GEODYNAMICS OF THE TAIWAN ARC-ARC COLLISION

Hsu and Sibuet (1995), on the basis of an overview of the satellite-de rived marine gravity anomalies, postulated that the Ryukyu subduction zone extended before the formation of Taiwan a few hundreds kilometres south of its present-day termination, and that Taiwan resulted from a n arc-arc collision rather than from an arc-continent collision. An an alysis of the structure and timing of rifting in the basins of the Sou theast Asia continental shelf offshore and onshore Taiwan shows that t hey are located within four belts parallel to the main China shoreline . Rifting occurred at the same time within basins belonging to each of these four belts and becomes younger oceanward for each belt. As a fi rst approximation, the four rifting phases occurred during Paleocene, Eocene, Oligocene to Early Miocene and early Middle Miocene times to P resent. Ridges with volcanic products are present between these belts. They seem to be the same age as basins located immediately northwest. We interpret these basins and associated ridges as relict backarc bas ins and arcs of the Ryukyu subduction system which were successively a ctive since the early Tertiary. The geographic distribution of basins and ridges suggests that the Ryukyu subduction zone extended from Japa n to southwest Taiwan from early Tertiary to Early Miocene times. Duri ng the early Middle Miocene, the southeast portion of the subduction z one facing the Taiwan basin and the future island Tainan became inacti ve. Southwest of the Tainan basin, the Pearl River basins are tensiona l basins formed during the rifting of the northern South China Sea mar gin. Consequently, the geology of the Southeast Asia continental shelf supports the existence of a former subduction zone with which the Luz on arc entered into collision in the Late Miocene. The kinematic evolu tion of the Southeast Asia region is compatible with such constraints. Such a detailed kinematic evolution of the collision between the Luzo n are and the former Ryukyu subduction zone is proposed both in plan v iews and in cross-sections. Collision started with the compression and uplift of the Hsuehshan trough backarc basins, where the continental crust and lithosphere were thin and weak, followed by the compression and uplift of the Luzon and Ryukyu arcs. The Lichi and Kenting melange s are explained in the framework of the arc-arc collision model.