PROTEROZOIC ASSEMBLY SIGNATURE AND ITS OVERPRINT IN LOWER CRUSTAL SEISMIC VELOCITY STRUCTURE ACROSS NORTH AMERICA

Growth of the North American crust during stitching of Archean blocks as well as during Proterozoic southward accretion likely included emplacement, differentiation, and thermal processes that generated lower crust with high seismic wavespeeds. The high seismic velocities can be due to mafic composition, the presence of garnet, or both. We seek to find such a seismic signature of assembly and attempt to differentiate it from Archean and post-Proterozoic overprint signatures. We review recent seismic results from the EarthScope Transportable Array from receiver functions and surface waves, compilations of active source studies, and selected xenolith studies to attempt to map the distribution of high-velocity lower crust across the continental U.S. Thin layers of high-velocity lower crust related to regional processes are found scattered throughout the continent. Thicker layers (6-25 km) in large areas are found in the central and eastern U.S. in areas with thick crust, with some NE-SW trending, possibly assembly-related features seen in their distribution. A first-order transition is at the Rocky Mountain Front, even though emplacement processes such as 1.4 Ga magmatism crossed this boundary. The difference between the two domains may reflect garnet growth with cooling and aging of continental crust in much of the central and eastern U.S., compared to more recent tectonic overprinting in most of the west.