NEOGENE EXTENSION ON SERAM: A NEW TECTONIC MODEL FOR THE NORTHERN BANDA ARC

摘要

It has long been argued that Seram’s Neogene tectonic evolution was dominated by thrusting and crustal shortening due to collision of the northern Banda Arc with the Australian passive continental margin. This collision model considers voluminous peridotites, which crop out in western and central Seram and on Ambon, to be part of a large dismembered ophiolite that was allegedly obducted from the Banda Sea, with associated granitoids explained as the melting beneath the ophiolite during emplacement. However, new field, geochronological, and thermobarometric data question this model, with potentially important consequences for the interpretation of sedimentary basin development both onshore and in the Seram Trough. Based primarily on structural mapping of western Seram, we interpret the peridotites to represent subcontinental lithospheric mantle (SCLM) beneath Seram, which was rapidly exhumed beneath low- angle detachment faults during a period of extreme crustal extension. Mantle exhumation is linked to widespread crustal anatexis and the formation of an extensive migmatite complex—the Kobipoto Complex— which records ultrahigh (UHT) temperatures of ~950°C at ~8 kbar pressure (25-30 km depth). Granitic rocks on Seram and Ambon, and unusual cordierite + garnet dacites (ambonites) also exposed on Ambon, all preserve evidence for their generation in the Kobipoto Complex at temperatures far in excess of that predicted by the ophiolite model. SHRIMP U-Pb zircon dating has revealed that melting and UHT metamorphism of the Kobipoto Complex was underway by 16 Ma, with granitoids produced episodically across Seram and Ambon from 16 Ma (Kobipoto Mountains, central Seram) until 3.5 Ma (Ambon). We propose that Seram has experienced a history of extreme extension by detachment faulting that is best explained by eastward rollback of the Banda slab into the Banda Embayment since 16 Ma.