Crust and lithosphere structure of the northwestern U.S. with ambient noise tomography: Terrane accretion and Cascade arc development

Abstract

To address the tectonic and magmatic modifications of the Pacific Northwest lithosphere, including transformation of the Farallon oceanic terrane “Siletzia” into continent, we study the crust and uppermost mantle of the Pacific Northwest with fundamental-mode Rayleigh-wave ambient noise tomography using periods 6–40 s, resolving isotropic shear-wave velocity structure from the surface to 70 km depth (3 crustal layers and 2 upper mantle layers). We optimize this estimate with the aid of a neighborhood search algorithm, which we also use with receiver functions to estimate Moho depth. Horizontal node spacing is 0.25°. The EarthScope Transportable Array, the Wallowa array, a portion of the High Lava Plains array, and seven permanent stations are joined to achieve high resolution. Very slow western Columbia Basin upper crust above very fast lower crust expresses the large Eocene sedimentary basins above a magmatically underplated crust of extended Siletzia lithosphere. High-velocity lower crust in adjacent areas to the east and south represents Siletzia thrust under the pre-accretion North America forearc. This interpretation is supported by an anomalous absence of post-accretion magmatism in these areas, implying an absence of slab removal. The southeast termination of the fast lower crust is especially strong and sharp about 35 km southeast of the Klamath-Blue Mountains gravity lineament, suggesting the Farallon slab to the southeast was torn away. The Columbia River Flood Basalts erupted at ~ 16 Ma, apparently creating a hole of diameter ~ 150 km in the edge of the underthrust Siletzia lithosphere. The magmatically active Oregon Cascade arc is slow at all depths, and the much less active Washington Cascades tend to have a volcano-centered structure that is slow in the lower crust but fast in the upper crust and upper mantle. This structure suggests that magmatic intrusion has increased upper crustal velocity, but that the higher temperatures beneath the active Oregon Cascades have a dominating effect and create low velocities.