A geomagnetic variation anomaly coincident with the Cascade Volcanic Belt

Abstract

A geomagnetic depth sounding profile in the western Cordillera has revealed concentrations of electric current related to the tectonics of the region. The east-west profile at approximately 46ø30'N extends from the coast to just east of the Cascade volcanic belt. In the western region of the profile the geomagnetic coast effect at a 50-min period was similar to results in other tectonically active areas such as California and southeastern Australia. Stations near the Cascade volcanic zone had anomalous response functions; a clearcut reversal in both the in-phase and quadrature components was observed in the direction of the induction arrows across the zone. Based on the similarity between the analyzed data curves and the response of a line current model, the anomaly can be approximated by a long, narrow conductor trending north-south in direction. This conductive zone does not extend as far north as a profile previously measured in southern British Columbia.Paper number 80B0813.

Interpretation of shallow electrical features from electromagnetic and magnetotelluric surveys at Mount Hood, Oregon

“…In this paper, the tipper is plotted so that it is a maximum in a direction parallel to the strike of a twodimensional structure. Figure 26), both the apparent resistivity principal directions and tipper strikes indicate a NW-SE regional strike, similar to the conductor strike direction indicated by the 300-s-period transfer function derived by Law et al [1980] from geomagnetic depth soundings across the Cascades in southern Washington. Tipper strikes are fairly consistent, but these directions show a curvature suggesting a transition to more resistive crust to the east and north of Mount Hood.…”

“…Law et al (1980) made deep geomagnetic depth soundings at six stations in an east-west line crossing the Cascades 120 km north of Mt.…”

On the synthesis of a large geomagnetic array from small overlapping arrays

“…The most significant anomaly, indicated by the large vertical fields which change sign across a line running north-south under the Cascade range, is evident for both source polarizations. The presence of a conductivity anomaly in this area is well documented, beginning with an initial report by Law, Auld & Booker (1980), and continuing with more detailed discussions by Stanley (1984) and Stanley, Finn & Plesha (1987) who used magnetotellurics to map a substantial crustal conductor, (the Southern Washington Cascades Conductor, SWCC), on two profiles crossing the Cascade volcanic arc in this area. Stanley et al (1987) have inferred that this structure represents a unit of sediments and other forearc basin rocks accreted and subsequently compressed against the pre-tertiary edge of the North American continent in the mid-Eocene.…”

Mount Rainier: A decade volcano

“…The relationship of the earthquakes to the west of the volcano is unclear. They occur near the northern end of a regionally exten sive, highly conductive geoelectrical body at midcrustal depth (the Southern Washington Cascades Conductor [SWCC] of Stanley et al [1987] and Law et al [1980]) ( Figure 1). These earthquakes may reflect regional tec tonic stresses and have little relation to the volcano, or they may indicate some un known connection between tectonic and magmatic processes.…”

“…Geophysical studies: State-of-the-art geo physical research is badly needed. The deep magnetotelluric studies of Booker and asso ciates [Law et al, 1980] andStanley et al [1987] demonstrate the importance of mod ern geophysical surveys in the Washington Cascades.…”