Late Miocene obduction and microplate migration around the southern Banda Sea and the closure of the Indonesian Seaway

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• 作者：Linthout ; Kees ; Helmers ; Henk ; Sopaheluwakan ; Jan
• 关键词：Miocene plate tectonics ; interarc spreading ; obduction ; Banda Arc ; Indonesian Seaway
• 刊名：Tectonophysics
• 出版年：1997
• 出版时间：November 15, 1997
• 年：1997
• 卷：281
• 期：1-2
• 页码：17-30
• 全文大小：1.25 M

文摘

Miocene shallowing and closure of the Indonesian Seaway between the Indian Ocean and the Pacific is related to plate-tectonic developments at the southern margins of the Banda Sea. Ultramafites, mainly lherzolite, closely associated with quartzo-pelitic metamorphic rocks, on the northern coast of Timor and on smaller islands in the southern Outer Banda Arc and also on southwest Seram in the northern Banda Arc, are fragments of Middle Miocene oceanic lithosphere obducted in the Late Miocene on sole rock of Australian continental origin. Initially cool sole rock was dynamo-thermally metamorphosed by heating from above by very hot, overriding oceanic lithosphere. Temperature equilibration between the sole and the Kaibobo lherzolitic complex (southwest Seram) took place at about 740°C and 4-5 kbar. Anatectic granitic magma formed in the sole and intruded in the ultramafite. ⁴⁰Ar-³⁹Ar determinations on muscovite and biotite from the sole and anatectic granite indicate that the system cooled through 400°C 6.0 Ma ago and through 320°C 0.5 Ma later. P-T-t modelling of the obduction/post-obduction exhumation curve, which is based on thermobarometry of the Kaibobo ultramafic complex and its sole, suggests that obduction started about 9.5 Ma, emplacement was completed about 8 Ma ago and that fast 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-continental Australian fragment) lay 8 Ma ago, as it migrated northward with the Australian Plate. The reconstructed obduction site of the southwest Seram ultramafites and those between northern Timor and Tanimbar determine an 850 km-long, ENE-trending zone along the southern margin of the Banda Sea. As the obduction of the lherzolitic complex on northern Timor also took place 8 Ma ago and cooling to 300°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 geochemical details suggest that the obducted ultramafites are remnants of weakly depleted, lherzolitic lithosphere, formed close to a passive margin by processes of very slow spreading. This notion and published palaeomagnetic data relating to the Neogene positions of Timor and the Australian continent as well as the types and ages of magmatic rocks in the Banda volcanic arc lead to a model for the Middle and Late Miocene geotectonic developments in the southern Banda Sea between the Eurasian and the Australian plates. The proposed model features interarc spreading in a short-lived, interarc Timor Plate (16-9.5 Ma) just north of the line Timor-Tanimbar, lengthening of the east Sunda volcanic arc by the creation of the volcanic Banda Arc, obduction-emplacement and exhumatian of ultramafic complexes, and migration of the Seram microcontinent. 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 biogeographic patterns and vertical thermal structure evolution of Miocene equatorial Pacific surface waters.