The dynamic nature of the continental crust-mantle boundary: Crustal evolution in the southern Rocky Mountain region as an example

Abstract

The evolution of continents involves modification of the lithosphere through time, including changes in crustal thickness and composition that create a dynamic crust-mantle boundary (Moho). The geological history of the southern Rocky Mountain region is relatively well understood and recent additions of modern seismic data provide an ideal opportunity to investigate the evolution of the crust. The results presented in this volume show that crust in the southern Rocky Mountains is relatively thick compared to the global average for the continents. The mafic lower crust and crust-mantle boundary of the Proterozoic provinces of the southwestern U.S. likely formed, and reformed, in several stages. Initial formation of juvenile continental crust took place by development and assembly of magmatic arcs between 1.8 and 1.6 Ga. Volcanic and plutonic rocks of this age record whole-crust differentiation and probably resulted in a mafic lower crustal residue. From 1.45 to 1.35 Ga, the crust underwent another period of differentiation leading to emplacement of A-type granites in the middle crust across southern Laurentia. Voluminous granitoid emplacement ca. 1.4 Ga, petrology of granitoids, widespread metamorphism, and 1.4 Ga lower crustal xenoliths are best explained by mafic underplating. Subsequent mafic additions to the lower crust likely took place at each of the times when basalts were emplaced in the Rocky Mountain region (1.1 Ga Grenville orogeny, Laramide orogeny, Oligocene ignimbrite flare-up and Neogene extensional tectonism), but these events were more local in distribution and not widespread enough to produce the thick, mafic crust observed over an extensive area.