Geology Reference
In-Depth Information
ocean crust as well as upper mantle rocks from more
than three miles below the seafloor.
The upper pillow basalt layers of an ophiolite
sequence are highly porous, and during the ocean
spreading process the convection of seawater in
through the ophiolitic rocks and out near the ridge
crest contributes to the precipitatation of massive
sulfide deposits. In this way the upper portion of the
ophiolite is richly mineralized with copper, lead, and
zinc with smaller amounts of gold, silver and platinum.
Nickel and chromium ores are primarily associated
with the deep periodite layer of the ophiolite sequence.
Basalts, ultramafics, and gabbros in the Klamath
ophiolites have been altered to form the low-grade
metamorphic rocks, greenstone and serpentine. The
word, "ophiolite", meaning snake, refers to the rock
serpentine which is completely fractured and broken up
by faults giving it a smooth surface.
The Western Paleozoic and Triassic belt is to the
southeast of the Western Klamath terrane. These two
terranes are separated by a fault marking the surface
along which the Western Klamath terrane was pushed
beneath the Western Paleozoic and Triassic terrane.
Studies of the magnetic alignments of mineral crystals
in the rocks of the Western Klamath terrane suggest
that it has been rotated less than 100 degrees in a
clockwise direction since its origin in the late Jurassic
to arrive at its present orientation of northeast by
southwest. Presently the Klamath Mountains are in
approximate alignment with the Blue Mountains. Both
provinces display extension and clockwise rotation
suggesting that the Klamaths may have connected with
the Blue Mountains beneath the Cascades.
The Western Klamath terrane has been
subdivided into six subterranes. From east to west, they
are the Condrey Mountain subterrane, the Smith River
subterrane, the Rogue Valley subterrane, the Briggs
Creek subterrane, and the Dry Butte and Elk subterra-
nes. Two of these, the Condrey Mountain and Smith
River subterranes, have been the focus of considerable
attention because of their geologic environment and
economic minerals.
Southwest of Ashland, the Condrey Mountain
subterrane, 90% of which projects across the border
into California, is an inverted drop-shaped exposure
formed during the late Jurassic between 146 to 148
million years ago. A hot oceanic slab and cooler ocean
sediments, brought together by thrusting plates, were
altered to the metamorphic rock, schist. Within the
body of the Condrey Mountain, the schists range from
the low-grade greenschist on the outside, to the middle
layer of graphite-rich blackschist, and the core of high-
grade blueschist, all of which have been folded with
tight crenulations. Intensive erosion of the dome today
Model for pre-rotation configuration of West Coast
tectonic terranes (after Miller, 1987)
Commonly occurring in Klamath terrane rocks,
ophiolites are layered rock sequences up to 3 miles in
thickness that develop in the deep ocean floors be-
tween two spreading tectonic plates. At the bottom of
an ophiolitic series, dark-colored ultramafic rocks of
peridotite are overlain by gabbros that form the base of
the ocean crust. These in turn grade upward into a
complex of sheeted dikes without an apparent host
rock. Layered over the intruded dikes are pillow basalts
which are lavas extruded underwater onto the sea floor.
These pillow-shaped blobs of lava are capped by deep
sea cherts and pelagic clays with fossils of radiolaria
and foraminifera typically found today in the open
ocean under thousands of feet of water. Ophiolites are
significant as they offer a chance to examine first hand
