The structure of Eastern Indonesia: An approach via gravity and other geophysical methods

Gravity and other geophysical data have been collected, processed and interpreted for the southern Banda Arcs, Eastern Indonesia. Land and marine data, from Timor in the west to the Kai Islands in the east, have been combined to allow examination of the crustal structure and tectonic evolution of the 3M.yr. old collision between the northward migrating Australian Plate and the Banda Sea micro-plate(s). Following collision in the Timor region, approximately 60 km of continental and volcanic margin crust may have been subducted. Further convergence caused the steepening of the Benioff Zone, resulting in the rupturing of the continental margin along new subduction decollements, thereby progressively isolating the continental crustal units in the north of Timor from the later formation of the southern imbricate wedge. Shortening between the inner arc and the suture zone, situated off-shore of north Timor, was possibly by eastward translation of crustal blocks, thickening of the arc and tectonic erosion. Timor was dissected by a number of large left-lateral faults during the collision process giving rise to a number of variably sized, crustal blocks. The same process was, and probably still is, active in the Tanimbar Islands, which has a similar gravity field to Timor. The Kai Islands, to the north of the Tanimbars, are part of a large, displaced, continental crustal block, with a geology similar to Tanimbar and Timor. The gravity field from Timor around the Banda Arc to Tanimbar has a common Bouguer anomaly profile, with values of +50mGal over the Australian Shelf decreasing to 0mGal over the Timor - Tanimbar - Aru Troughs, before decreasing further to -30 to -50 mGal over the thickened crust of the forearc. Anomaly values in the north of the forearc create a steep, northerly positive, gradient reaching 180 to 200mGal at the junction of the continental and arc crusts. Gravity profiles over the Kai block also have a form common to the Banda Arcs except for the eastern margin where instead of values decreasing away from the Australian Shelf they rise steeply to 150 to 200mGal. This elongated high is probably due to thin crust related to present-day crustal extension in Eastern Indonesia. The curved form of the Banda Arcs probably results from the NNE-SSW pincer convergence of the Irian Jaya continental crustal block from the north, the NNE convergence of the Australian Continental margin from Timor to Tanimbar and the presence of the New Guinea Continental block to the east. The NNE-SSW convergence has led to ESE-WNW extension of the Banda Sea region, particularly in the Weber - Kai -Aru region, which has tectonically overprinted the earlier, arc/continental collision, compressional phase. Strike-slip faulting and associated rotation and translation of crustal blocks is at a maximum in this eastern region