Abstract

The tectonic and volcanological history of the central New Hebrides Island Arc since late Miocene time is reflected in the sedimentology of two Ocean Drilling Program (ODP) drill sites in the intra-arc North Aoba Basin. Sites 832 and 833 are located on the floor and the eastern margin, respectively, of a summit basin with water depth of over 3000 m. At each site, drilling reached depths greater than 1000 meters below seafloor (mbsf), and the cores represent the first opportunity to examine a thick section of intra-arc basin sediments. The cores are notable for thick sequences of volcanogenic sediments. Volcanism produced layers up to 42 m thick of unadulterated volcanic material, in sequences that grade upward from lapilli, to thick layers of volcanic sand, to fine ash and clay. The rapid erosion that accompanied volcanism and tectonic uplift of islands produced thick layers of breccia and conglomerate, featuring a wide variety of clasts of terrigenous and shallow-water origin, which serve as the basal layers of fining upward sequences. The sediments reflect four periods of island building. Volcaniclastic debris flows and coarse-grained turbidites, overlain by calcareous, fine-grained turbidites and hemipelagic sediments, were recovered from the deepest 240 m of Site 832. These sediments resulted from island building that I interpret as volcanism and uplift on Maewo Island in the late Miocene, with contributions of eroded sediment from Espiritu Santo Island. During the Pliocene, continuing erosion on Maewo Island and volcanism in the area contributed calcareous and volcanogenic debris to the hemipelagics, silty turbidites, and tephra collecting at both sites. Uplift and extension during the middle Pliocene are suggested by normal faulting in the sediments, and by an unconformity at Site 832. From latest Pliocene to early Pleistocene times, renewed uplift and volcanism on surrounding islands revived debris-flow deposition. Late Pleistocene sedimentation was dominated by volcanic ashflows as Central Chain volcanoes built depositional aprons. Tephra falls interbedded with hemipelagic sediments have dominated sedimentation in recent times.

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