Crustal Structure and Thickness Along the Yellowstone Hot Spot Track: Evidence for Lower Crustal Outflow from Beneath the Eastern Snake River Plain

Receiver functions from seismic stations about the Yellowstone hot spot track are migrated to depth using a Vp/Vs map constructed from stacking of the direct and free surface Moho reverberations (i.e., H-K analysis) and a shear velocity tomogram constructed from surface wave measurements. The thickest crust (48-54 km) resides in the Wyoming province beneath the sampled Laramide age blocks, and the thinnest crust (32-37 km) resides in the Montana Basin and Range province. The eastern Snake River Plain (ESRP) crust is thickest (47 km) at its NE end beneath the young calderas and thinnest (40 km) at its SW end beneath the older Twin Falls caldera. Two ESRP crustal thickness domains are found: (1) at the older Twin Falls and Picabo calderas, the mean ESRP crust is 4 km thicker with respect to its margins and (2) adjacent to the Heise caldera field, the mean ESRP crust is 4 km thicker with respect to its SE margin crust but no thicker with respect to its NW margin crust. This lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the Heise caldera field. Confirmation of these crustal thickness variations is provided by inspection of common conversion point (CCP) stacks that delineate several secondary features: the top of a thick high-velocity (3.9 km/s) lower crust layer within the Wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the Yellowstone Caldera. This paired arrival would be consistent with a low-velocity zone perhaps associated with magma staging beneath the caldera. Our most important finding is that the magmatic loads injected into the ESRP crust over the last 4-12 Myr, in tandem with the ESRP crustal viscosity structure, have been sufficient to drive significant outflow of the ESRP lower crust.