The “death” of the Sevier-Laramide orogen: Gravitational collapse of the crust or something else?

Abstract

ABSTRACT The Basin and Range Province is considered to be one of the most iconic continental rift provinces that postdates a prolonged orogeny. Here, I present evidence that challenges all the assumptions that lead to the long-held notion that gravitational collapse of thickened (55–65-km-thick) continental crust was a major driver of Basin and Range extension. This study focused on integrating the regional tectonic and magmatic history of the northeastern region of the Basin and Range (centered on the Albion–Raft River–Grouse Creek metamorphic core complex) and combines insights from a compilation of data from metamorphic core complexes worldwide to illustrate the effect of accounting for the magmatic histories when estimating pre-extensional crustal thickness. In the region of the Albion–Raft River–Grouse Creek metamorphic core complex, there is evidence of three Cenozoic extensional events and three coeval magmatic events. By taking into account the regional magmatic activity during the Cenozoic (Paleogene, Neogene, and Quaternary magmatism), and the inferred mantle-derived magmatic volume added to the crust during the process of extension, it is shown that the pre-extensional crustal thickness cannot have been more than ~53 km, and it was more likely close to ~46 km. This estimate is consistent with Eocene igneous geochemistry estimates of crustal thickness and with crustal thickness estimates from shortening of ~30-km-thick mid-Jurassic crust. During the Cenozoic evolution of the northeastern Basin and Range, the crust in the area of study thinned from ~46 km to ~32 km, and the elevation of the pre-extensional plateau collapsed from ~2.5 km to its present-day average of ~1.8 km. This study concludes that an alternative mechanism to predominantly gravitational crustal collapse is required to explain the extension in the region of the Albion–Raft River–Grouse Creek metamorphic core complex. I support recent interpretations that this mechanism involved the complex interaction of the removal of the Farallon flat slab (by slab roll-back or delamination of the slab) with the impingement of the Snake River Plain–Yellowstone mantle anomaly. The switch in the stress regime from compression (during the slab subduction) to a complex regime during slab roll-back, followed by extension (in the middle Miocene), and the associated mantle-derived magmatism, led to the thinning of the subcontinental lithospheric mantle, thermal weakening of the crust, and the thinning of the crust during the Cenozoic. This crustal extension is expressed as regional Basin and Range normal faulting and local vertical flow and exhumation of the mobilized middle crust at metamorphic core complexes like the Albion–Raft River–Grouse Creek complex.