Abstract
THE drilling sites for leg 34 of the Deep Sea Drilling Project (DSDP) were in the Nazca Plate (Fig. 1), which was formed by the spreading from two ridges, the fossil Galapagos Rise and the currently active East Pacific Rise. Spreading half rates on the East Pacific Rise vary from 8 to 10 cm−1 (refs 1–3) and are the highest on any presently active ridge. Fast spreading is associated with subdued basement topography, a situation that enables structural provinces within the Nazca Plate to be clearly delineated. The near equatorial latitudes of the sites with poor magnetic anomaly development mean, however, that the determination of the ages of sites 319 and 320 with respect to the magnetic reversal time scale will be precluded. Table 1 shows the basement ages for the sites, estimated from the overlying sediments.
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