A magnetotelluric (MT) transect has been obtained near latitude 45"N from the active Juan de Fuca spreading center, across the subduction ,..one and Cascades volcanic arc, and into the back arc Deschutes Basin region. This paper presents the MT dat.a set and describes ~ts major characteristics as they pertain to the resistivity of the subduction system. In addition, we discu~s the measurement and processing procedures employed as well as important concerns in data interpretation. Broadband audiomagnetotelluric (AM1)/MT soundings (approx. 0.01-500 s period) were collectei on land with considerable redundancy in site location, and from which 39 sites were selected which con~train upper crustal heterogeneity but sense also into the upper mantle. Fifteen long-period MT recordings (about 50-10,000 s) on land confinn the broadband responses in their common period range and extend the d,:pths 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 200-1OS 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 WillanIette 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-surfac.e 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.

Two‐dimensional, Backus‐Gilbert inversion of the EMSLAB land magnetotelluric (MT) data along the 200‐km‐long Lincoln Line has yielded optimally smooth geoelectric sections. Inversions were performed on the apparent resistivity and impedance phase data approximating the transverse magnetic (TM) mode. The land portion of the Lincoln Line traverses the edge of the North American plate that is being underthrust by the Juan de Fuca plate system. The inversion reveals three centralized conductive zones in the depth range of 20–40 km. A slightly conducting (<100 S) zone is centered at 30–35 km depth under the Oregon Coast Range; this feature may be the top of the subducting Juan de Fuca plate since there is complementary evidence here from Consortium for Continental Reflection Profiling seismic data. A prominent conduction zone of several hundred Siemens (S) is also detected at 30–35 km depth under the very resistive (>1000 ohm m) Western Cascades. Here the depth is too shallow for the zone to be the subducting plate. There is also evidence for a highly conducting (>1000 S) lower crust east of the High Cascades on the east end of the Lincoln Line. Two vertical conductive regions are also exposed in the inversion model. One occurs at 70–80 km from the coast under the Willamette Valley where a postulated Eocene trench may have left a suture zone. The second region is coincident with surface hydrothermal activity along the Western‐High Cascades boundary. There are ample sources of water in the crust, e.g., in subducted sediments, from dehydration reactions along the upper plate boundary, and in volcanic arc magmas, to lead us to believe that hot, saline water is the major source of the conductive occurrences along the Lincoln Line. However, the various zones appear to be distinct, and the water may be trapped by different mechanisms.

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We report a remote reference magnetotelluric survey performed in La Presita area, in the nighborhood of Culiacán, Sinaloa (México).  Fifteen sounding were carried out along three lines, two transversal and one parallel to the geological strike of the region. Due to excessive industrial and agricultural activity and the high electrical conductivities found in the coastal plain, signals are generally low in amplitude and contaminated with noise.  On account of the use of the remote reference station, and a special processing of the data, consisting of an adequate selection of segments with high coherencies foro each frequency window, it has been possible to obtain information with good statistical properties. In order to perform the interpretation of the data, we first identify the polarization mode of each apparent resistivity and phase curves by using ipedance and tipper polar diagrams, and induction arrows. Finally, a two-dimensional modelling is included in the interpretation, taking into account all the geological and geophysical knowledge of the area.  We have modified, by numerical experiments, our final model with realistic bounds:  there are no evidences of the xistence of geothermal resources in this area.