Abstract
J. Makris Institut fiir Geophysik der Universit~it, W-2000 Hamburg, Bundesrepublik Deutschland Back-arc spreading in the Okinawa Trough is evidenced from elongated local ridges in central graben structures. It is believed to be in a nascent stage, to have initiated in the southern trough, and to migrate north at present. The transition between arcand back-arc volcanism is probably represented in the middle Okinawa Trough [3]. During cruises SO 55 and SO 56, several target areas were investigated in the Okinawa Trough (Fig. 1). Methane concentrations in seawater samples were determined on board using a modification of the technique described in [4, 5]. The method is based on inert gas stripping followed by gas chromatography. Shore-based helium and neon measurements were realized on a Clarke-type double-collecting mass
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