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In-Depth Information
Table 12.3
Misfits of longitudinal phases
Station
1
2
5
6
7
8
9
11
12
14
15
, deg
4.0
2.6
3.1
4.6
5.5
4.0
2.0
4.1
2.5
2.3
4.9
, deg
23
21
21
23
45
38
29
42
20
29
41
of their low-frequency branches). The remarkable accord between the model and
the observation catches the eye.
In its oceanic segment, the EMSLAB-III model is similar to EMSLAB-I and
EMSLAB-II models, resolving a thick oceanic asthenosphere in a depth range of
37-110 km.
In the continental segment, the EMSLAB-III model clearly defines a crustal
conductive layer (25-40 km,
=
20 Ohm
·
m) and a conductive asthenosphere
(100-155 km,
m). The crustal and asthenosphere conductors are con-
nected by a columnar conductive body (
=
30 Ohm
·
m) that penetrates through
the lithosphere and reaches a depth of about 7 km under the volcanic zone of the
High Cascades.
The downgoing oceanic plate in a depth range of 4-40 km contains a thin inclined
conductor (
=
20-30 Ohm
·
=20Ohm
·
m) separated from the crustal conductive layer by a zone of
higher resistivity (
m). To all appearance, the fluids in the crustal
conductive layer are of internal or mantle origin.
We would like to stress that EMSLAB-III model clearly reflects the fluid regime
of the subduction zone. The downgoing plate drags low-resistivity water-saturated
rocks of the ocean floor. As the plate sinks, free water is driven out and migrates
through the shear zone (at the contact between the subducting oceanic and sta-
ble continental plates). Dehydration (release of combined water), beginning at
depths of about 30-40 km in the downgoing plate supplies fluids to the mantle
and causes wet melting of asthenospheric material. Low-resistivity melts migrate
upward through the lithosphere and form the volcanic arc. The heating of the
lithosphere activates dehydration in the lower crust, forming the crustal conductive
layer.
The new geoelectric model EMSLAB-III of the Cascadia subduction zone recov-
ered from the magnetovariational and magnetelluric data with magnetovariational
priority fills the most essential gaps in the previous EMSLAB-I and EMSLAB-II
models and is in excellent agreement with the predictive geothermal and petro-
logical CASCADIA model behind which are present-day concepts of geodynamic
history of the plate subduction. It seems that the MV-sounding with its rather high
sensitivity to horizontal and vertical conductivity variations and rather high immu-
nity to near-surface galvanic distortions can be considered as an efficient tool of the
modern geoelectrics. Today the geophysicists have every reason to develop a new
magnetotelluric strategy that would realize these attractive potentials of the mag-
netovariational sounding, especially in the deep geoelectric studies. Do not forget
that many things worth doing in the world had been declared impossible before they
were done.
= 200-500 Ohm
·
