Conductive structures and tectonics beneath the EMSLAB land array

Abstract

Magnetovariation fields were recorded by the land part of the EMSLAB array, August‐September 1985. Data from two complete days and two shorter sequences, chosen for moderate magnetic activity, are here used to map induced currents, which flow preferentially in the more conductive rocks, in Washington and Oregon and adjoining areas. Fourier transform anomaly maps, induction vectors, and hypothetical event anomaly maps are used to delineate conductive structures. These include the ocean and suboceanic asthenosphere and extended regions of attenuated vertical component caused by highly conductive layers under the Basin and Range Province and the Canadian Cordillera, both considered associated with partial melting and hydrothermal water. The Cascade Range of volcanoes has a north‐south strip of high conductivity some 100 km wide beneath it, which is named the Deep Cascades conductor. Between 46° and 47°N the Deep Cascades currents appear to flow up through a narrow upper crustal conductor, well known from prior work by others, into Puget Sound. The Deep Cascades conductor correlates with high heat flow and probably delineates the partial melting beneath the volcanic arc. At its southern end it appears to be continuous with the Basin and Range regional conductor. Northward it terminates between 47.5° and 48°N, south of Mounts Baker and Garibaldi. The Blue Mountains of NE Oregon and SE Washington coincide approximately with a kidney‐shaped block of resistive lithosphere, which lies between the Klamath‐Blue Mountain Lineament and the conductive mantle of the Basin and Range. The Columbia Embayment and plateau basalts occupy a triangular block of lithosphere more resistive than the Canadian Cordilleran Regional Conductor, which abuts against it, but less resistive than the Blue Mountains. Boundaries between these tectonic units are mapped from the associated magnetovariation anomalies, that is, in terms of electrical conductivity.

Electrical structure of the central Cascadia subduction zone: The EMSLAB Lincoln Line revisited

“…A large onshore-offshore magnetotelluric experiment, EMSLAB, completed in the late 1980s, is an example of the focus given to Cascadia Gough et al, 1989;Wannamaker et al, 1989;Jiracek et al, 1989;Hellfrich, 2003). At the time it was completed, it was the largest and most ambitious experiment of its kind, and was a consortium effort of 12 institutions in four countries ).…”

“…A regional magnetovariational (MV) study, which uses the time-varying magnetic field alone to infer structure, identified several distinct tectonic regimes including the northwestern Basin and Range and segmentation of the volcanic chain near Mt St. Helens and Mt Adams. The MV data also demonstrated a fundamentally N-S preferred orientation to structure over most of the survey domain (Gough et al, 1989). A detailed central transect of MT soundings, the Lincoln Line, incorporated both seafloor and land stations with data to periods of 10,000 s over a length of nearly 700 km.…”

Modelling electromagnetic responses of 2-D structures due to spatially non-uniform inducing fields. Analysis of magnetotelluric source effects at coastlines

“…One example is the EMSLAB array (Gough et al 1989). Some decades ago, various large 1-D or bi-dimensional (2-D) arrays of magnetometers were deployed around the world to study both the electrical structure of the crust and upper mantle and the morphology of the external magnetic field.…”

The BC87 Dataset: Tectonic Setting, Previous EM Results, and Recorded MT Data.

“…6 does not support the presence of an elongated conductor, although a very localized one around one station in Alberta is apparent. The EMSLAB array maps (GOUGH et al, 1989) are equivocal on this question, although GOUGH et al (1989) consider that they support the existence of the SABC conductor. They do, however, express surprise that "the (SABC) currents stopped at the border".…”