Abstract
Abstract. The island of Seram, which lies in the northern part of the 180°-curved Banda Arc, has previously been interpreted as a fold-and-thrust belt formed during arc-continent collision, which incorporates ophiolites intruded by granites thought to have been produced by anatexis within a metamorphic sole. However, new geological mapping and a re-examination of the field relations cause us to question this model. We instead propose that there is evidence for recent and rapid N–S extension that has caused the high-temperature exhumation of lherzolites beneath low-angle lithospheric detachment faults that induced high-temperature metamorphism and melting in overlying crustal rocks. These "Kobipoto Complex" migmatites include highly residual Al–Mg-rich garnet + cordierite + sillimanite + spinel + corundum granulites (exposed in the Kobipoto Mountains) which contain coexisting spinel + quartz, indicating that peak metamorphic temperatures likely approached 900 °C. Associated with these residual granulites are voluminous Mio-Pliocene granitic diatexites, or "cordierite granites", which crop out on Ambon, western Seram, and in the Kobipoto Mountains and incorporate abundant schlieren of spinel- and sillimanite-bearing residuum. Quaternary "ambonites" (cordierite + garnet dacites) emplaced on Ambon were also evidently sourced from the Kobipoto Complex migmatites as demonstrated by granulite-inherited xenoliths. Exhumation of the hot peridotites and granulite-facies Kobipoto Complex migmatites to shallower structural levels caused greenschist- to lower-amphibolite facies metapelites and amphibolites of the Tehoru Formation to be overprinted by sillimanite-grade metamorphism, migmatisation, and limited localised anatexis to form the Taunusa Complex. The extreme extension required to have driven Kobipoto Complex exhumation evidently occurred throughout Seram and along much of the northern Banda Arc. The lherzolites must have been juxtaposed against the crust at typical lithospheric mantle temperatures in order to account for such high-temperature metamorphism and therefore could not have been part of a cooled ophiolite. In central Seram, lenses of peridotites are incorporated with a major left-lateral strike-slip shear zone (the "Kawa Shear Zone"), demonstrating that strike-slip motions likely initiated shortly after the mantle had been partly exhumed by detachment faulting and that the main strike-slip faults may themselves be reactivated and steepened low-angle detachments. The geodynamic driver for mantle exhumation along the detachment faults and strike-slip faulting in central Seram is very likely the same; we interpret the extreme extension to be the result of eastward slab rollback into the Banda Embayment as outlined by the latest plate reconstructions for Banda Arc evolution.
Highlights
Norvick during the 1975 Seram Expedition (Audley-Charles et al, 1979), from observations made by Rutten and Hotz in 1918 (Germeraad, 1946), and from faults inferred from ASTER digital elevation model (DEM) interpretation
The aim of our field-based reinvestigation into the tectonics of Seram and Ambon was to test the latest hypotheses for Banda Arc evolution that have invoked slab rollback and extension (Hall, 1996, 2002, 2011, 2012; Milsom et al, 2001; Spakman and Hall, 2010) and compare them to previous models that have outlined a thrusting scenario for the islands (Audley-Charles et al, 1979; Linthout et al, 1989, 1991)
The peridotites show no compelling evidence that they belong to an ophiolite – the dominantly fertile lherzolitic compositions and the absence of products from partial melting including basaltic dyke complexes and large-volume gabbros suggest that a subcontinental lithospheric mantle (SCLM) origin is much more likely
Summary
Eastern Indonesia (Fig. 1) is the product of complex collision between Eurasia, Australia, and the Pacific (Caroline and Philippine Sea plates) since the Late Oligocene (van Bemmelen, 1949; Hamilton, 1979; Bowin et al, 1980; Katili, 1989; Milsom et al, 2001; Hall, 2002, 2012; Hinschberger et al, 2005; Harris, 2006; Nugroho et al, 2009; Villeneuve et al, 2010; Watkinson et al, 2012). The old, cold, and negatively buoyant Jurassic oceanic lithosphere which occupied the Banda Embayment, termed the Proto-Banda Sea, is thought to have readily collapsed upon invasion by the advancing Java subduction zone at 16 Ma, causing eastward rollback of the subduction zone into the Banda Embayment (Spakman and Hall, 2010; Hall, 2012) These authors proposed that during this event, the encircling buoyant continental crust of the Sula Spur and NW Australian margin would have resisted subduction, thereby forcing the slab to assume its extreme curvature and likely dictating its rollback velocity. 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. 3)
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