The structure of continental crust during and after formation

Abstract:

Continents are regions that ride near or above sea-level. Today, the continents are underlain by at least 30 km of felsic crust, under which lies cold peridotitic mantle lithosphere that can extend to depths of 200 km in the oldest cores of continents. Making large amounts of felsic magmas requires crystal fractionation from hydrous parental basaltic magmas. Hydrous parental magmas require water to be introduced into the mantle, presumably by subduction. One place where felsic continental crust is being formed is in continental arcs. As basaltic magmas rise upwards through the cold thermal boundary layer, they cool, crystallize and fractionate, resulting in felsic magmas rising towards the surface and more mafic cumulates remaining at depth. We show that the shallow magma chambers that give rise to dacitic to rhyolitic eruptions are actually felsic cumulates with 30% trapped melt. Mafic cumulates exist in the lower crust and in the lithospheric mantle, with lesser amounts of trapped melt. Active continental arcs should be characterized by partially molten mushes at intermediate crustal depths (30 % trapped melt), at the Moho, and just below the Moho (<10% melt) and should thus give distinct seismic signals. After crystallization, most mafic cumulates will have velocities comparable to or greater than that of peridotite. However, the most primitive cumulates, which form beneath the Moho, may have velocities less than peridotite, resulting in long-lived low velocity layers within the lithospheric mantle. Finally, we show that elevations of mountain belts correlate well with crustal thickness, indicating that the isostatic compensation of mountains is controlled by the crust. This suggests that beneath thickened crust, the lithospheric mantle is thin and does not contribute to topography. Craton elevations, however, are lower than that predicted by their crustal thicknesses, requiring sub-Moho mass excess. While peridotitic mantle underlying cratons are chemically buoyant, the anomalously low elevations suggests that cratons are underlain by thick thermal boundary layers, whose negative thermal buoyancy slightly exceeds the chemical buoyancy. Alternatively, high density rocks, such as cumulate pyroxenites formed during continent formation, may reside within the cratonic mantle, increasing its density.