LATE MIOCENE OBDUCTION AND MICROPLATE MIGRATION AROUND THE SOUTHERN BANDA SEA AND THE CLOSURE OF THE INDONESIAN SEAWAY

Miocene shallowing and closure of the Indonesian Seaway between the In dian Ocean and the Pacific is related to plate-tectonic developments a t the southern margins of the Banda Sea. Ultramafites, mainly Iherzoli te, closely associated with quartzo-pelitic metamorphic rocks, on the northern coast of Timer and on smaller islands in the southern Outer B anda Arc and also on southwest Seram in the northern Banda Are, are fr agments of Middle Miocene oceanic lithosphere obducted in the Late Mio cene on sole rock of Australian continental origin. Initially cool sol e rock was dynamo-thermally metamorphosed by heating from above by ver y hot, overriding oceanic lithosphere. Temperature equilibration betwe en the sole and the Kaibobo lherzolitic complex (southwest Seram) took place at about 740 degrees C and 4-5 kbar. Anatectic granitic magma f ormed in the sole and intruded in the ultramafite. Ar-40-Ar-39 determi nations on muscovite and biotite from the sole and anatectic granite i ndicate that the system cooled through 400 degrees C 6.0 Ma ago and th rough 320 degrees C 0.5 Ma later. P-T-t modelling of the obduction/pos t-obduction exhumation curve, which is based on thermobarometry of the Kaibobo ultramafic complex and its sole, suggests that obduction star ted about 9.5 Ma, emplacement was completed about 8 Ma ago and that fa st vertical movements continued until about 7 Ma. The obduction of the Kaibobo lherzolitic complex actually took place in an area northeast of the current position of Tanimbar, where Seram (as a micro-continent al Australian fragment) lay 8 Ma ago, as it migrated northward with th e Australian Plate. The reconstructed obduction site of the southwest Seram ultramafites and those between northern Timer and Tanimbar deter mine an 850 km-long, ENE-trending zone along the southern margin of th e Banda Sea. As the obduction of the lherzolitic complex on northern T imer also took place 8 Ma ago and cooling to 300 degrees C occurred 5. 5 Ma ago, a similar time setting to Kaibobo is inferred. It is argued that oceanic lithosphere was being formed in the Early Miocene (about 6 Ma prior to the start of obduction) along this zone in the southern Banda Sea. The lherzolitic composition and petrographical and geochemi cal details suggest that the obducted ultramafites are remnants of wea kly depleted, lherzolitic lithosphere, formed close to a passive margi n by processes of very slow spreading. This notion and published palae omagnetic data relating to the Neogene positions of Timer and the Aust ralian continent as well as the types and ages of magmatic rocks in th e Banda volcanic arc lead to a model for the Middle and Late Miocene g eotectonic developments in the southern Banda Sea between the Eurasian and the Australian plates. The proposed model features interarc sprea ding in a short-lived, interarc Timer Plate (16-9.5 Ma) just north of the line Timor-Tanimbar, lengthening of the east Sunda volcanic are by the creation of the volcanic Banda Arc, obduction-emplacement and exh umatian of ultramafic complexes, and migration of the Seram microconti nent. The model is in good agreement, in space and in time, with the 9 .9-7.5 Ma history of shallowing and eventual closure of the Indonesian Seaway, as inferred in the literature from developments in biogeograp hic patterns and vertical thermal structure evolution of Miocene equat orial Pacific surface waters.