Continental Lower Crust

Abstract

The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10–20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO2. Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO2 contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust.