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magnetotellurics can serve as a productive tool for studying the subduction zone.
Presently, we recall these results with an understanding of the important role that
they played in the history of deep geoelectric studies.
The 1990s were marked by a rapid development of computational geoelectrics
(Zhdanov and Spichak, 1992; Mackie and Madden, 1993; Varentsov, 1999;
Siripunvaraporn and Egbert, 2000; Novozhynski and Pushkarev, 2001). The cre-
ation of computational programs that enable the automatized inversion of the MT
and MV response functions in complex media opened the way toward the improve-
ment of the EMSLAB results (Zhdanov and Spichak, 1992; Berdichevsky et al.,
1992a; Varentsov et al., 1996). This discussion set the stage for revising ideas of the
Cascadia subduction zone. Let us follow (Pushkarev, 2002; Pushkarev et al., 2002;
Vanyan et al., 2002a) and consider a new model of the Cascadia subduction zone
constructed in the hypoteses test mode with the priority of MV soundings.
12.7.1 Brief Geological Description of the Cascadia
Subduction Zone
The region under study is a part of the Pacific orogenic belt, characterized by intense
Tertiary and Quaternary volcanism. The main geological structures of the region
originate from the subduction and accompanying volcanism (Khain and Lomize,
1995). They extend S-N for up to 300-500 km. The S-N (
x
) and W-E (
y
) directions
can be regarded as the major tectonic directions, defining the longitudinal (
||
) and
transverse (
) components of the impedance tensor.
The Juan de Fuca spreading ridge, giving rise to the Juan de Fuca plate, is located
at a short distance from the coast (about 500 km). Moving eastward, we cross the
following structures: (1) the abyssal basin of the Juan de Fuca plate, (2) the continen-
tal slope composed of compacted sediments of the accretionary prism, (3) the shelf
covered by loose sediments, (4) the Coast Range consisting of volcanic-sedimentary
rocks, (5) the gently dipping Willamette Valley filled with a thick sequence of sed-
iments and basaltic intrusions, (6) the Western (older) and (7) the High (younger)
Cascade mountains composed of volcanic and volcanic-sedimentary rocks typical
of present-day active volcanic arcs, and (8) the lava-covered Deschutes Plateau.
The oceanic crust within the abyssal basin of the Juan de Fuca plate has a struc-
ture typical of the Pacific. It comprises three layers: (1) a sedimentary layer 1-2
km thick, (2) a 1.5-2 km thick layer of basalts (pillow lavas) and basaltic flows
with dolerite dikes, and (3) a layer of fully crystalline igneous rocks (gabbro and
ultramafic varieties) 3-4 km thick.
The Cascade Range includes high peaks and sharply defined mountain crests.
The highest peaks are volcanic cones developed on the ancient basement. The moun-
tain structures are composed of Oligocene-Pliocene volcanic rocks represented by
lava flows and significant amounts of breccias, tuffs, and mudflow deposits. The
structure of the Cascade Range is complicated by the emplacement of intrusive
massifs.
The more easterly plateau is also dominated by volcanic rocks with prevailing
Pliocene and Pleistocene lavas.
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