Magnetotelluric observations across the Juan de Fuca Subduction System in the EMSLAB Project

Abstract

A magnetotelluric (MT) transect has been obtained near latitude 45°N from the active Juan de Fuca spreading center, across the subduction zone and Cascades volcanic arc, and into the back arc Deschutes Basin region. This paper presents the MT data set and describes its major characteristics as they pertain to the resistivity of the subduction system. In addition, we discuss the measurement and processing procedures employed as well as important concerns in data interpretation. Broadband audiomagnetotelluric (AMT)/MT soundings (approx. 0.01–500 s period) were collected on land with considerable redundancy in site location, and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle. Fifteen long‐period MT recordings (about 50–10,000 s) on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers. On the Juan de Fuca plate offshore, 33 out of 39 sea floor instruments at 19 locations gave good results. Of these locations, five magnetotelluric soundings plus two additional geomagnetic variation sites, covering the period range 200–105 s approximately, constitute the ocean bottom segment of our profile. The feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30–50 s period. This phase anomaly, with a corresponding inflection in the apparent resistivity, is continuous eastward from the seacoast and ends abruptly at the High Cascades. It signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle, with the layer conductance decreasing eastward from the coast to a minimum under the Coast Range but increasing suddenly to the east of the central Willamette Basin. The higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100–1000 s over the same distance. The transverse electric mode apparent resistivity and phase on the land display a variety of three‐dimensional effects which make their interpretation difficult. Conversely, both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge, indicating that the measurements here are reflecting primarily the large‐scale tectonic structures of interest and are little disturbed by small near‐surface inhomogeneities. The impedance data near the ridge are strongly suggestive of a low‐resistivity asthenosphere beneath resistive Juan de Fuca plate lithosphere. Approaching the coastline to the east, both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater.