Reinterpretation of the geology of Seram: implications for the Banda Arcs and northern Australia

Abstract

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 ...

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

“…Three subdivisions have usually been drawn (in stratigraphic order and decreasing metamorphic grade): (1) The Old Crystalline Schists of Valk (1945) and Germeraad (1946), later named the "Kobipoto Complex" by Audley-Charles et al (1979), which includes granulite facies metapelites, migmatites, and gneisses that are present in the Kobipoto Mountains; (2) Widespread mica schists, amphibolites, and phyllites of the Tehoru and Taunusa Complexes, proposed by AudleyCharles et al (1979) and Tjokrosapoetro and Budhitrisna (1982) respectively, which are equivalent to the Formation of Crystalline Schists and Phyllites of Valk (1945); and (3) Low-grade slates and marbles ascribed to the Saku Formation of Tjokrosapoetro and Budhitrisna (1982), equivalent to the Graywacke and Glossy Slate Formationof Valk (1945) …”

“…The Kobipoto Complex is best exposed in the remote Kobipoto Mountains of central Seram (Rutten, 1918b;Germeraad, 1946;Audley-Charles et al, 1979;de Smet and Barber, 1992;Tjokrosapoetro et al, 1993a) and the vicinity of Gunung Langget in southwest Seram (van der Sluis, 1950;Gafoer et al, 1993). High-grade metamorphic rocks are found in association with variably serpentinised peridotites.…”

“…High-grade metamorphic rocks are found in association with variably serpentinised peridotites. Audley-Charles et al (1979) made the distinction between two types of gneiss from the Kobipoto Mountains found as boulders in the gorges of the Wai (river) Isal and Wai Tuh: (a) metatexites 1 with quartzofeldspathic segregations and complex folding containing large garnets and green spinel; and (b) coarser-grained granitic diatexites containing biotite, cordierite, garnet, staurolite, and andalusite, with xenoliths containing sillimanite and green spinel. AudleyCharles et al (1979) suggested both types were metamorphosed under granulite facies conditions in the lower crust until the onset of anatexis, with the aluminous xenoliths representing residua after partial melting.…”

“…Some authors, largely based on interpretation of seismic and geochemical trends, have proposed that the geometry of the Banda Arc is created by two separate slabs -a southwarddipping slab beneath Seram forming the northern portion of the arc, and a northward-dipping slab beneath Timor forming the southern portion -which have coincidentally sutured along their junction (Cardwell and Isacks, 1978;McCaffrey, 1988McCaffrey, , 1989Stevens et al, 2002;Bock et al, 2003;Das, 2004;Hinschberger et al, 2005;Fichtner et al, 2010;Špičák et al, 2013). Although more plausible than early models that proposed the Banda Arc represents an eastward continuation of the Sunda Arc subsequently flexed through 180 • (Audley- Charles et al, 1972;Katili, 1975), these two-slab hypotheses run into problems when placed into a regional plate-tectonic context (Hall, 2002Spakman and Hall, 2010). Specifically, they require prior existence of oceanic crust between Seram and the Bird's Head (West Papua), at least 2000 km of N-S shortening across the Banda region between Australia and Halmahera to have occurred in the last 6 Ma, and a westward continuation of the Tarera-Aiduna Fault from south of the Bird's Head to have sutured the two slabs and to have accommodated their differences in motion.…”

“…We first provide a synthesis of previously published field reports (Valk, 1945;Germeraad, 1946;van der Sluis, 1950;Audley-Charles et al, 1979;Tjokrosapoetro and Budhitrisna, 1982;de Smet and Barber, 1992) and geophysical surveys (Milsom, 1979;Milsom et al, 2001;Bock et al, 2003;Das, 2004;Widiyantoro and van der Hilst, 1997;Spakman and Hall, 2010) combined with ASTER digital elevation model (DEM) interpretation. We present a series of new small-scale geological maps for a number of key areas around Seram alongside a revised geological sketch-map for the whole island (Fig.…”