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

Reconnaissance field traverses in Seram have led to major revisions in the stratigraphy, structure, tectonic history and geological maps. The island is composed of 4 principal stratigraphical-structural elements:(1) metamorphic continental -I of uncertain structural status and palaeogeographical affinity, (2) an entirely marine early TiiassicMiocene imbricate succession regarded as para-autochthonous, (3) an allochthon composed of several different thrust sheets, including metamorphic rocks, Triassic limestone and a late Miocene olistostrome, (4) a Plio-Pleistocene post-orogenic autochthon. The apparently overthrust slices of metamorphic basement complex can be interpreted as derived from either the Asian or Australian craton. The Australian shelf, slope and rise sediments, possibly including some oceanic sediment, are regarded as para-autochthonous. Remarkably close correlation is demonstrated between the stratigraphical breaks reported from the Mesozoic-Cenozoic succession of the NW Australian shelf, from Misool, and the para-autochthonous rocks of Seram and Timor. This emphasizes the presence of the Australian craton underlying these 3 islands. Close correlation is also found between the allochthonous rocks of Seram and Timor. Some of these thrust sheets are interpreted as having been derived from the Asian continental margin. The ultrabasic rocks of SW Seram and Ambon seem to form the highest thrust sheets. The main period of orogenesis, involving over-thrusting, olistostrome emplacement and imbrication of the underlying Australian cover-rock sequences occurred in the late Miocene-early Pliocene (N.18). The structural position of the volcanic rocks of Ambon is uncertain. A tentative interpretation is that they are in sim, having been extruded from deep-seated fractures that penetrate the 'Asian' thrust sheets and the underlying Australian continental basement.In our continuing study of the Banda Arcs, Seram ( Fig. 1) was considered the most important island to investigate after Timor, since it lies on the opposite side of the 180" curved Banda Arc, is the next largest island (being 400 X 75 km), and has geological sketch map coverage (with sample descriptions), which allow reinterpretation on the basis of airphotos, ERTS photos and field studies. The geology of Seram is a mirror image of the geology of Timor in many important respects, such as the supposed directions of overthrusting and the contrasting Mesozcic faunas and facies showing affinities with either Australian (basement) or 'Asian' (overthrust) elements. Published reports on the geology of Seram (Valk 1945; Germeraad 1946; van der Sluis 1950; Zillman & Paten 1975) suggested that lithologies and faunas were similar to those in Timor. Compared with Timor (AudleyCharles 1968; Carter et al. 1976; Barber er al. 1977), accounts of the pre-Neogene stratigraphy and structure of Seram seem confused, with apparently unrelated stratigraphical divisions and dissimilar structural elements lumped together unconvincingly. Furthermore, no unambiguous evidence of ...

Abstract.Analysis of block assemblages, matrix clay mineral composition and microfauna, and offshore seismic reflection profiles reveal that the Bobonaro melange in the Timor region is sourced from mud-rich Australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement. Melange facies include broken formation, matrix-rich mud injections, and classic mixed block-in-clay facies. Each of these are distinguished by varying degrees of remobilization, mixing, and dispersion at different structural positions across the orogenic wedge. The most important structural control is whether melange was generated beneath

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