Geoscience Reference
In-Depth Information
Figure 9.61.
Structure
across the Cascadia
subduction zone of
western Canada where
the Juan de Fuca plate is
descending beneath the
North American plate.
Land is shown by solid
horizontal bar. (a) Surface
heat-flow measurements.
(b) Estimated isotherms,
earthquake foci (dots),
strongest E reflectors
(short lines) and zone of
high electrical
conductivity assumed to
be associated with water
(shaded zone). (From
Lewis
et al
.(1988) and
Hyndman (1988).)
(a)
100
80
60
40
20
0
?
0
100
150
200
250
50
Distance (km)
(b)
Georgia
Strait
Vancouver Island
Shelf
Mainland
0
200
20
400
E
40
600
60
?
80
trench and the marginal basin, but not beneath Honshu. There the highest velocity
measured was 7.5 km s
−
1
. This low velocity is characteristic of the asthenosphere.
The density model for the Chile Trench and Andes was constrained by refrac-
tion data, which indicate that the crust beneath the Andes is some 60 km thick, the
upper 30 km having a seismic P-wave velocity of 6.0 km s
−
1
and the lower 30 km
avelocity of 6.8 km s
−
1
. This thick crust appears to have grown from underneath
by the addition of andesitic material from the subduction zone rather than by
compression and deformation of sediments and pre-existing crustal material.
The convergent plate boundary off the west coast of North America, where
the North American plate is overriding the young Juan de Fuca plate (Fig. 2.2)at
about 2 cm yr
−
1
,isanexample of a subduction zone with no easily distinguishable
bathymetric trench. The dip of the subducting plate here is very shallow, about
15
◦
. The subducting oceanic plate is overlain by a complex of accreted terranes,
which are exposed on Vancouver Island and the mainland. This is, therefore, not
a simple subduction zone but one where subduction has assembled a complex
assortment of materials and pushed or welded them onto the North American con-
tinent. Figure 9.60(a) shows that there is a major high-seismic-velocity anomaly
extending to at least 1500 km depth, namely the subducted Farallon plate, of
which now the Juan de Fuca plate is all that remains at the surface (Sections 3.3.4
and 8.1.4). Figures 9.61 and 9.62(b) show the thermal structure and the seismic
P-wave velocity structure across this subduction zone. The subducting oceanic
plate and the 35-km-thick continental crust are clearly visible in the seismic
model. One unusual feature of the velocity model is the wedges of somewhat
higher-velocity material and bands of low-velocity material immediately above
the Juan de Fuca plate. One interpretation is that the low-velocity material is
