CENOZOIC EVOLUTION OF THE SAVU BASIN, INDONESIA - FORE-ARC BASIN RESPONSE TO ARC-CONTINENT COLLISION

A seismo-stratigraphic analysis of the Savu Basin relates the evolution of a complexly evolved forearc basin to the late Miocene-Recent collision with Australia, initial early Miocene subsidence of the outer forearc basin and the subsequent development of a middle-late Miocene volcanic 'proto' are, located in the south of the basin, both resulted from the late Oligocene-early Miocene eastward propagation of the Java/Timor Trench. Block-faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle-late Miocene volcanic centres. Just north of the accretionary prism, the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism. The southern forearc basement probably acted as a barrier against compression. The thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision. Owing to its strength, the Savu Basin did not react to collision by the development of northward verging 'inter-arc' thrusts. In contrast, it responded to the underthrusting of continental crust by reactivation of the basement ridges. This resulted in the differentiation of the forearc basin into a presently extinct and uplifted Miocene South Savu Basin and a Pliocene-Recent active North Savu Basin. The simultaneous late Miocene-Recent uplift of large segments of the outer forearc and subsidence of the North Savu Basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust. The relatively recent decoupling of this crust from the oceanic slab, and the subsequent isostatic rebound, may superimpose the initial effects of post-collision extension on are-continent collision.