Non-Isostatic Slab-Support for Island Arcs and Back-Arc Basins

Abstract

Most of active island arcs are characterized by large positive gravity anomalies existing along the central ridge of arc, whereas the accompanying deep-sea trenches are associated with negative free-air anomalies. Crustal and upper mantle structure revealed by seismological investigation has indicated that a pair of positive and negative gravity anomalies are non-isostatically supported by a solid slab of lithosphere being subducted beneath the arc and trench system. Some active back-arc basins and back-arc rifts such as the Lau Basin, Manus Basin and Okinawa Trough are also non-isostatically held by the underlying slab. In contrast the actively spreading Mariana Trough has water depths greater than 3,000 m with nearly zero free-air anomalies. This seems to be caused by sharp bending of the deep-focus earthquake zone beneath the Mariana Trough in such a way that no slab exists under the back-arc region. Extinct back-arc basins such as the West Philippine Basin has water depths greater than 6,000 m with zero free-air anomalies except for minor topographies like seamounts and depressions with horizontal sizes smaller than 100 km. Free-air anomalies of the Shikoku Basin are zero except for its eastern limb where the Izu-Bonin subduction slab has much thermal and mechanical influences. In a geological period around 15 Ma the subducted slab beneath the NE Honshu, Japan is presumed to have been in a situation similar to that of the present Mariana Trough. Most of the present NE Honshu subsided to the sea bottom deeper than 1,000 m at that time, as revealed by widespread marine deposits, several boreholes and hydrothermal ores. Timing of onset of subsidence seems to coincide with start of opening of the backarc basins.