Crustal Structure and Tectono-Magmatic Processes of the Yellowstone-Snake River Plain System From Gravity-Density Measurements and Strength Models Employing Seismic Constraints

The structure and composition of the Yellowstone-Snake River Plain (YSRP) system were analyzed from integrated modeling of gravity data from 33,209 stations in the YSRP and surrounding region. Recently derived tomographic velocity models, heat flow and temperature information, GPS-determined strain rates, and earthquake locations were used to constrain 3D density models. The density data were also constrained by velocity-density analyses based on petrologic information. These results were augmented by 1D strength profiles from representative tectonic and volcanic areas that were compared with earthquake focal depths. Results of this study suggest that the SRP lower crust has been thickened by the addition of an underplated layer 3 km thick composed primarily of clinopyroxene, with a density of 3.20 g/cm3. A mid crustal high-velocity sill occurs throughout the SRP, and is interpreted to be composed of a series of gabbroic lenses inter-fingering with the granitic upper crust. This geometry yields a bulk composition comparable to diorite and a density of 2.90 g/cm3. The sill varies laterally along the SRP from 4 to 11 km in thickness, resulting in the series of SW-NE trending gravity anomalies observed in the SRP. In Yellowstone, the density model is characterized by an upper- crustal partial melt 10 km beneath the caldera, 7 km beneath the northeastern side of the Yellowstone caldera, and extending up to 20 km north of the caldera boundary. The partial melt has a density of 2.52 g/cm3 for the caldera body and a significantly lower value of 2.47 g/cm3 for the northeastern caldera anomaly. Southwest of Yellowstone, the mid crustal SRP sill transitions to the Yellowstone partial melt. The transitional body has a density of 2.82 g/cm3. Strength models reveal that temperature has the greatest effect on crustal rheology of the region. The YSRP crust becomes progressively cooler with increasing distance from Yellowstone, and the shear strength increases from ~30 MPa in the Yellowstone caldera to ~50 MPa in the eastern SRP. The average thermal gradient in the upper crust decreases from 55 C/km in the Yellowstone caldera to 35 C/km in the eastern SRP (temperature data courtesy of David Blackwell, Southern Methodist University). In Yellowstone, the brittle ductile transition (based on mapping the 80th percentile maximum focal depths) is at 4 km depth and transitions into the Snake River Plain where the brittle ductile transition is at 8 km, coincident with the transition from the active volcanic Yellowstone system to the cool and stable SRP crust.