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ABSTRACT: Deep Structure of the North Aoba Intra-Arc Basin, Vanuatu

B. Pointoise, P. Charvis, J. Y. Collot

The intra-arc North Aoba basin (NAB) lies along the summit of the New Hebrides island arc in the southwest Pacific Ocean. Five ocean-bottom seismometers (OBSs) were deployed along two perpendicular lines within the basin. OBS refraction data show that basin fill is less than 4 km thick. P-wave velocities in the fill ranges from 1.7 to 4.5 km/s. Four record sections show a shadow zone that we believe results from a high velocity (3-4 km/s) rock layer that is interbedded with basin fill at a depth of l-1.5 km. This high velocity layer is evident in all but the westernmost parts of the basin, and we believe that the layer represents a major volcanic event.

The crust beneath the NAB is characterized by a 2-3 km thick upper layer with a velocity between 5.5 and 6.2 km/s. This layer overlies a 4-5 km thick rock unit having a velocity between 6.6 and 7.0 km/s. Pn arrivals observed on 3 OBSs indicate velocities higher than 8.2 km/s at a depth of 16 km.

These velocities indicate that the NAB overlies either thinned island arc crust or a trapped fragment of oceanic crust. We prefer the second possibility because: (1) the velocity structure of the NAB crust is similar to that under the North Fiji Basin, (2) the Moho under the NAB is flat and the crust-mantle transition at 20 km is sharp, and (3) observed Pn velocities (8.2 km/s) are higher than expected for a recent island arc.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990