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Imaging and Interpreting the Pacific Northwest
Eugene Humphreys (University of Oregon), Brandon Schmandt (University of Oregon)
Improved tomographic resolution of Pacific Northwest (PNW) upper mantle distinguishes two high velocity features in addition
to subducted Juan de Fuca (JdF) ocean lithosphere:
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). Flatslab
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.
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.
References
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.
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 crosssection
(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.
II-172 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics