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

Accretion of the Siletzia oceanic terrane to the Pacific Northwest of the United States ~50 Ma led to a series of tectonic and magmatic adjustments including the establishment and heterogeneous development of the Cascade volcanic arc across Siletzia, and large-scale back-arc crustal extension. To address volcanic arc development and the tectonic and magmatic transformation of oceanic lithosphere into continental lithosphere, we study the crust and uppermost mantle of the Pacific Northwest with fundamental-mode Rayleigh-wave ambient noise tomography from periods 6-40 seconds, resolving isotropic shear-wave velocity structure from the surface to 70 km depth. Imaged structure is complex but geologically coherent. The Cascade arc has three domains. The old western Cascades are fast through the mid-crust; the magmatically active Oregon Cascades are slow along strike 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. Coherence of the NE-trending Klamath-Blue Mountains lineament, observed both with gravity and seismology, supports the hypothesis that this lineament represents a transform suture between Siletzia and older North American continent. The northern suture of Siletzia, a paleo-subduction zone, is poorly defined seismically. The paleo-forearc, now occupying much of the southeast quarter of Washington, is seismically fast near the Moho, low in elevation and nearly tectonically rigid, suggesting that Siletzia oceanic lithosphere underlies this area. Beneath the western Columbia Basin, upper crustal velocities are very slow and lower crustal velocities are very fast, consistent with the hypothesis that strong Eocene crustal extension created ~10-km-deep sedimentary basins and resulted in magmatic underplating. Other areas imaged include: the Columbia River Flood Basalt eruptive source area in northeastern Oregon, which has fast upper and middle crust, slow lower crust and a complex upper mantle; the Snake River and High Lava Plains, where uppermost mantle is uniformly slow, but their middle to lower crusts are heterogeneous; and the region near the Washington coastline, which is slow above the shallow slab interface and fast both to the east of the near-coast areas and near and below the slab interface. This low-velocity volume is consistent with the crust being composed of off-scraped subducted sediments backstopped by the upturned Crescent basalts of Siletzia.