Structural and Kinematic Evolutions of the Okinawa Trough Backarc Basin

Abstract

Refraction data acquired in the Okinawa Trough show that the crust is of continental origin and that its thickness increases from 10 km in the southern Okinawa Trough to 30 km in the northern Okinawa Trough. The Okinawa Trough is still in a rifting stage. In the southern Okinawa Trough, magnetic anomalies are linked either to limited arc volcanic intrusions located on the northern side of the Ryukyu arc, to the axis of the main depressions where basaltic intrusions with volcanic arc affinities are present, or to volcanic products lying at different depths within the thinned continental crust. A link among partial melting occurring just above the Benioff zone, ascent of volcanic products through extended continental crust, and the amount of crustal extension are proposed. We also suggest that both arc and backarc basin volcanism are derived from the same source located above the Benioff zone. Melt preferentially rises through the fissures and faults created by extension in the continental crust. The total amount of extension across the Okinawa Trough has been estimated from refraction and gravity data. It decreases slightly from 80 km in the southern Okinawa Trough to 74 km in the northern Okinawa Trough. Parameters of rotation have been established for the entire opening of the Okinawa Trough and for the three phases of extension already identified: (1) The late Pleistocene to Recent phase of extension is characterized by normal faults with vertical offsets of a few meters changing progressively in direction along the Okinawa Trough. The amount of extension is about 5 km in the middle Okinawa Trough. (2) The early Pleistocene phase of extension is characterized by large tilted blocks which affect late Pliocene-early Pleistocene sediments. Directions of these normal faults change progressively along the Okinawa Trough. Extension in the northern Okinawa Trough is estimated as 25 km. The difference in azimuth of the normal faults for these two tectonic phases increases toward the northern Okinawa Trough. (3) The early Miocene phase of extension is poorly characterized from geological data but has been calculated from parameters of other rotations. The corresponding pole of rotation is located 1500 km northeast of Kyushu; 50 km of extension in the northern Okinawa Trough and 75 km in the southern Okinawa Trough are expected. The proposed kinematic evolution of the Okinawa Trough differs considerably from the previous reconstructions and raises numerous questions, as for example, the interpretation of paleomagnetic measurements obtained in the southeastern Ryukyu Islands. The bending of the Okinawa Trough developed since the onset of the backarc basin extension, except for the southwestern portion of the Okinawa platelet whose pronounced bending is due to the collision in Taiwan.