Okinawa Trough: Origin of a back-arc basin

Abstract

The sequence of back-arc basin development in the southwestern part of Okinawa Trough has apparently been crustal thinning, involving normal faulting and fault block rotation, rifting, and crustal separation with associated magmatic intrusion. Seismic-refraction data shows a mantle depth of about 15 km beneath the central rift of Okinawa Trough while gravity data suggests that the crust thickens away from the central region of Okinawa Trough towards both the Ryukyu Arc and the continental shelf. Seismic-reflection profiles across Okinawa Trough reveal a generally continuous, undisturbed upper sediment section beneath the trough floor, unconformably overlying a highly faulted lower section which has a velocity of 4.9 km/sec. Exceptions to this structural relationship are fault grabens cutting the upper section along the axial and southeastern regions of the trough and normal faults at the extreme margins of the trough. Correlation of similar structural relationships on Taiwan suggests that extension produced normal faults in the lower section during Miocene time creating the initial expression of Okinawa Trough, and that the unconformity between the upper and lower sections is of Miocene age. Linear magnetic anomalies and high-velocity crustal rocks associated with the central trough region are interpreted as due to Pliocene to Recent magmatic intrusion associated with crustal separation. Offsets in the magnetic lineations indicate that the crust is separating along a series of short spreading centers and transform offsets.