Report on DELP 1984 Cruises in the Middle Okinawa Trough : Part V : Topography and Geology of the Central Grabens and Their Vicinity

Detailed survey by means of PDR, single channel water-gun and air-gun, multi-channel air-gun, proton and three component magnetometers and dredger revealed five segments of central grabens which are located in the axis of the Okinawa Trough of the study area. These grabens are aligned en echelon. The "Iheya" Graben is the most pronounced. No positive evidence was found for the existence of transform faults connecting the en echelon grabens. A basaltic ridge and knolls were newly found in the central axis of these grabens. From these central knolls and the ridge pieces of olivine basalt were dredged. These are island arc type high alumina basalts with K-Ar ages younger than 1 Ma. Some rhyolite, dacite and a small quantity of dacitic andesite younger than 1 Ma (K-Ar and fission track ages) were also dredged in the vicinity, indicating bimodal volcanism. Thus, the petrologic evidence does not support the existence of an oceanic crust but it suggests an extensional stress field in the middle Okinawa Trough. Based upon the evidence, the middle Okinawa Trough is concluded to be in a rifting stage of an incipient spreading process in the continental margin