Cenozoic tectonics of the Pacific Northwest (PNW) and the associated mantle structures are remarkable, the latter revealed by EarthScope seismic data. In this thesis we model teleseismic body waves constrained by ambient-noise surface waves and teleseismic receiver function analysis in order to recover better-controlled higher resolution images of the PNW continuously from the surface of the crust to the base of the upper mantle. We focus on and have clearly imaged two major upper mantle structures: (1) the high-velocity Farallon slab (the "Siletzia curtain") extending vertically beneath the Challis-Kamloops-Absaroka volcanic flareup (~53-47 Ma) of western Idaho and central Washington; and (2) a high-velocity anomaly beneath the Wallowa Mountains of northeast Oregon associated with the main Columbia River flood basalts source region. The proximity of these two structures along with the tectono-magmatic history of the PNW leads us to reexamine the origin of the Columbia River Basalts ~ 16 Ma. This thesis includes co-authored material submitted for publication.

<p dir="ltr">This dissertation uses a multidisciplinary basin analysis approach to document the sedimentary, structural, and volcanic response to Cenozoic oceanic plateau collision and translation along the northwestern Cordillera. During this time, two fragments of oceanic plateau accreted in the Pacific Northwest (Siletzia terrane) and in southeastern Alaska (Yakutat terrane). My research aims to test if the Siletzia and Yakutat terranes have an early shared history as the same spreading ridge-centered plateau in the Pacific Northwest and constrain timing of breakup and translation of the Yakutat terrane to southeastern Alaska. Chapter 2 focuses on development of a new U-Pb zircon geochronology technique to aid in a more accurate and precise understanding of sediment routing systems. The goal of developing this technique is to utilize it in pinpointing the source regions along the northwestern Cordillera supplying sediment to the basin on the Yakutat terrane as it made its northward journey to southeastern Alaska. Chapter 3 focuses on creating a regional chronostratigraphy for deep-marine Cenozoic sedimentary and volcanic rocks of the peripheral rock sequence on the northern Olympic Peninsula in Washington. These sedimentary rocks directly overlie the Siletzia plateau and record basin evolution before, during, and following its collision to the continental margin. Chapter 4 uses detailed lithofacies mapping and U-Pb geochronology of metasedimentary and volcanic rocks in the Olympic subduction complex to document the structural response to seamount subduction in the Eocene. Seamount subduction began shortly after collision of the Siletzia oceanic plateau to the Pacific Northwest and played a critical role in development of the early forearc region and Ancestral Cascades arc. Chapter 5 compares the lowermost sedimentary and geochronologic basin record on both Siletzia and Yakutat terranes to test if they have an early shared history in the Pacific Northwest.</p>