Cenozoic inversion structures in a back-arc setting, Western Flores Sea, Indonesia
Emmet, Peter Anthony
Bally, Albert W.
Doctor of Philosophy
A geophysical and geological study utilizing high quality seismic and well log data was undertaken of a marginal basin in the Western Flores Sea, Indonesia, to document the evolution of Paleogene extensional basins and their transformation during the Neogene into compressional uplifts. These are referred to as inversion structures because they begin as extensional half-grabens and are transformed by compression into structural highs. The crust underlying the area is transitional between continental crust of the Sunda craton to the west and oceanic crust of the Banda back-arc region to the east. Half-grabens began to form in the middle Eocene by extensional reactivation of thrusts and bedding planes within the deformed and peneplained basement complex which was an accretionary prism during the Cretaceous. Extension and regional subsidence continued until early Miocene time, when compression began to reactivate the extensional bounding faults of the half-grabens as thrusts. Compressional growth of the inversion structures was most dramatic during the late Miocene and Pliocene and continues today. The Paleogene grabens appear to have resulted from essentially orthogonal extension, oriented roughly N-S with respect to present geography. During the Neogene, the axis of compression which caused the inversion of the extensional structures appears to have been essentially the same as that which caused the extensional structures. Seismic interpretation of the Neogene units attempted the resolution of individual depositional sequences related to changes in relative sea level. The sequences were recognized by their constituent lowstand, transgressive and highstand systems tracts, where present, as defined by stratal termination patterns, truncational relationships and incision. The number of Neogene sequences, their ages as constrained by biostratigraphic data, relative amounts of incision and truncation at specific sequence boundaries, and the stacking patterns of the sequences were compared to those predicted by the published, reputedly globally-correlative, sea level chart of Haq and others (1987). The comparison is generally favorable, with the greatest variances noted in the ages of individual sequences, and the greatest similarity noted in the long-term stacking patterns of the sequences, especially for tectonically quiescent parts of the basin.