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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. INTRODUCTION The primary objective of Leg 134 was the study of the collisional tectonic history of the central New Hebrides Island Arc system (Vanuatu) in the southwestern Pacific Ocean. Eastward subduction of the Australia-India Plate beneath the New Hebrides Island Arc has been active since the late Miocene. The complex tectonics of the New Hebrides Island Arc involve the d'Entrecasteaux zone (DEZ), which is composed of two aseismic ridges that are colliding with the central part of the arc (Collot, Greene, Stokking, et al., 1992). Rapid subduction of the North d'Entrecasteaux Ridge (2500 m high) uplifts a group of islands in the frontand backarc, with simultaneous subsidence of the intra-arc Aoba Basins (Collot et al., 1985). In the North Aoba Basin, Sites 832 and 833 penetrated more than 1000 m of upper Miocene to Holocene volcaniclastic and carbonate sediment, as well as basaltic sills. Magnetic directions in these two sites were obtained by measuring archive half sections using the shipboard pass-through cryogenic magnetometer measurements and by progressive thermal and AF demagnetization experiments of discrete samples on shore. More than 1000 discrete samples were selected to determine the characteristic directions. The measurement of discrete samples acts as a valuable calibration of the results obtained using the pass-through 1 Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994. Proc. ODP, Sci. Results, 134: College Station, TX (Ocean Drilling Program). 2 Earth Science Department, U.C. Santa Cruz, CA 95064, U.S.A. 3 ORSTOM, BP 48, 06230 Villefranche-sur-Mer, France. 4 ODP, Texas A&M University, College Station, TX 77845, U.S.A. cryogenic magnetometer onboard the ship. The magnetic polarity patterns contained within cores from these two sites, when unambiguously correlated with the sequence of magnetic polarity chrons of the geomagnetic polarity time scale (GPTS) (Berggren et al., 1985), should serve as both an important tool for estimating age and for stratigraphic correlations between the two sites and as additional data that may help improve the calibration of the geomagnetic polarity chrons. In this paper we describe and discuss the paleomagnetic analysis of the sediment and volcanic rocks recovered from these two sites, focusing on the intervals that provided the most reliable data, and then interpret the polarity sequences used in conjunction with biostratigraphy and the GPTS to present a magnetostratigraphy for the two basin sites. SITE SETTING AND STRATIGRAPHY The site locations and tectonic history of the North Aoba Basin have been documented in detail in the 134 Initial Reports volume and appropriate papers (Collot, Greene, Stokking, et al., 1992; Greene et al., this volume) and are only briefly summarized here. Site 832 is located at 14°47'S, 167°34'E in a water depth of 3089 m below sea level (mbsl). This site is centered on the flat basin floor of the North Aoba Basin, approximately 50 km northeast of the northern tip of the Queiros Peninsula of Espiritu Santo Island, and 45 km south of the active volcanic island of Santa Maria (Fig. 1). At Site 832, two holes were drilled that penetrated more than 1100 m of basin-fill deposits. Seven lithostratigraphic units were identified at Site 832. The uppermost lithostratigraphic Unit I is a 385.6-m-thick Pleistocene sequence of sandy to clayey volcanic silt with interbedded volcanic

Magnetostratigraphy of the North Aoba Basin