Abstract
The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system. Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N150° for the Pleistocene phase of extension (2–0.1 Ma) and N170° for the late Pleistocene-Holocene phase of extension (0.1–0 Ma). The present-day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80–100 km above the Wadati-Benioff zone, the minimum depth for emplacement of arc magmatism. Scarce present-day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions. A N045° oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N-S linear volcanic feature of the Philippine Sea plate. Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross-backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80–100 km beneath the backarc basin. A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW-ESE 400-km-long volcanic feature of the Philippine Sea plate. We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor. This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts. The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity.