STRUCTURE AND TECTONICS OF THE PACIFIC NORTHWEST FROM EarthScope MAGNETOTELLURIC DATA

The magnetotelluric (MT) component of the Earthscope Transportable Array has covered nearly 25% of the continental United States. Three-dimensional modeling of the MT data provides important constraints on lithospheric structure as well as the distribution of fluids and melt within the crust and mantle. Resistivity models of the Pacific Northwest image a first-order change in the upper mantle across the Klamath Mountains-Blue Mountains lineament parallel to both the direction of Juan de Fuca plate convergence and the fast axis of shear-wave splitting measurements. Southeast of this boundary, conductive upper mantle is characteristic of the Basin and Range province, and suggests that this province extends northwest of its physiographic expression. The lithosphere is resistive beneath eastern Washington, and may reflect the western edge of cratonic North America as suggested by some seismic studies.

A modest enhancement in conductivity near the top of the Juan de Fuca slab at depths of 30-40 km may indicate fluid release from metamorphic reactions or pore collapse. The Cascades magmatic arc is associated with enhanced crustal conductivity, however no indication of the Newberry Hotspot track is observed. The intrusive root of the Western Cascades arc is imaged as a linear resistive feature that is rotated clockwise relative to the modern arc, consistent with evidence for post-Miocene rotation of the Eocene Siletz terrane.

Within the crustal section, the outlines of sedimentary basins beneath the Columbia River Basalts are imaged. In addition, a zone of enhanced conductivity is co-located with the Chief Joseph Dike Swarm associated with the ~17 Ma basaltic outpouring. A thickened crust is evident beneath the Columbia River Plateau, and enhanced conductivity at its western edge (the previously identified Southwest Washington Crustal Conductor) dips to the northeast, and may reflect the suture between the Siletz terrane and North America.