Structural expression of forearc crust uplift due to subducting asperity

Abstract

New structural observations and mapping of reefal terraces, carried out both on the field and on satellite remote sensing data, indicate that Sumba Island is presently undergoing a large amount of extension, associated with a significant regional uplift. This crustal uplift may have been created by a major thrust emerging in the South of the island. The uplift, which partly accommodated the Australian plate–South West Banda Arc convergence, is associated with the general northeastward tilt of the island. The consequent anomalous positive topography along the southern coast of the island is being compensated by significant tectonic erosion along large-scale curvilinear normal faults in the southeastern half of the island. The most important expression of this gravitational collapse is located at the receding side of an advancing circular dome, showing striking similarities with accretionary wedges being affected by seamount subduction. The part of the forearc basin known as the Savu Basin is moderately deformed (mostly in its central part) and appears to act as a rigid buttress in the convergence between the Banda Arc and the Australian plate. As a result the convergence appears to be transferred northward within the actively-shortening back-arc domain, which goes from the north of the Flores Island to the southwest of the Timor block. The convergent plate boundary shows a transition between a stable domain (West of Sumba) and a tectonized domain (East of Sumba), the latter coinciding with the subduction of the outer Australian passive margin. The subduction of the ocean–continent boundary of the Austral-Indian plate below the Banda arc since the Lower Pliocene may have incorporated some crustal fragments in the plunging Benioff zone. Most likely, the integration of the stretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain, exhibiting inherited structures of the upper plate.