Geology Reference
In-Depth Information
are separate groups of rocks formed in an open oceanic
or coastal environment, each group with its own
layered sequence of distinctive rocks and fossils by
which it is recognized. These slabs of oceanic rock were
rafted toward North America from the west where they
were bent, folded, and broken upon collision. As they
were accreted to North America, the succeeding
terranes were thrust beneath each other like shingles
on a roof with the oldest to the east and the youngest
to the west. All of the terranes are separated from each
other by fault zones.
During the Paleozoic and Mesozoic more than
250 million years ago, the Klamaths began as an ocean
chain or island archipelago that extended in a north-
west line down from British Columbia and Washington
into Idaho and California. Assembled very close to the
North American West Coast, terranes were accreted to
North America in the middle and later Jurassic. In
middle Jurassic time, a subduction zone between two
tectonic plates generated a series of volcanoes atop
these older, accreted terranes. As the volcanic arc
separated from the landmass and migrated westward
away from North America, a backarc basin developed
between the older volcanic chain and the eruptive
centers situated above the subduction zone. During the
late Jurassic, sills and dikes were intruded into the
basin. In the final stage, the arc, basin, and remnants of
the older volcanic chain migrated toward North Ameri-
ca where they were accreted and imbricated in thrust
sheets over each other.
From south to north the provinces today
include the Sierra Nevada in California, the Klamath
and Blue Mountains in Oregon, Idaho, and Washing-
ton, and the Cache Creek area in central British
Columbia. Terrane rocks of the Klamath province are
linked to those of the Sierras of California and the
Blue Mountains in northeast Oregon by strikingly
similar fossils and rock layers. Even though the rela-
tionship between these regions is still not well under-
stood, all three form a discontinuous belt of Paleozoic
and Mesozoic rocks. This 1,000 mile long ancient
mountain chain bends in a northeast direction across
the state beneath the Cascades from the Klamaths to
the Blue Mountains before turning back toward the
north in the Blue Mountains to continue into north-
western Washington. This kink is thought to have
developed during early Tertiary with rotation and
westward displacement of the Klamath and Blue
Mountains provinces.
After initial contact of the exotic blocks to the
North American continent, the Klamath block was
rotated into a final position. It is particularly important
to fix the time the terrane was accreted to the main
Distribution of terranes in southwest Oregon and
northern California (after Irwin, 1985)
westward by a well-developed riverine system. Draining
the Idaho batholith and Klamaths, the rivers deposited
the sediments in the forearc basin along the margin of
the mountains. Uplift in the Miocene resulted in
extensive erosion of the region. The Pleistocene
brought only small glaciers to these mountains, while
continuing uplift along with rising and falling sea level
produced coastal terraces.
In their long history as part of the sea floor as
well as during the emplacement of granitic intrusions,
Klamath terrane rocks were enriched with a diversity of
economic minerals including gold, copper, nickel, and
chromite.
Geology
Because the Klamath Mountains are made up
of composite belts of rocks formerly part of an ocean
environment, the concept of displaced accreted terranes
is fundamental to understanding geology here. Terranes
