Geophysical Field Character And Tectonic Evolution Of Eastern Waters Off Taiwan Island

Taiwan is one of the typical mountain belts in the world, which is located near the junction of the Luzon arc, the Ryukyu arc, the Philippine plate and the Eurasia plate, and its formation and evolution are influenced by the formation and evolution of the Okinawa trough and the Manila trench. Because of its particular tectonic environment and the topographic character, Taiwan has been paid great attention.The studied area is eastern waters off Taiwan island(21.5°N~24.5°N, 122°E~ 126°E). Because of the reasons of geographic location, politics and history, eastern waters off Taiwan island is not studied systematically and all sidedly. With the development of marine geophysical reconnaissance technique and methods and the more evidences, it is important to find out the regional tectonics and the characteristics of the crust structure of eastern waters off Taiwan island and to discuss the deep crust structure and its formation and evolution.In this paper, data from FIO,SOA and NGDC are used to describe the geographic field character of eastern waters off Taiwan island, then tectonic interpretation is proceeded. On the basis of it, the typical tectonic units of eastern waters off Taiwan island are explained synthetically by both geological and geophysical methods. At last, the formation and evolution of eastern waters off Taiwan island is discussed in this paper. The results show:1. the gravitational field, magnetic field and seismic strata interpretation of eastern waters off Taiwan island all show that the tectonizations are complicated in this region, and all suggest that the deep tectonics are controlled by the subduction of plate and faults of the studied area.2. Gagua Ridge is the uplift of the broken oceanic crust as the role of faults. Basements of Gagua Ridge and fault-subsidence basin at both E&W sidesand of the Huatung basin are the oceanic crusts of the uplifts of the west Philippine sea plate. The reason for the uplift of the Gagua Ridge is that the belt between west Philippine basin and the Huatung basin leads to the collapse as the adjustment of the isostatic compensation effect in the course of Philippine plate drifting towards NW. The weak tectonic belt below the Gagua Ridge leads to the subduction of the eastern oceanic crust and the relative uplift of the western oceanic crust as the role of the squeeze.3. By the boundary of the Gagua Ridge, Eastern Ryukyu trench is different from western Ryukyu trench in tectonic environment. Although they show the subduction towards north, both the topographic truncation caused by the northern subduction of Gagua Ridge and the oceanic crusts at both E&W sides have the different geotectonic features, which cause the difference of the geophysical field characters, seismic strata structures and material compositions of seafloor sediments4. The formation and evolution of eastern waters off Taiwan island undergone the subduction of the Philippine plate towards NW, the subduction of the South China Sea towards east from early Miocene to middle Miocene, then the arc-continent collision between Luzon arc and Eurasia continent in the late Miocene and the subduction and consumption of Philippine plate from Pliocene to Holocene. Thus, the tectonics framework of eastern waters off Taiwan island is formed gradually.