Paleomagnetism and Tectonics of the D'Entrecasteaux Collision Zone (Sites 827-831)

Abstract

During Leg 134 of the Ocean Drilling Program 11 holes were drilled at 5 sites to study the process of subduction of oceanic ridges in the d'Entrecasteaux Zone and seamounts beneath the New Hebrides Island Arc. The upper 10 to 100 m of sediment are composed of volcanic silts characterized by high magnetic susceptibilities (>IO~2 S1). The magnetic susceptibility reflects the amount of volcanogenic materials in the silts. The large differences in magnetic susceptibility between sites suggest that the sediment source is related to the uplift and erosion of Espiritu Santo Island and that currents and topography control the location and rate of sediment deposition as indicated by the very low proportion of volcanogenic sediment on top of the Bougainville Guyot. Only a normal component of magnetization was identified in the volcanic silts indicating deposition of the sediment during the Brunhes Chron. We were unable to determine a magnetostratigraphy in the deformed substratum of the New Hebrides Island Arc (Sites 827, 829, and 830). At Site 828, located on the North d'Entrecasteaux Ridge, pass-through whole-core cryogenic and discrete sample measurements define a straightforward magnetostratig raphy in a 25-m sequence of nannofossil-chalk sediment below the Pleistocene silts. Correlations with the geomagnetic reversal time scale and biostratigraphic markers indicate that these pelagic sediments were deposited during the Oligocene from 30 to 36 Ma. In 35 samples of Oligocene age, a reliable magnetization was observed with a mean inclination of 37.4°. This mean inclination corresponds to a paleolatitude of 20.9°S, which is about 10° farther north than the expected latitude based upon the hotspot reconstruction model. Paleocene to Eocene volcanic rocks were recovered at Site 831 below the carbonate cap of the Bougainville Guyot, but the brecciated nature of the volcanic rocks prevented the determination of a paleolatitude.