Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data

Abstract

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Joint geologic interpretation of the two data sets provides constraints on the nature of the crust not available from either data set alone. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2–20 ohm m) zone that occurs at depths of 6–20 km and largely within a midcrustal velocity layer of 6.4–6.6 km/s, overlying a lower crust with velocities of 7.0–7.4 km/s. Although this conductor and the midcrustal zone of 6.4–6.6 km/s velocities are generally rather horizontal, important structures do occur. For instance, near the boundary of Western Cascades and High Cascades the MT midcrustal conductor rises to within 6 km of the surface. In addition, on the coincident MT‐refraction profile in northern California a significant westward downdip occurs on both the MT deep conductor and the 6.4‐km/s velocity layer, with both occurring at very similar depths. However, in the Columbia Plateau of Washington, no deep crustal conductors occur shallower than 25 km; also, the velocity structure is quite different, with a 6.8‐km/s midcrust and a 7.5‐km/s lower crust. Complex accretionary structures occur on MT models for the southern Washington Cascades, quite unlike the more horizontal structures of the Oregon and California Cascades. The accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, we propose a model involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor. The higher midcrustal velocities and larger depth to deep MT conductors under the Columbia Plateau are explained by a more mafic crustal composition and lower heat flow.