Geology Reference
In-Depth Information
The distance between the subduction
trench and volcanic archipelago is
believed to be a function of the relative
age of the crust being subducted and
the rate of plate movement. Young,
warm crust moves rapidly, is buoyant,
and assumes a low angle of subduction
with considerable distance between the
arc and trench. Older, cooler crust
moves more slowly, is less buoyant,
and droops at a pronounced angle
upon subduction bringing the arc very
close to the trench.
As an alternative to the entire Coast Range
block swinging like a clock hand, the "ball-
bearing" model accommodates the measured
rock magnetic rotations by a simple rifting
mechanism [after Wells and Heller, 1988].
this proto-Cascade volcanic belt suggests that the
underlying subducting slab was at a shallow angle and
that the rate of collision between the North American
and Farallon plates was rapid.
Today the origin of the Cascade range is still
not well understood. One suggestion for the growth of
the range is that a volcanic chain above an actively
subducting ocean trench developed behind a slowly
rotating Coast Range tectonic block. Breaking loose
from the mainland along the Olympic-Wallowa linea-
ment, the block rotated almost 50 degrees clockwise
with a pivotal point in the northern Washington Coast
Range to its present position. An alternate proposal is
that Cascade volcanism was produced by a steepening
subducting slab due to a marked decrease in the
convergence rate between the North American and
Farallon plates.
area of the Willamette Valley and just to the west of
the volcanic vents along the Cascade archipelago.
Volcanic ash from the vents was flushed by streams
into shallow marine basins along the coast where thick
forests of tropical to subtropical plants on the low
coastal plain are now preserved as wood, leaves, and
pollen. Coal-producing swamps and estuaries formed
the alluvial deposits of the Colestin Formation, the
oldest rocks in the Cascades. Nonmarine volcanic tuffs,
sandstones, breccias, and lava flows of the Colestin
reach a thickness of over 4,000 feet along the Cascades
from Lane County south to California. From Eugene
northward to Molalla, upper continental shelf sands
and silts were the final marine sediments to be deposit-
ed along the retreating ocean shoreline. Molluscs and
a variety of other invertebrates preserved in these rocks
of the Eugene, Illahee, and Scotts Mills formations
reflect warm subtropical to temperate waters. Broken,
worn fossil fragments in some areas testify to stormy
coastal conditions, while local accumulations of barna-
cle shells are so rich in many of these near-shore
Cenozoic
During the Eocene and Oligocene epochs the
coastline angled northwest by southeast through the
