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Fig. 12.49 Conductance S of the upper layer along the Lincoln line: 1-observations, 2-spline
approximation
Figure 12.49 plots the conductance S of the upper layer along the Lincoln line.
The plot is based on bathymetric data and MT soundings from oceanic and con-
tinental segments of the profile. The conductance S in the deep ocean is of the
order of 10000 S. The Coast Range, composed of Early Tertiary sediments and
volcanic rocks, is characterized by a conductance of about 100-150 S. The volcanic-
sedimentary complex of the western part of the Western Cascades and the thick
sequence of Tertiary deposits filling the Willamette Valley are characterized by con-
ductances reaching 250-300 S. In the eastern part of the Western Cascades, where
these deposits wedge out, the conductance drops to 10 S. Beneath the High Cascades
and the backarc plateau, the S values once again increase, reflecting the development
of volcanic-sedimentary rocks underlying Late Tertiary and Quaternary volcanic
formations.
Figure 12.50 presents a map of the conductance S of the upper layer encompass-
ing the Juan de Fuca Ridge and plate with adjacent continental areas. In constructing
this map, the data on sediments thickness and their average resistivities have been
used. The Juan de Fuca Ridge (I) and the abyssal basin of the Juan de Fuca plate
(II) are clearly seen in this map. The following structures of submeridional strike
are outlined on the continent: the Coast Range (III), the Willamette Valley (IV) and
the Puget Sound lowland (V), the Western Cascades (VI) and the High Cascades
(VII). This map has been incorporated into a less detailed map covering the entire
northwestern United States (1280 km
×
1280 km) and superimposed on a uniform
background of S
10000 S. Estimates indicate that, in modeling the magnetotel-
luric field in the central part of this map, the influence of its edges can be neglected.
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