Petrophysical modeling of high seismic velocity crust at the Namibian volcanic margin

Abstract

Two recent onshore-offshore seismic transects across the Namibian passive margin reveal a thick (to 20 km) prism of material at the base of the crust with high seismic velocity (Vp = 7.1-7.6 km/s). To better understand the nature of this material and the processes that formed it, we estimate the bulk chemical composition of the high-velocity crust by relating its seismic velocity to a petrophysical model that links basalt composition and conditions of partial melting of peridotite. Observed average seismic velocities in the igneous crust are consistent with basaltic material with ∼14-18 wt% MgO. This conclusion is not affected by the presence of cumulate minerals because it integrates over the full thickness of the body; however, the highest Vp values of 7.6 km/s are consistent with velocities expected for cumulate minerals produced by fractional crystallization of a 14%-18% MgO parental melt. The subsolidus growth of garnet is unlikely to be a significant factor for the crustal velocity above the Moho depth of 30 km. Garnet growth in a magnesium-rich basaltic crust can be expected to limit the crustal thickness to ∼30 km because bulk densities at deeper levels may exceed those of the peridotite mantle. The relationship between MgO content of partial melts and the potential temperature of a fertile peridotite source suggests that the estimated 14-18 wt% MgO basalts were generated from mantle at ∼1440-1560 °C potential temperature, which may be a good estimate for the potential temperature of the ancestral Tristan mantle plume at the Namibian margin. The igneous crust has the greatest volume, highest MgO contents, and highest inferred mantle potential temperatures at the location of the northern transect, which is closest to the Walvis Ridge hotspot trace. The mantle potential temperature estimated for the southern transect is 50-100 °C lower, suggesting cooling of the plume material during its flow southward.