Crustal structure of the Columbia Plateau: Evidence for continental rifting

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Abstract
Data from a 260‐km‐long seismic refraction profile provide a detailed look at the crustal structure beneath the central Columbia Plateau (CCP). The CCP profile, centered on the Pasco Basin in eastern Washington, trends approximately N50° E between the towns of Wasco, Oregon, and Warden, Washington. The upper crust above the crystalline basement consists of between 5 and 12 km of the Columbia River Basalt Group (CRBG) and underlying sediments. The CRBG is laterally variable in thickness, ranging from 3 to 6 km over the length of the CCP seismic profile, but is thickest near the center of the Columbia Plateau. A thick, but laterally varying, accumulation of sediments underlies the CRBG with the thickest accumulation of sediments (5.0 km/s) occurring beneath the Pasco Basin in what we interpret to be a graben. Sediments there extend from the base of the CRBG to basement (6.3 km/s) at depths between 10 and 12 km. We recognize two layers between the basement and the upper mantle with seismic velocities of 6.8 and 7.5 km/s, located at 18‐ and 25‐km depths, respectively. The 6.8‐km/s layer thins beneath the graben, where it has apparently been replaced by the 7.5‐km/s layer, creating the characteristic geometry and velocity of the “rift pillow” layer previously observed in some other recognized continental rifts. The Mono (8.4 km/s upper mantle) is located at about 40‐km depth; an apparent localized, low‐velocity layer is observed within the upper mantle at a depth of about 50 km. Thus the deep crustal and upper mantle structure beneath the central Columbia Plateau is complex and is atypical of normal deep continental crust. The presence of an upper crustal graben and lower crustal rift pillow, as inferred from the seismic refraction data, combined with gravity, earthquake, electrical, and geologic data, suggest that Eocene (and possibly later) continental rifting occurred prior to deposition of the CRBG.