Resistivity cross section through the Juan de Fuca Subduction System and its tectonic implications

A resistivity cross section to depths exceeding 200 km has been derived from magnetotelluric observations along a profile near latitude 45°N from the Juan de Fuca spreading center, across the coastal subduction complex, the High Cascades volcanic arc, and into the back‐arc Deschutes Basin region. In this two‐dimensional interpretation, emphasis was placed on data approximating the transverse magnetic mode since these data are relatively robust to common departures from the two‐dimensional assumption. The vertical magnetic field, however, has been very valuable in defining structure of the offshore sediments, of the oceanic asthenosphere and below the arc volcanics of the Western and High Cascades. The transverse electric data on land suffer a variety of three‐dimensional effects, making their interpretation very difficult. In contrast, the greater uniformity of upper crustal conditions on the seafloor allowed a good fit to both modes of the impedance plus the vertical magnetic field at least down to 10 4 s. Important components of our model resistivity structure and their hypothesized tectonic implications are as follows. Measurements on the seafloor indicate very low resistivity in the eastward thickening sedimentary wedge of the abyssal Cascadia Basin provided its depth extent is constrained. The majority of these sediments are inferred to be off‐scraped or lose most of their interstitial water before being carried more than ten or so kilometers down the trench. In the oceanic upper mantle of the Juan de Fuca plate, moderately low resistivities from about 35–215 km depth (nominal) presumably reflect up to several percent partial melt attending regional upwelling in the vicinity of the ridge. Very low resistivities below 215 km depth in the oceanic mantle are difficult to explain physically, but problems with very long‐period source field effects or bias error may hamper the modeling here. Under the Oregon Coast Range, a low‐resistivity layer dips inland at approximately 20°. Its conductance decreases by an order of magnitude from the coast to around 60 km inland and its position appears consistent with the Juan de Fuca plate subduction decollement. Possible causes of the low‐resistivity include residual sediments, pore waters, perhaps sulfides, and dehydrating oceanic crust carried down the subduction zone. East of about 60 km inland, there is a much stronger, subhorizontal conductor with a depth to top of around 25 km. To explain it, a massive breakdown of greenschist minerals is hypothesized to liberate most of the water carried in the altered ocean crust well before the volcanic arc is reached. The resistivity manifestation of the arc magmatism itself is not strong but consists of a low‐resistivity axis in the middle crust below and to the west of the arc at the surface. It presumably represents fluids released from crystallizing magmas. East of the High Cascades, both the MT and geomagnetic data detect a strong gradient in resistivity with lower values to the south. It is suggested that the Brothers fault zone signifies an electrical, thus possibly a thermal and magmatic, boundary with the northern Basin and Range.