ABSTRACT: Results of ODP Leg 125 Drilling in the Mariana/Izu-Bonin Forearcs
P. Fryer, J. A. Pearce, L. Stokking
ODP Leg 125 drilled a total of nine sites in the Mariana and Izu-Bonin forearcs, the regions between the active volcanic arc and the trench axis. Six sites were located on or adjacent to serpentine seamounts, four of these (Sites 778-781) on Conical Seamount in the Mariana forearc, and two others (783 and 784) on the Torishima Forearc Seamount in the Izu-Bonin forearc. The remaining sites (Sites 782, 785, and 786) were drilled into volcanic sequences along the eastern edge of the Izu-Bonin forearc basin.
The principal results of the drilling were to achieve (1) the recovery of the first evidence for Pliocene or younger magmatic activity in an extant intraoceanic forearc terrain; (2) the first deep penetration of the Eocene basement of the Izu-Bonin outer-arc high to recover 650 m of boninite flows and hyaloclastite, andesite-dacite flows, breccias, sills, and dikes; (3) the confirmation that some forearc serpentine seamounts can form by flows of clast-bearing serpentine mud from a central conduit, as do mud volcanoes; (4) the discovery of mafic clasts within the serpentine mud flows that have both IAT and MORB affinities, that are metamorphosed in the low to moderate temperature/pressure regimes: (5) the confirmation of high-pH, low-chlorinity fluids at shallow levels near the summit f the seamount which probably originated beneath the forearc and are subduction-related; (6) the recovery of complex hydrocarbon-rich gases also of probable subduction related origin within the Mariana serpentine seamount; and (7) the identification of numerous ash layers within the Izu-Bonin forearc basin that indicate peaks of volcanic activity in the Eocene-Oligocene and from the late Miocene to the Holocene.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990