GLOBAL LOWER CRUSTAL DENSITIES FROM CONVERSION OF SEISMIC BODY WAVES

Crustal characteristics constrain the silicate mantle's temporal evolution and the extraction of the Earth’s crust. We developed a new method for determining lower crustal densities from observations of P-to-S converted phase amplitudes. We constructed 86,987 receiver functions from seismic waves recorded by 16,086 seismic stations globally. Lower crustal and upper mantle body wave velocities - necessary for calculating density - derived from the Litho1.0 global seismic velocity model. We assumed a constant upper mantle density – 3.35 g/cm³, consistent with parameter sensitivity tests we carried out. We thus calculated 5,685 lower crustal density estimates. The mean density is 3.0 ± 0.3 g/cm³, consistent with a mafic lower crustal composition. We divided our lower crustal density estimates into 22 different terrane types according to the global analyses of Hasterok et. al. (2022). Mann-Whitney-Wilcoxon U rank tests of lower crustal densities of these terranes, applied pairwise, allow us to discriminate between the two samples, providing a probability estimate that the lower crustal densities derive from the same parent distribution. Our results show that Atlantic-Indian Ocean hotspots have a different lower crustal density distribution, relative to all the other subterranes, than the Pacific oceanic hotspot distribution. Archean craton lower crustal densities show a clear affinity for those of arc terranes. The lower crust of younger shield areas is more like that of orogenic belts and magmatic provinces.