Transcontinental crustal and upper‐mantle structure

Abstract

Transcontinental seismic, aeromagnetic, and gravity measurements, together with geologic observations, suggest that the conterminous United States is divided by the Rocky Mountain system into two crustal and upper‐mantle superprovinces. In the eastern superprovince, the velocity of compressional waves in the upper‐mantle rocks is everywhere greater than 8 km/sec, the mean crustal velocity is generally greater than 6.4 km/sec, and the crust is generally thicker than 40 km. In the western superprovince, the velocity of compressional waves in the upper‐mantle rocks is everywhere less than 8 km/sec (except along the margin of the Pacific Ocean basin), the mean crustal velocity is generally less than 6.4 km/sec, and the crust is generally thinner than 40 km. Aeromagnetic data are characterized by anomalies of large amplitude in the eastern superprovince, indicating an abundance of magnetic minerals, whereas the magnetic field in the western superprovince is relatively featureless. The weakly magnetic crust of the western superprovince is relatively devoid of magnetic minerals at shallow depths and may be above the Curie temperature for magnetite (578°C) and therefore nonmagnetic at depth. Gravity measurements and considerations of isostasy indicate that crustal and upper‐mantle densities vary with velocity. These observations, together with the Cenozoic geologic record of diastrophism and volcanism in the western superprovince and relative Cenozoic inactivity in the eastern superprovince, suggest a mobile upper mantle in the west and a predominantly silicic crust that is now receiving mafic and probably also silicic material from the mantle, whereas the upper mantle in the east is relatively stable and the now predominantly mafic crust has been extensively intruded with mafic material from the mantle; additional mafic material has been added by extrusion of lava. The primitive continental crust that evolved from the mantle was probably silicic, and it has been made slowly more mafic by addition of mafic material from the mantle and removal of silicic material from the continental surface by erosion and stream transport.