Stratigraphical analysis of island arc—continental margin collision in eastern Indonesia

Abstract

Eastern Indonesia is a sinuous plate margin interpreted as a collision between the northern margin of the Australian continent and an Asian island arc. Detailed stratigraphical analysis of the collision zone in Timor reveals that pre-Pliocene deformation phases affected members of the allochthon before they were overthrust together onto the Australian margin in the middle Pliocene. The Australian facies para-autochthon below the thrust sheets was not involved in these pre-Pliocene deformations. The distinction within the collision zone of elements having different structural histories and opposite facies polarity permits identification of the plate margin which has been overthrust and folded after the initial collision. The lowest thrust sheet emplaced in Timor and other islands of the Outer Banda Arc appears to be part of an Asian outer arc ridge overthrust by fragments of a continental margin metamorphic basement and volcanic-sedimentary cover. Stratigraphical studies provide the basis for an explanation of the origin, transport and emplacement of a major olistostrome, for the sequence of allochthonous elements emplaced on the underthrusting continental margin and for post-collision deformation phases in the arc. A speculative model interprets a progressive Mio-Pliocene collision between the Australian margin and an island arc migrating away from SE. Asia by spreading of the marginal-type Banda Sea. The Asian arc was underthrust by the Australian continental margin but buoyancy probably restricted the process to overthrusting slivers of rocks from the trench and trench-arc gap. This model could be tested by seismic and magnetic studies of the Banda Sea region. The Indonesian