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Question

No two subduction zones are identical. Their differences may include the width
of the accretionary prism, the rate of subduction, and the steepness of the sub-
ducted plate as it moves into the mantle. Earthquakes occur in the subducted plate
as it moves, concentrated in what geologists call a Wadati-Benioff zone. We can track
the plate as it is subducted by the depths and locations of earthquake foci (points
where energy is released).
FIGURE 2.12 Anatomy of an island arc-trench system.
Arc trench gap
Volcanic island arc
Accretionary
prism
Trench
Fore-arc
basin
Oceananic lithosphere
Asthenosphere
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEA
Even without any earthquake information, it is possible to estimate the steepness
of a subduction zone from its anatomy. All you need is an accurate map of the sea-
floor and a little geologic reasoning based on your knowledge of arc-trench systems.
EXERCISE 2.9
Estimating the Steepness of Subduction Zones (continued)
We will see how this is done in Exercise 2.9
Name:
Section:_
EXERCISE 2.9
Estimating the Steepness of Subduction Zones
Course:
Date
The boundaries of the Wadati-Benioff zone track the progress of the subducted slab of lithosphere into the mantle, and
Name:
Course:
Section:
the island arc sits directly above the point at which the mantle above the subducted plate begins to melt.
(c) At what depth does melting apparently occur beneath the Amchitka segment of the arc? _
guns 150
Date:
- km
(a) Based on island arc-trench geometry (see Fig. 2.12) and the fact that melting typically occurs at about the same
(d) How does the point at which melting occurs control the distance of the volcanic arc from the trench?
depth in all subduction zones, what is the major factor that controls the width of the arc-trench gap? Explain.
(b) Based on your answer to question (a), sketch two island arc-trench systems, one with a wider arc-trench gap than
(e) Sketch the outlines of the subducted plate for each segment on its profile.
the other.
Hints:
. Assume that melting occurs at the same depth in each segment.
Draw a vertical line from the volcanic arc to the melting depth.
Draw a horizontal line to represent the melting depth.
Draw the subducted slab so that its upper surface passes through the intersection point.
Using your protractor, measure the angle of subduction downward from the horizontal.
(f) For each segment, record the following data in the table provided:
. The width of the arc-trench gap
The angle of subduction
Narrow arc-trench gap
Wide arc-trench gap
The Aleutian arc extends westward from mainland Alaska into the Pacific Ocean. Profiles across four segments of the
Amchitka
Shumagin Islands
Cook Inlet
kwentna
Aleutian arc are given in the graphs that follow, which show the positions of the volcanic arc and trench for each segment.
Gray dots in the profile of the Amchitka segment show the earthquakes in the Wadati-Benioff zone.
Arc-trench gap
Subduction angle
Volcanic arc
Volcanic arc
Trench
(g) Based on these data, what is the relationship between the steepness of subduction and the width of the gap
Trench
between the volcanic arc and the trench?
100
100
Depth (km)
200
200
Depth (km
200
Depth of
zone
melting
300
Amchitka segment
Shumagin Islands segment
200
400
600
800
200
400
600
800
Distance (km)
Distance (km)
Volcanic ard
Trench
Volcanic arc
Trench
TTTTTTTTTTTTTTT
2.5.4 Transform Faults
We have thus far looked at transform faults only in the oceans-the oceanic frac-
10
ture zones-but some transform faults also cut continental lithosphere. The most
Depth (km)
200
epth (km)
famous of these faults constitutes the San Andreas fault system of California, which
200
has caused major damage and loss of life over the past 100 years. The San Andreas
fault system (shown in Exercise 2.10) is not a single fault, but rather a zone several
300
300
miles wide containing numerous active transform faults. It extends for more than
Cook Inlet segment
Skwentna segment
1,000 km, connecting segments of the Juan de Fuca Ridge and Cascade Trench at
200
400
600
800
its northern end to an unnamed mid-ocean ridge segment in the Gulf of California
200
400
600
800
Distance (km)
Distance (km)
to the south.
(continued)
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEATURES
43

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