r I I â Ç Structural and Kinematic Evolutions of the Okinawa Trough Backarc Basin Jean-Claude Sibuet, Shu-Kun Hsu, Chuen-Tien Shyu, and Char-Shine Liu 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, [o 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 occur- ring 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 conti- nental 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 middte 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 Jean-Claude Sibuet and Shu-Kun Hsu . IFREMER, Centre de Brest, 29280 Plouzané, France. Chuen' Tien Shyu and Char-Shine Liu . Institute of Oceanography, National Taiwan University, Taipei, Taiwan, Republic of China. Backarc Basins: Tectonics and Magmatism, edited by Brian Taylor, Plenum Press, New York, 1995.