The distribution and composition of high-velocity lower crust across the continental U.S. Comparison of seismic and xenolith data and implications for lithospheric dynamics and history

The composition of the continental lower crust is not well known. High seismic wave speeds may indicate mafic or garnet-bearing material, with implications for emplacement history, evolution, and rheological and dynamic behavior. In this contribution, we use recent seismic results from the EarthScope Transportable Array, 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. and assess its relationship to proposed emplacement- and destruction-related mechanisms such as underplating and intraplating, collision, extension, heating, cooling, hydration, and delamination. Thin layers of high-velocity lower crust related to regional processes are found scattered throughout the continent. Thicker layers in large areas are found in the central and eastern U.S. in areas with thick crust, bounded roughly by the Rocky Mountain Front. Emplacement processes likely originally spanned this boundary, and 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., while crustal thickness and lithospheric temperatures in the western U.S. are unfavorable for growth and maintenance of thick layers of high-velocity garnet-bearing lower crust.