Marine Geosciences

F. Klingelhoefer, T. A. Minshull, D. K. Blackman, P. Harben, and V. Childers

The study of the internal structure volcanic islands is important for our understanding of how such islands form and how the lithosphere deforms beneath them. Studies to date have focused on very large volcanic edifices (Hawaiian Islands, Canary Islands, Marquesas, Reunion), but less attention has been paid to smaller islands, which are more common. Ascension Island, a 4 km high volcanic edifice with a basal diameter of 60 km, is located in the equatorial Atlantic (8 deg. S), 90 km west of the Mid-Atlantic Ridge on 7 Ma oceanic lithosphere. Geological evidence suggests that the island has been recently active.  Gravity modeling suggests that the effective elastic thickness is low, with an Airy type isostatic compensation, but gravity data alone cannot distinguish the effects of isostatic depression of the crust from those of magmatic underplating.  We present initial results from a seismic tomographic study of the island conducted during a 4 day geophysical survey aboard RRS James Clark Ross in May 1999.  Seismic tomography can in principle distinguish the different possible interpretations of gravity anomalies and identify any low velocity zone associated with partial melt, if present.  A further objective was to determine how T-phases in the ocean sound channel interact with the island slopes and propagate to monitoring stations on the island, by using known, calibrated acoustic sources.  The seismic source was an 11-airgun array with a total volume of 6186 cu. in. (101 l).  A total of around 3600 shots from this array were recorded on 4 ocean bottom hydrophones, 30 sonobuoys and 10 land stations.  Preliminary results from two-dimensional travel-time inversion of a 90 km long NE-SW profile indicate good ray penetration including energy undershooting the volcanic edifice, so velocites for the entire structure will be constrained.

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