EXTENSIONAL FAULT-BEND FOLDING AND SYNRIFT DEPOSITION - AN EXAMPLE FROM THE CENTRAL SUMATRA BASIN, INDONESIA

We describe an analytical method for interpreting the geometry and str uctural history of asymmetric half grabens in rift basins with extensi onal fault-bend fold theory. Using seismic reflection profiles from th e Central Sumatra basin and balanced forward models, we show how local variations in tectonic subsidence relative to deposition rates yield distinctive patterns of folded synrift strata and unconformities that record basin history. If the deposition rate exceeds the local subside nce rate, folded growth strata form upwardly narrowing kink bands that have been described previously as growth triangles, In contrast, if t he deposition rate is less than or equals the local subsidence rate, g rowth strata are folded and truncated at the surface on half-graben ma rgins. Subsequent increases in deposition rate relative to subsidence rate form angular unconformities near the half-graben margins. These u nconformities develop without the necessity of erosion and are folded by continuing fault slip. Strata above and below the unconformities ar e concordant in the deeper parts of the half grabens. Thus, angular un conformities on half-graben margins are helpful for defining sequence boundaries that may reflect changes in deposition and tectonic subside nce rates. In addition, fault-bend fold interpretations yield fault ge ometry and measures of horizontal extension, both of which control thr ee-dimensional half-graben geometry and accommodation space. We show h ow along-strike variations in fault geometry produce intrabasinal stru ctures that may form prospective fairways or local depocenters.