Magnetotelluric soundings across the Pemberton Volcanic Belt, British Columbia

Abstract

Magnetotelluric soundings were carried out at three sites about 30 km apart in a line crossing the Lillooet Valley near Pemberton, British Columbia, to determine the regional conductivity structure and its relationship to the Meager Creek geothermal area. Telluric data were limited to a period band of 10–500 s, while geomagnetic data were recorded in this band as well as a longer period band of 5 min–3 h.Single-station vertical transfer functions indicate that no simple, two-dimensional conductivity discontinuity crosses the profile line or its immediate extension. Thus, the Pemberton volcanic line of plutons does not have an associated two-dimensional continuous conductivity anomaly. Impedance tensors calculated from the short period data are dominated by three-dimensional effects and surface structure effects at the western and eastern sites respectively. The results from the central site indicate a macro-anisotropy in horizontal conductivities, possibly produced by local fracture zones. One-dimensional modelling suggests a vertical conductivity structure consisting of a highly resistive upper crust 15–25 km thick, underlain by a conductive region. Though this conductive layer beneath Pemberton is more resistive, a similar layer has been noted in previous studies in southern British Columbia and appears to be a feature of the western Cordillera.