Magnetostratigraphy of the North Aoba Basin

Abstract

The goal of Ocean Drilling Program (ODP) Leg 134 was to study the tectonic history of the central New Hebrides Island Arc system (Vanuatu). ODP Sites 832 and 833 were drilled in the intra-arc North Aoba Basin. Volcanic sandstone and siltstone and clayey silty limestone make up most of the recovered basin-fill sediment. The magnetic susceptibility in the sediment is high, varying on average from 0.005 to 0.03 S1 because of the high volcanic influxes derived either from erosion of volcanic rocks from nearby islands or from the output of active volcanoes. Determination of the timing of the tectonic and volcanic events recorded in the sedimentary sequences requires the establishment of a high-resolution stratigraphic framework. The magnetostratigraphic data presented here are based on a combination of shipboard whole-core pass-through cryogenic measurements, made at 5-cm intervals on all archive core halves after alternating field (AF) demagnetization (usually 10 mT), and shipboard and post-cruise progressive AF and thermal demagnetization analyses of 466 (Site 832) and 715 (Site 833) discrete samples. The volcanic-rich sediment yielded complex remanent magnetizations having widespread drilling-induced magnetic overprints. Detailed AF and thermal demagnetizations of discrete samples indicate that the volcanic sandstones have a stable, normal-polarity magnetization that is probably of viscous origin acquired during the late Brunhes Chron. At Site 832, in the north central part of the basin, a major stratigraphic unconformity is found at 700 meters below sea floor (mbsf). Above the breccia unit that defines the unconformity, numerous slump structures have disturbed primary depositional remanent magnetizations, so a reliable magnetostratigraphy is not possible from 150 to 700 mbsf. The Brunhes/Matuyama boundary was not identified. Below the unconformity, a reliable magnetostratigraphic record from 700 to 825 mbsf was obtained in silty limestone. In conjunction with biostratigraphic data, correlation of the magnetostratigraphic sequence with the geomagnetic polarity time scale indicates that the sediments were deposited from the late Miocene (Chron 5) to the Pliocene/Pleistocene boundary. The inteΦretation of the magnetostratigraphic record in turn suggests that the volcanic breccia unit accumulated during early Pleistocene time. At Site 833, located on the eastern flank of the basin, the Brunhes/Matuyama boundary and the Jaramillo Subchron can be recognized from 200 to 260 mbsf. From 260 to 650 mbsf, numerous volcanic sandstone layers, which have relative high magnetic susceptibilities (0.01-0.03 S1), record a predominant normal polarity of possibly viscous origin that makes the establishment of a reliable magnetostratigraphic record difficult. The Mammoth subchron tentatively has been recognized from 650 to 700 mbsf. The magnetostratigraphy at Site 833 implies a sedimentation rate of about 10 cm/k.y. during the middle Pliocene, which is in contrast to the low sedimentation rate (about 1.5 cm/k.y.) estimated at Site 832 for the same period.