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

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 hightemperature 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 AlMg-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 MioPliocene granitic diatexites, or "cordierite granites", which crop out on Ambon, western Seram, and in the Kobipoto Mountains and incorporate abundant schlieren of spineland 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 greenschistto 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 lowangle 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.

Mantle structure and tectonic history of SE Asia

“…10). The slab has the form of a lithospheric fold that plunges west, that has been described as resembling a spoon-shape or the prow of a boat (Pownall et al, 2013). There is a prominent tear in the slab on its north side, beneath Buru and west Seram, that narrows eastwards.…”

UHT metamorphism on Seram, eastern Indonesia: reaction microstructures and P–T evolution of spinel‐bearing garnet–sillimanite granulites from the Kobipoto Complex

“…25–23 Ma (Hall, ), continues to drive the tectonics of Eastern Indonesia (Bock et al ., ; Nugroho et al ., ). Several active subduction zones currently facilitate this convergence, but there is no consensus on the exact mechanisms that led to the present‐day tectonic configuration (see previous discussions by Hall & Wilson, ; Milsom, ; Hinschberger et al ., ; Harris, ; Spakman & Hall, ; Hall, , ; Pownall et al ., ; Zahirovic et al ., ). Conflicting models often differ in their interpretation of the highly curved Banda Arc (Fig.…”

“…Although some authors’ models require the subduction of two separate slabs to explain this geometry (e.g. Cardwell & Isacks, ; McCaffrey, ; Hinschberger et al ., ), others have argued that the Banda Arc's tight curvature resulted from subduction rollback of a single slab (Hamilton, ; Hall, , , , ; Milsom, ; Spakman & Hall, ; Pownall et al ., , ). Recent field‐based geological studies on the islands of Seram and Ambon (Watkinson et al ., ; Pownall et al ., , ; Pownall & Hall, ) have shown evidence indicating a dominantly extensional Neogene tectonic setting, thereby substantiating earlier proposals by Hall & Wilson () and Spakman & Hall () for Banda slab rollback having driven extreme lithospheric extension across the northern part of the arc.…”

“…Pownall et al . (, ) reported a new discovery of granulites that contain coexisting spinel + quartz from the Kobipoto Mountains of Seram in eastern Indonesia (Figs & ), inferred using phase equilibria modelling to have reached peak metamorphic conditions of ~950 °C and ~8 kbar. Sensitive high‐resolution ion microprobe U–Pb dating of zircon rims interpreted by Pownall et al .…”

“…Importantly, the Seram granulites are interpreted to be one of very few examples to have formed in a modern tectonic setting for which the mechanism that drove high‐ T metamorphism is preserved in the recent geological record. Field relations on Seram (Pownall et al ., ) demonstrate that anatexis and granulite facies metamorphism were driven by extreme lithospheric extension, linked to slab rollback of the Banda Arc (Spakman & Hall, ), that caused the exhumation and emplacement of hot subcontinental lithospheric mantle (SCLM) against the mid crust. This paper aims to expand on the work of Pownall et al .…”