Geology Reference
In-Depth Information
In order to explain the simultaneous eastward
advancement and narrowing of the Cascades volca-
nic front, a popular synthesis calls for a "knee-bend"
in the subducting slab that is swinging upward (after
Priest, 1990; Duncan and Kulm, 1989; Verplanck and
Duncan, 1987).
uplift, what was once a moist tropical climate in the
eastern part of the state was transformed to the high
desert environment of today. The uplift was accompa-
nied by intensive downcutting by streams and erosion
of the mountains so that today the older Cascades
display deeply cut ridges of resistant lava rather than
the rounded volcanoes that once existed.
Concurrent with and resulting from High
Cascade volcanic activity was the formation of struc-
tural grabens, lying within the eastern margin of the
Cascade range. The word "graben" derives from the
German "grave" and refers to the sunken trough of
earth above a collapsed buried coffin. Structurally a
graben is a lowered block sitting between two faults.
Beneath much of the younger High Cascades enor-
mous, discontinuous grabens, 10 to 20 miles wide,
extend for 300 miles from north to south. In the
central Cascades, where volcanoes are densely clus-
tered, both the east and west sides of the graben are
well-developed. At the northern and southern borders
of the state, where volcanoes are more sparce, the
graben is either incomplete or only a "half graben.
Movements of the various faults that make up the east
and west sides of the trough are estimated to have
dropped down an average of 2,000 feet. The sinking of
the grabens, which began between the late Miocene
and very early Pliocene, 5 to 10 million years ago, has
continued well into the late Pliocene. Collapse of the
grabens followed the eruptions of volcanoes pumping
out magma that left many chambers beneath the area.
With the outpouring of basalts and growth of broad
shield volcanoes, the heavy volcanic "loading" of the
incipient High Cascades may also have contributed to
the downward rupture of the grabens.
The Cascade ranges are still too young for
erosion to have extensively exposed the batholiths that
fed the volcanic eruptions. The siliceous composition
of some of the High Cascade volcanoes, however,
suggest that these chambers of magma were close to
the surface because the highly viscous, molten rock,
which forms the massive batholiths, has difficulty
travelling upward very far into the crust before cooling.
Although these intrusive bodies should theoretically be
present at shallow depths beneath the High Cascades,
geophysical probes of the range suggest the source
magmas are deeper in the lower crust and upper
mantle.
Within the Western Cascades, intrusive batho-
lithic rocks are situated in two north-south belts
running from Canada across Washington and Oregon.
Having been emplaced between 35 and 15 million years
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