IMAGING CRUSTAL STRUCTURE IN SOUTHWESTERN WASHINGTON WITH SMALL MAGNETOMETER ARRAYS

We use data from a series of small (three to five stations) overlapping magnetovariational (MV) arrays to image variations of vertically integrated electrical conductivity in the crust of southwestern Washington. Two principal structures are revealed: a large north-south trending anomaly (the southern Washington Cascades Conductor (SWCC), which has been detected by several previous induction experiments), and a smaller anomaly which branches off of the SWCC just north of Mount St. Helens and trends westward beneath the Chehallis Basin. A weaker east-west trending anomaly is evident farther to the north beneath southern Puget Sound. The MV results concerning the SWCC are reasonably consistent with the model of Stanley et al. (1987), who interpret the anomaly as a suture zone of mid-late Eocene age, but the array data allows us to map the horizontal extent and complex three-dimensional character of the SWCC in greater detail. We suggest that the SWCC represents a section of the early Cenozoic subduction zone which is analogous to the present-day Olympic Peninsula. In the region west of the Cascades, the array data show that crustal conductivity is distinctly three dimensional, consisting of highly resistive blocks (crystalline rocks) separated (in the upper 5-10 km at least) by interconnected narrow regions of higher conductivity (sedimentary units). This pattern of conductivity variations is consistent with the inferred origin of the region as a seamount complex, which was subsequently broken into discrete blocks which have been thrust together during and after accretion to the North American continent. The distribution of anomalous electric currents and our model for crustal conductance are in striking agreement with a variety of other geophysical constraints, including gravity, magnetics, present crustal seismicity, and the pattern of recent volcanic vents. The St. Helens seismic zone (SHZ), which coincides with the western edge of the broad southern portion of the SWCC, is abruptly terminated in the north by the smaller east-west trending conductive zone. North of the SHZ near Mount Rainier, seismicity is concentrated in a narrow band coincident with the very narrow northern portion of the SWCC. In addition, volcanic vents are concentrated around the edges of the SWCC but are rare in the interior of the zone of high conductivity. The magnetometer array data thus suggest that present patterns of crustal deformation and volcanism are in part controlled by the complex tectonic history (and resulting crustal structure) of the region.