Lithospheric disruption beneath the Columbia River Basalt province from Rayleigh wave seismic tomography

The Columbia River Basalts (CRB) designated as one of the global Large Igneous Provinces (LIP) may have erupted rapidly in as little as 1 My. There are several hypotheses proposed to explain the eruption of the basalt groups such as the Yellowstone mantle plume, isostatic rebound and delamination that induces decompression melting, and passive back-arc processes that tap ancient enriched reservoirs. These models predict different sources of the magma, degrees of thermal heating, chemical melting, or disruption of the lithosphere as well as final thickness of the lithosphere. Here we test these model predictions using Raleigh wave tomography. We will present 2D phase velocity maps and 1D shear wave velocity results to distinguish between the geodynamic models for CRB emplacement. We measure Rayleigh waves from 18 to 91 seconds using a subset of 44 USArray TA stations and local Passcal seismic experiment in the Wallowa mountains of NE Oregon and SE Washington. Preliminary phase velocities averaged over the study area increase steeply from 18 s to 25 s associated with crustal structure. Above 30 seconds phase velocities are significantly lower than typical continental velocities in eastern North America, for all periods above 30 s by as much as 4.2%. The dispersion curve above 30 s, is steadily increasing up to 91 seconds. The low phase velocities at higher periods may indicate lithospheric properties that produce lower velocities such as a thinner lithosphere, lower densities, or higher temperatures at depths shallower than 120 km. Azimuthal anisotropy averaged over the multi-region study area is 0.5% and oriented in an East-West direction. All regions have 3-5% lower velocities than typical continental values at periods greater than 50 s. These low velocities indicate thermal heating, melt percolation or perhaps disruption of the lithosphere. The phase velocities of the Precambrian region are consistently higher than the Phanerozoic region west of the continental margin for periods above 50 s and may be consistent with previous observations of a high velocity curtain east of the CRB's.