Crustal structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland

Results from the Reykjanes-Iceland Seismic Experiment (RISE) show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63°N, from 12.7 km thick at o Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9-11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (≥7.5 km s^-1), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis tnan beneath older crust, consistent with lithospheric cooling with age.