No abstract

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.

We resolved the architecture of the early Proterozoic Penokean orogen suture and late middle Proterozoic (Keweenawan) Midcontinent rift system magmatic overprint in east-central Minnesota and western Wisconsin through recovery and analysis of a legacy magnetotelluric (MT) data set. We digitized printed plots of off-diagonal MT and controlled-source audio (CSA) MT responses, including error intervals, to provide 22 soundings along a profile of ~225 km length extending from north of Lake Mille Lacs, Minnesota, southeastward to Flambeau Ridge, Wisconsin. The MT data were inverted to a smoothed electrical resistivity structure using a two-dimensional finite-element, regularized Gauss-Newton algorithm emphasizing the transverse magnetic (TM) mode data subset. Our model reveals a major electrically conductive zone dipping moderately to the southeast for >50 km in the 5-35 km depth range, which marks the probable Penokean suture in easternmost Minnesota. We interpret the conductor to reflect a package of graphitized metasediments of the former Archean continental margin and near foreland zone, underthrust as the Penokean terrane collided with the Superior Province. The large-scale conductor is now hidden beneath mainly Yavapai-aged plutonic rocks of the East-Central Minnesota batholith. Below the axis of the later Midcontinent rift (St. Croix horst, subsequently), a compact resistive body ranging from 5 to 20 km deep overlies the large conductor. We interpret this resistor to be mafic volcanic and intrusive rocks of the Midcontinent rift event, which correlate spatially with a high Bouguer gravity anomaly similarly modeled. The rift here coincides with the lower-crustal reaches of the suture, but the specific influence of the suture on rift emplacement is unclear.

As part of EMSLAB, broadband magnetotelluric data have been collected and analyzed at 11 sites along an east‐west transect in the Oregon Coast Range. Using one‐ and two‐dimensional modelling, the Coast Range was found to consist of marine sediments about 4 km deep near the coast and volcanic rocks about 8 km deep farther inland. Willamette Valley consists of surficial conductive sediments to a depth of 1 km, which overlie an eastern extension of the rocks that compose the Coast Range. An east dipping conductor, at a depth of approximately 20 km, is associated with the upper surface of the subducting Juan de Fuca plate. Possible mechanisms for the high conductance are water‐saturated fractured basalts forming the surface of the plate, subducted sediments, and/or the dehydration of the subducting slab trapped by metamorphic layering within the lower crust. This conductor correlates with an east dipping reflector, 35–40 km below the eastern flank of the Coast Range, defined by COCORP profiling and thought to represent the surface of the subducting Juan de Fuca plate.

Given the degree of complexity of modern magnetotelluric (MT) instrumentation, comparison of the total performance for two or more systems is an important verification test. This paper compares the processed data from five MT systems which were designed and constructed separately, and which employ different electrode types, electrode separations, magnetometers, and methods of signal processing. The comparison shows that there is a high degree of agreement among the data from the different systems. The study also demonstrates the compatibility and reliability of the MT systems employed as part of EMSLAB Juan de Fuca (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate). This project, proposed by a consortium of institutions, involves not only magnetotellurics studies but also studies of magnetic variation, on land and on the sea bottom. The project calls for the real‐time MT systems to occupy stations along segments of a profile in Oregon. A composite profile will be created from the segments. Prior to commencing the main MT profiling phase, one week was set aside in August, 1984, for all groups to record and process MT data sequentially at six sites in diverse geologic terrains; this experiment was called mini‐EMSLAB.