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MAGNETOVARIATIONAL SOUNDING: NEW POSSIBILITIES N -50
18 30 75 78

717 200 S y, km
90

0
83 83 82

50
78 32 29 21

100
21 35 37

150
43 45 45

66

71

1 2
601 150 270 86 000 246 44 600

5 10 20 50 100 200 500 km Crustal conductor 100 m
120 481

A
>1000 108 199 97 >1000 23 11 77 120

B
56 5 7 >1000 33 39 >1000 223 24 904 160 7423 9 15 37

C
65 >1000 121 10 66 41 >1000 77 29

99

90

Fig. 15. The PI model: parallel inversion of Re Wzy, Im Wzy, ||, , and using the II2DC program; resistivity values (in m) are shown within blocks, and the region of lower crustal resistivities is shaded (cf. Fig. 4).
Abyssal basin Coast Range Willamette Valley Western Cascades High Cascades

100
250Аы

200

Coast

400

Distance, km

Depth, km

Harzburgite replaced with depth by spinel-garnet lherzolite

250Аы

900Аы (dry lherzolite solidus)

500Аы 900Аы 750Аы 500Аы 750Аы

50

Asthenosphere (lherzolite with ~2% melt)
Neogene sediments of a continental basin Loose/compacted oceanic sediments Post-Eocene accreted sediments Pre-Eocene accreted oceanic sediments Oceanic basalts/gabbros Mafic eclogites Pre-Eocene continental crust, mostly composed of accreted terranes and subjected to Tertiary amphibolite/granulite metamorphism

900Аы

H2O

750Аы 1250Аы

100

Late Paleocene-Mid-Eocene mafic basalts and gabbroid rocks intruded into the accretionary prism or coastal portions of the pre-Tertiary crust Late Eocene-Mid-Miocene volcanic-sedimentary complex of the Western Cascades Late Eocene-Mid-Miocene basalts/gabbroids intruded into the pre-Tertiary crust Neogene andesitic basalts Recent amphibolite/granulite metamorphism Melting of wet/dry peridotites Subducting sediments and fragments of the oceanic and continental crust Prehnite-pumpellyite/blueschist metamorphism in the contact zone Melting of wet acidic rocks/eclogitization of basic rocks Serpentinization of mantle peridotites in the mantle wedge Volcanoes

H2O

150

Fig. 16. Predictive geothermal and petrological CASCADIA model, constructed along an E-W profile across central Oregon [Romanyuk et al., 2001]. IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 39 No. 9 2003


718

BERDICHEVSKY et al. (a) CR 0 50

CB 150 4:1 100 50

NB

WV 100

WC 150

HC

DP 200 km

0 5 25 50

1:1 100

150 200 1:4 300 400 km (b) 250 10
-1

200

CB 150 100

NB 50 0

CR 50

WV WC HC 100 150 km 10
-1

10

0

10

0

10

1

10

1

10

2

10

2

km Resistivity, m <1 <3 < 10 < 30 < 100 < 300 < 1000 < 3000

km

Fig. 17. Geoelectric models of the Cascadian subduction zone: (a) EMSLAB-I [Wannamaker et al., 1989b]; (b) EMSLAB-II [Varentsov et al., 1996]. CB, Cascadia basin; NB, Newport basin; CR, Coast Range; WV, Willamette Valley; WC, Western Cascades; HC, High Cascades; DP, Deschutes Plateau.

from the 2-D pattern. The EMSLAB-I model minimizes the misfits of the curves and and ignores the curves || and ||. Its main elements are (1) the upper conductive part of the plate, sinking at a low angle beneath the Coast Range; (2) a subhorizontal conducting layer in the middle continental crust broadening in the area of the High Cascades; and (3) a well-developed conductive asthenosphere beneath the ocean. The problem of the junction between the slab and the crustal

conductor remains open in this model. The continental asthenosphere is not present in this model, although the shape of the experimental curves || and || suggests a low resistivity of the upper mantle. The absence of large divergences between the model values of Re Wzy and Im Wzy, on the one hand, and the experimental data, on the other hand, is considered by the authors as evidence of the reliability of the model.
Vol. 39 No. 9 2003

IZVESTIYA, PHYSICS OF THE SOLID EARTH