The mechanics of continental extension in western North America: implications for the magmatic and structural evolution of the Great Basin

Abstract A finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the Early Tertiary North American Cordillera. Crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the Great Basin during extension. Marginal highlands, corresponding to the Sierra Nevada and Colorado Plateau, develop near the edges of the weakened region, where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain. The great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the Great Basin, favoring the formation of metamorphic core complexes during the early stages of extension. Ductile flow becomes inhibited as the crust thins and cools, possibly contributing to the change in the style of extension from low-angle detachment faulting to high-angle normal faulting during the Middle Miocene. As the lithosphere cools, the strength of the uppermost mantle increases. After about 10 m.y. the upper mantle becomes stronger beneath the Great Basin than beneath adjacent unextended regions. Strain then begins to migrate outward from the interior of the Great Basin onto the marginal highlands, and their interior edges begin to collapse and are incorporated into the widening Great Basin. After 40 m.y. of extension the highest rates of strain are localized at the edges of the Great Basin, in positions corresponding to the Walker Lane tectonic belt of western Nevada and the Colorado Plateau transition zone. The weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle. Portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions, creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies. Rapid thinning of the model lithosphere during the first 10 m.y. of extension results in nearly adiabatic decompression of the lithospheric mantle. Partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present. This mechanism may account for widespread silicic magmatism in the Great Basin during the Middle Tertiary. As the region undergoing extension widens, the rate of lithospheric thinning decreases and the mantle beneath the Great Basin begins to cool. This leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15–20 m.y. after the onset of extension. The appearance of predominantly basaltic and bimodal volcanic suites in the Great Basin during the last 5–10 Ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350°C (approximately 70°C warmer than typical mantle potential temperature).