Upper crustal structure of the Yellowstone Caldera from seismic delay time analyses and gravity correlations
Authors: Lehman Jay A. , Smith Robert B. , Schilly Michael M. , Braile Lawrence W. 
Published: February 2008

The 1978 Yellowstone‐Snake River Plain seismic experiment provided detailed refraction data that were recorded across a two‐dimensional array of seismographs in Yellowstone National Park. A delay time analysis was applied to 173 crystalline basement Pg arrivals from these data to determine the three‐dimensional distribution of velocities and the layer configuration of the upper crust beneath the Yellowstone caldera. The P wave velocity structure of the caldera is characterized by a surface layer of combined sediments and rhyolite flows, averaging 2.8 km/s, that range in thickness from 1.5 to 2.0 km. Adjacent to the caldera, the crystalline upper crustal layer has a velocity of 6.05±0.01 km/s, but this layer decreases by 6% to 5.70 km/s beneath the caldera and extends northeast 15 km beyond the caldera. Smaller zones of very low P velocity, 4.0 km/s, a 30% velocity reduction compared to the 6.05 km/s layer, occur in the upper crust beneath the northeastern caldera rim and beneath the southwest caldera in the vicinity of the Upper and Midway Geyser basins. A three‐dimensional gravity interpretation based upon densities derived from the seismic model suggests that the regional gravity low of −60 mGal over the caldera correlates directly with (1) the surface layer of combined sediments and rhyolite flows, (2) the low‐velocity, 5.7‐km/s, upper crustal layer, and (3) the 4.0‐km/s low‐velocity zone beneath the northeastern caldera rim. An interpretation of the seismic velocities and densities, based on experimental data and theoretical models, suggests that the 6.05‐km/s (ρ = 2.70 g/cm3) regional Pg velocity is characteristic of crystalline basement rocks such as the Precambrian granitic gneisses that are exposed north and south of the Yellowstone caldera. The 5.7‐km/s velocity layer (ρ = 2.65 g/cm3) is interpreted to be a 10‐km‐thick hot body of granitic composition. The 4.0‐km/s low‐velocity body (ρ = 2.40 g/cm3) beneath the northeastern caldera rim is associated with a local −20 mGal gravity anomaly with a range of interpretations from a large steam‐dominated system to a felsic body of 10 to 50% partial melt. The postulated 4.0‐km/s low‐velocity layer beneath the southwestern caldera does not have a corresponding gravity signature and may reflect a zone of high seismic attenuation attributed to a saturated fracture zone. This seismic velocity model of the Yellowstone caldera suggests that the source of the Quaternary silicic volcanic rocks corresponds to the 5.7‐km/s layer in the upper crust that appears to contribute the principal source of heat to the Yellowstone hydrothermal systems. The anomalous low‐velocity body in the northeastern caldera rim area may be a cupola of partial melt with an overlying vapor‐dominated carapace and may represent the waning stages of the third cycle of Quaternary volcanism or the beginning of a new cycle of volcanism that will continue the northeastward progression of the silicic volcanism of the Yellowstone‐Snake River Plain propagating system.