Abstract:
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 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 he 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 terminatedi n 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.
