M. SIAHAAN, and A. D. WIRABUDI, no affiliation

In the Bengkulu PSC of onshore and offshore Southwest Sumatra, localized basins containing four distinct seismic megasequences are recognized.

The basal, probably Paleogene, megasequence was deposited mainly as a syn-rift unit within a system of northeast-trending half grabens probably segmented by northwest-trending transfer faults. A major unconformity separates this unit from a late Paleogene to early Miocene megasequence, and appears to mark a change in the basin forming mechanism from Paleogene extension to possible oblique slip. According to this model, the transfer faults of the rift system were rejuvenated by right-lateral oblique slip in the late Paleogene to early Miocene, thereby superimposing local pull-apart basins on the underlying grabens.

These units are succeeded with strong unconformity by an early to middle Miocene megasequence marking the waning of the localized basin-forming tectonism and the onset- of major transgression associated with a unified, open marine basin. Finally, this unit was unconformably succeeded by a dominantly regressive, progradational middle Miocene to Recent megasequence resulting from uplift and erosion of the Barisan Mountains. The associated inversion of the earlier basin megasequences increases in intensity from offshore toward this mountain belt. Furthermore, although within the PSC this inversion was initiated during the early Miocene, its completion was diachronous toward the Barisan Orogen, within which the youngest (neotectonic) structures are developed.

These results imply that far from accommodating a simple, homogeneous fore-arc basin, the Sumatran fore-arc is tectonically heterogeneous with considerable potential for localized Paleogene and early Neogene basins.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)