THE EVIDENCES OF THE BACKARC SPREADING AND DISCUSSION ON THE KEY ISSUES IN THE OKINAWA TROUGH

The typical tectonic morphology including submarine canyon,seamount,tectonic depression and tectonic platform has been described and analysed,based on the latest,high-precision multi-beam sounding data with full-coverage in the Okinawa Trough,as well as the three-dimensional submarine topographic maps.There are two kinds of tectonic morphologic features.One is approximately parallel to the axis of the Okinawa Trough and the other perpendicular to the axis.The mechanism of the formation of the tectonic morphology is inferred as: the collision between the Philippine plate and in turn,the Eurasian plate causes the backarc spreading and the tectonic morphology perpendicular to the axis of the Okinawa Trough,and in turn,the backarc spreading causes the linear seamounts and echelon tectonic depressions along the axis.The effects of plate collision together with backarc spreading created the above tectonic morphologic types in the middle and southern Okinawa Trough.Several key issues of tectonic researches in the Okinawa Trough are discussed: (1) The viewpoint of backarc spreading in the Okinawa Trough is supported by the evidences of tectonic morphology,multi-beam sounding data and seismic profiles and 3.5 kHz profiles from the Sino-French cruise in 1996,and the evidences of strong magnetic anomalies,high heat flow,strong volcanism,strong magmatism and frequent earthquakes.The directions,amount and rates of the tectonic movements with three stages are estimated quantitatively.(2) Several models of the tectonic evolution in the Okinawa Trough are summarized,and the evolution process of the Trench-Arc- Backarc basin system are discussed geodynamically.(3) The evolution of the tectonic movements with three stages in the Okinawa Trough is proposed based on the collected evidences,namely,the uplift and rift of Miocene,the passive extention from Pliocene to early Pleistocene,and active backarc spreading from late Pleistocene to Holocene.There are obvious differences among the directions,features and mechanism of the tectonic movements with three stages.The changes of extentions in the trough created the tectonic morphology with several kinds of strike,and recent spreading of the Okinawa Trough is becoming more active and faster.