Imaging and Interpreting the Pacific Northwest

Imaging and Interpreting the Pacific Northwest Upper and lower left panels show P tomography in map and cross-section, respectively. Center and right columns shows tectonic cartoons in map (upper) and cross- section (lower) of the PNW just before and just after Siletzia accretion. Dark gray = Siletzia. Light gray = Siletzia forearc. Bottom center panel shows a topographic map of CRB source area, with the high-standing Wallowa Mountains in the center of a topographic bull’s eye.
Improved tomographic resolution of Pacific Northwest (PNW) upper mantle distinguishes two high velocity features in addition to subducted Juan de Fuca (JdF) ocean lithosphere:
</p><p>50 m.y. old slab (Siletzia “curtain”). The large volume of this nearly vertical high-velocity “curtain” can only be attributed to subducted slab, and it lies where the Farallon ocean lithosphere subducted prior to Siletzia accretion at ~53 Ma (see figure). Flat- slab subduction probably occurred prior to accretion (55 Ma), during the amagmatic and compressive Laramide orogeny. We infer that the imaged curtain is Farallon slab that rolled back following accretion, leading to early ignimbrite flareup magmatism (the Challis trend) and core-complex extension that were active at 50 Ma.
</p><p>Post-flood basalt drip (Wallowa “drip”). A spherical-shaped high-velocity body about 100 km across and centered at 250 km depth is imaged directly beneath the source area for the Columbia River flood basalts (CRB) and the resulting topographic bull’s eye (see topography map). This feature could be downwelling North American lithosphere or basalt-depleted asthenosphere. Note that the CRB event occurred adjacent to the Siletzia curtain.
</p><p>Schmandt, B. and Humphreys, E. 2010. Complex subduction and small-scale convection revealed by body-wave tomography of the western U.S. upper mantle. Earth Planet. Sci. Lett., accepted.</p>


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