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low resistivity in the upper mantle. The absence of gross discrepancies between
the observational and model values of Re
W
zy
,
Im
W
zy
is regarded as evidence of
reliability of the model.
Unfortunately, the EMSLAB-I model is vulnerable to criticism. A resistive (cold)
continental mantle contradicts the modern geodynamic concepts of the Cascadia
subduction zone (compare EMSLAB-I model with the predictive model CASCA-
DIA shown in Fig. 12.43). It is natural to think that the EMSLAB-I model does
not show the continental asthenosphere because of a low sensitivity of TM-mode to
mantle conductors. The point is that in the Cascadia subduction zone, only bimodal
inversion using both modes (TE + TM) can provide a key to studying the astheno-
sphere (Berdichevsky et al
.
, 1992).
Experiments on the bimodal interpretation of MT and MV data from the Cas-
cadia subduction zone resulted in the EMSLAB-II model (Fig. 12.53). This two-
dimensional model has been constructed with the INV2D-FG program designed for
automated inversion (Varentsov et al
.
, 1996). The program enabled the optimization
of resistivities in 20 blocks with a fixed geometry. The following response functions
were used in the course of bimodal inversions of MT and MV data: (1)
⊥
and
(minimum
weight). The EMSLAB-II model has much in common with the EMSLAB-I model:
the same oceanic asthenosphere, the same downgoing plate, and the same crustal
conductive layer. However, the plate is connected with the crustal conductor, and
the continental mantle contains a conductive asthenosphere (!) separated from the
oceanic asthenosphere. Thus, a new evidence for partial melting in the continental
mantle has been obtained.
and
⊥
(normal weight), and (3)
Re
W
zy
(maximum weight), (2)
CB
NB
CR
WV
WC
HC
250
200
150
100
50
0
50
100
150
km
0.1
0.1
1
1
10
10
100
100
km
km
resistivity (Ohm·m)
<1
<3
<10
<30
<100
<300
<1000
<3000
Fig. 12.53
The EMSLAB II model. Notation is the same as in Fig. 12.52 (Varentsov et al., 1996)
