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

摘要

It has long been argued that Seram’s Neogenerntectonic evolution was dominated by thrusting andrncrustal shortening due to collision of the northernrnBanda Arc with the Australian passive continentalrnmargin. This collision model considers voluminousrnperidotites, which crop out in western and centralrnSeram and on Ambon, to be part of a largerndismembered ophiolite that was allegedly obductedrnfrom the Banda Sea, with associated granitoidsrnexplained as the melting beneath the ophioliternduring emplacement.rnHowever, new field, geochronological, andrnthermobarometric data question this model, withrnpotentially important consequences for therninterpretation of sedimentary basin developmentrnboth onshore and in the Seram Trough. Basedrnprimarily on structural mapping of western Seram,rnwe interpret the peridotites to representrnsubcontinental lithospheric mantle (SCLM) beneathrnSeram, which was rapidly exhumed beneath lowanglerndetachment faults during a period of extremerncrustal extension.rnMantle exhumation is linked to widespread crustalrnanatexis and the formation of an extensivernmigmatite complex—the Kobipoto Complex—rnwhich records ultrahigh (UHT) temperatures ofrn~950℃ at ~8 kbar pressure (25–30 km depth).rnGranitic rocks on Seram and Ambon, and unusualrncordierite + garnet dacites (ambonites) also exposedrnon Ambon, all preserve evidence for theirrngeneration in the Kobipoto Complex atrntemperatures far in excess of that predicted by thernophiolite model. SHRIMP U–Pb zircon dating hasrnrevealed that melting and UHT metamorphism ofrnthe Kobipoto Complex was underway by 16 Ma,rnwith granitoids produced episodically across Seramrnand Ambon from 16 Ma (Kobipoto Mountains,rncentral Seram) until 3.5 Ma (Ambon). We propose that Seram has experienced a history ofrnextreme extension by detachment faulting that isrnbest explained by eastward rollback of the Bandarnslab into the Banda Embayment since 16 Ma.