Shear wave structure of the Southern Volcanic Plateau of Oregon and Idaho and the Northern Great Basin of Nevada from surface wave dispersion

Abstract

Fundamental mode Rayleigh wave phase velocities (to 100‐s period) have been measured for two paths across the northern Great Basin of Nevada and Utah and southern Volcanic Plateau of Oregon and Idaho. The average dispersion for these regions is similar to that previously observed for the central Great Basin of Nevada and Utah. Average structural models were derived for these paths from the observed dispersion data and existing refraction data. The model for the southern Volcanic Plateau and western Snake River consists of a three‐layer, 40‐km‐thick crust and a 10±10 km thick high‐velocity mantle lid above a broad low‐velocity zone for shear waves. Higher phase velocities are observed at long periods (>40 s) for the path through the northern Great Basin than for parallel paths in the central Great Basin or Volcanic Plateau, indicating higher average mantle shear wave velocities. The model for the northern Great Basin consists of a three‐layer 30‐km‐thick crust and a 30±10 km thick high‐velocity mantle lid above a low‐velocity zone for shear waves. The average shear wave velocity within the low‐velocity zone beneath the northern Great Basin is 4.3 km/s compared with 4.1 km/s within the low‐velocity zone beneath the central Great Basin. These higher mantle shear wave velocities postulated for the northern Great Basin are at odds with the observed heat flow but in agreement with teleseismic P wave results. Both the higher velocities and the high heat flow may be the result of upper mantle differentiation during a recent volcanic event.