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about 10-20 km. Ingebritsen et al. (1989) believe that heat rises from great depths
through a relatively narrow zone.
Figure 12.44 shows a predictive geothermal and petrological model Cascadia,
generalizing current ideas of the subduction zone and its fluid regime (Romanyuk
et al., 2001b). The prediction is based on the existing estimates of the heat flow
and depths of the Curie isotherm (
500
◦
C) as well as petrological analysis of mag-
mas, and some other data. The continental crust above the downgoing Juan de Fuca
plate in the near-shore area is characterized by lower temperatures. A subvertical
zone of higher temperatures reaching the melting point of wet peridotite (
∼
900
◦
C)
has been outlined beneath the High Cascades. The release of fluids in the upper
part of the subducting plate can be associated with several mechanisms. First, free
water of micropores and microfractures is released under the action of increasing
lithostatic pressure at depths of up to 30 km. Then, at depths of 30-50 km, where
the temperature exceeds 400
◦
C, dehydration of minerals such as talc, serpentine,
and chlorite starts. Finally, basalt is transformed into eclogite at depths greater than
75 km, and amphibolite exsolution can take place at depths exceeding 90 km. All
these processes are accompanied by release of fluids. Supposedly, fluids released
at shallow depths migrate through the contact zone between the subducting and
continental plates. At greater depths, fluids can be absorbed by mantle peridotites
(serpentinization) at lower temperatures and disturb phase equilibria at higher tem-
peratures, giving rise to wet melting. Melts migrate upward to the Earth's surface,
resulting in the formation of a volcanic arc.
In conclusion, we note the results of sea-floor frequency sounding on the Pacific
Plate (Vanyan, 1997). The upper part of the oceanic crust consisting of sediments
and basaltic pillow lavas is characterized by a higher porosity and has a resistivity
of 3-10 Ohm
∼
·
m. Below, the resistivity markedly increases, reaching at least 10 000
Ohm
m.
Such is the a priori geological and geophysical information on the basis of which
we will interpret geoelectric data obtained on the Lincoln line.
·
12.7.3 MT and MV Soundings on the Ocean Coast
The resistivity contrast on the ocean coast, reaching three and even four orders of
magnitude, causes a strong MT anomaly referred to as the
coast effect
. This anomaly
has galvanic and inductive components.
The galvanic anomalies arise when the electric current flows perpendicular to the
shoreline (the TM-mode). The shoreward oceanic electric current divides into two
branches.
One part of the current flows into the continental sedimentary cover. Sediments
capture the oceanic current and channel it far from the coast, with slow leakage
into the crystalline basement and deep conductive zones. This phenomenon can be
referred to as a
continental-trap effect
. The size of the continental trap is of the
order of several adjustment distances
√
S
1
R
2
, where
S
1
=
h
1
/
1
is the average
