Large amounts of mafic magma transferred from mantle to crust during passage of the Yellowstone hotspot across the eastern Snake River Plain. Total mass is estimated from petrologic evidence to be equivalent to a 12 km thick layer of gabbro, yet the crust appears to have thinned not thickened during hotspot tectonism. Previous work indicates two crustal thinning mechanisms operated during hotspot magmatism: (1) the ESRP and flanking Basin-Range crust extended ~25% in a ENE-direction, and (2) the ESRP lower crust flowed outward (NNW and SSE) into Basin-Range crust in response to upper crust densification and subsidence. We investigate if these two mechanisms could sufficiently thin the ESRP crust and make room for a 12 km thick mafic layer.

If accommodated by faulting and ductile flow, extension should have proportionately reduced crustal thickness. Published geophysical studies indicate the ESRP crust is ~42 km thick, adjacent Basin-Range crust is generally 42-37 km thick, and nearby unextended craton is 47 km thick. Assuming a pre-extension thickness of 47 km for ESRP crust, 25% extension would have reduced crustal thickness to ~37 km. The 5 km of ESRP crust in excess of 37 km is interpreted to be additional mantle mass in the form of mafic intrusions and volcanic rocks.

If accommodated by dike injection, extension of the ESRP would not involve crustal thinning. Dikes would contain the equivalent of an 8 km thick mafic layer, while overlying volcanic rocks would account for the equivalent of an additional 3 km thick mafic layer. To yield the modern ESRP crustal thickness, 8 km (47 + 3 – 42) of crustal thinning by density-driven lower crustal flow would be needed.

Assuming Basin-Range crust was uniformly thinned from 47 to ~37 km during extension and experienced no mantle influx from below, crust in excess of ~37 km contains density-driven mass from the ESRP. Depending on small variations in estimated Moho depth, estimates of excess mass in the Basin-Range vary from a 2-6 km thick equivalent layer beneath the ESRP.

Overall, the observed amounts of crustal flow and extension could generate (1) a 42 km thick ESRP crust with room for the equivalent of a 7-11 km thick mafic layer, if extension involved crustal thinning, or (2) a 44-48 km thick ESRP crust with room for the equivalent of an 11 km thick mafic layer, if extension was by dike injection.