The tectonic setting of the southern Washington Cascades has been studied with the aid of magnetotelluric (MT) and other geophysical data. The main feature of interest in the geophysical data is a broad high-conductivity anomaly mapped with MT and geomagnetic variation (GMV) data. This anomaly is located roughly within the triangle formed by the volcanoes Mount Rainier, Mount St. Helens, and Mount Adams but exceeds beyond Mount Rainier to the northwest. We interpret the cause of the anomaly to be conductive rocks with resistivities of 1–4 ohm m and thicknesses possibly greater than 15 km. These conductive rocks are found 2–8 km beneath the overlying less conductive volcanic and sedimentary rocks at the surface. Two aeromagnetic lows follow the trend of the conductivity anomaly, and linear belts of strike-slip seismicity are coincident with both these magnetic lows. One of the aeromagnetic lows is coincident with the western margin of the conductivity anomaly. The geophysical data appear to outline a suture zone of probable Eocene age caused by accretion of a large seamount complex (Siletzia) and that may contain large thicknesses of compressed forearc basin and accretionary prism sedimentary rocks of Cretaceous to Eocene age. Part of the shallower conductive rocks may be associated with carbonaceous continental and transitional marine sedimentary rocks of the Puget Group. The contact between the hypothesized compressed basin and the accreted terranes to the west may localize the release of shear stresses in this region of oblique subduction. Several possible explanations for the conductivity anomaly are considered in addition to the compressed basin hypothesis.