The Okinawa Trough is a marginal sea that opened when the Ryukyu Arc was rifted away from the Asian mainland. Geophysical surveys in the southwestern portion of the Okinawa Trough have delineated two grabens formed in the basement and overlying 2.5 seconds of sediment. One graben extends along the trend of the basin from 124°E to 125°E longitude, to the east and west of which the well-developed graben structure is replaced by a more dispersed pattern of normal faults. About 40 km north of this broadening of the fault pattern, a second graben is found which continues to the northeast where it, too, is replaced by basin-wide normal faulting.

The sediments in the basin can be divided into two formations. Widespread deformation by folding and faulting is found in the lower unit, whereas the unfolded upper unit is primarily deformed by the faulting associated with the graben. A tentative scheme for the evolution of the graben would have the lower unit deposited in the basin while extension was active, with the upper unit deposited after the main phase of spreading had ceased and as a result, subject only to limited deformation. Faulting in the grabens is presently active although extension is occurring at a very slow rate. These faults have offsets which in many cases increase with depth to at least the bottom of the upper unit. The dip of the bedding beneath the graben also increases with depth, indicating that incipient sp eading has been occurring since the beginning of the deposition of the upper unit (at rates on the order of a few mm/yr).

An east-west trending ridge between the two grabens was dredge-sampled and found to be composed of biotite-rich quartz diorite and metamorphosed pillow basalts. It is thought to be a silver of the Ryukyu Arc stranded in the basin during the rifting event which formed the Okinawa Trough.

In spite of the extremely slow spreading rate in the Okinawa Trough, two new heat flow measurements of 126 and 443 mW/sq m, in addition to previously reported values, show the heat flow to be high in the basin.