Download Eastern Klamath Mountains - College of the Siskiyous

Geologic Overview of the
Eastern Klamath Mountains
William Hirt
Department of Natural Sciences
College of the Siskiyous
Weed, California
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Hirt–Eastern Klamath Mountains
Hirt–Eastern Klamath Mountains
INTRODUCTION
The Klamath Mountains of northern
California and southern Oregon (Fig. 1) are
one of the best studied and most accessible
assemblages of accreted terranes in the
world. This paper will introduce the plate
tectonic setting, rock types, geologic history,
and mineral deposits of the oldest terrane in
the range—the Eastern Klamath terrane—
as preparation for our upcoming field trip.
The definitions of words shown in italics
are given in a glossary that follows the
references.
PLATE TECTONICS
In order to understand the origin of the
Klamath Mountains we need to review
a bit about the outer part of the Earth.
In particular, we need to know what
lithospheric plates are, how they are formed
at divergent boundaries and consumed at
covergent ones, and how plate convergence
can lead to terrane accretion and the
production of magmas along subduction
zones.
Earth’s internal structure
Earth’s interior is divided into three major
parts: the core, mantle, and crust. Of
these, only the upper mantle and crust
play major roles in our discussion of the
Klamath Mountains. The crust and the cold,
rigid upper mantle move together as a
single sheet called the lithosphere. Earth’s
lithosphere is broken into about a dozen
large plates and a number of smaller ones
(Fig. 2). Each of these 100 to 150 km-thick
plates is composed mostly of peridotite, a
magnesium-rich rock that makes up Earth’s
mantle. A 4 km-thick layer of slightly less
dense mafic rock (mostly basalt) caps
oceanic lithosphere, whereas a 45 km-thick
layer of even less dense felsic rock (mostly
Figure 1. Map of California’s geomorphic provinces
showing the Klamath Mountains in the northwestern part of the state. From the California Geological
Survey website.
granite) lies atop continental lithosphere.
Beneath the lithosphere is a soft region of
the upper mantle called the asthenosphere
that extends to a depth of about 350 km.
Because the rock here is hotter than that
in the overlying lithosphere, it contains a
small percentage of melt that lubricates
the movement of its grains. As a result, the
asthenosphere flows relatively easily and
accommodates the horizontal and vertical
motions of the overlying lithospheric plates.
Boundaries between the plates are the sites
of most of the volcanic and seismic activity
on Earth. Based on the relative motions
of the plates that adjoin them, these
boundaries are classified as being either:
divergent, where two plates separate;
convergent, where they move together; or
shear, where they slide horizontally past one
Figure 2. Simplified map of Earth’s lithospheric plates. Divergent boundaries are shown by solid gray lines,
convergent boundaries by gray lines with sawteeth in the direction of subduction, and shear boundaries by
dotted black lines. Diagonally ruled areas mark broad plate boundary zones, and dots mark hotspots. From
Simkin et al. (1994).
another. Here in the Pacific Northwest, for
example, the boundary between the North
American and adjoining oceanic plates is a
convergent boundary called the Cascadia
subduction zone. To the west lies a divergent
boundary, the Gorda-Juan de Fuca ridge;
and to the south a shear boundary, the San
Andreas fault.
Most divergent boundaries are midocean ridges where two oceanic plates
are separated by narrow zones of
extensional faults. As the plates pull apart
the asthenosphere wells up, undergoes
partial melting due to decompression,
and produces basalt magmas that rise
and erupt to build new oceanic crust (Fig.
3a). Although most oceanic lithosphere is
subsequently subducted (see below), some
fragments have been added onto continents
along convergent or shear boundaries. These
fragments of oceanic lithosphere, known as
ophiolites, grade downward from seafloor
“pillow lavas”, through “sheeted” basalt
dikes and gabbro, to mantle peridotite (Fig.
3b). All of these rock types are typically
altered by hot water that circulates through
the lithosphere near a mid-ocean ridge
to produce greenstone (altered basalts
and gabbros) and serpentinite (altered
peridotite).
Most convergent boundaries are subduction
zones where one plate of oceanic lithosphere sinks into the mantle beneath an
overriding plate. Only oceanic lithosphere is
able to subduct because continental lithosphere, with its less-dense felsic crust, is too
buoyant to sink into the mantle. Here in the
Pacific Northwest, the Gorda and Juan de
Fuca plates are sinking eastward beneath
the North American plate. Volatiles bound
into the downgoing plates is released as they
are heated within the Earth (Fig. 4). This
water causes the hot rock of the asthenosphere to undergo additional melting and
to produce mafic magmas that rise slowly towards Earth’s surface. Only about 10% of this
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Hirt–Eastern Klamath Mountains
Hirt–Eastern Klamath Mountains
do know that this terrane had begun to
“backstop” subsequent accretion events by
between 400-380 Ma (Irwin and Wooden,
1999). During approximately the next 250
million years a series of eight collisions built
the Klamath Mountains by suturing a succession of terranes to the western margin of the
North American plate (Fig. 7).
Figure 4. Block diagram of a continental-margin suduction zone like the Cascadia subduction zone here
in the Pacific Northwest. Note that in the ancient
Trinity subduction zone both the subducting and
overriding plates are thought to have been oceanic
lithosphere.
Figure 3. (a) Block diagram of a mid-ocean ridge.
(b) Schematic cross-section of the oceanic lithosphere. If such lithosphere is accreted to a continent
it it referred to as an ophiolite.
magma reachs the surface and erupts, however. The rest stalls in the crust, crystallizes,
and releases heat that melts the surrounding
crustal rocks and produces felsic magma.
Some of this felsic magma reachs the surface, but most cools slowly and crystallizes
underground to form bodies of granite and
related rocks, called plutons, that dot the
Klamath Mountains (Fig. 1). The Castle Crags
pluton (Fig. 12) is one such body.
Finally, although much of Earth’s oceanic
lithosphere is formed at mid-ocean ridges,
it can also be formed above a subduction
zone where extensional stresses pull the
lithosphere apart beneath an oceanic arc
and enable rising asthenospheric and
subduction-generated magmas to widen
and thicken the lithosphere. The Trinity
ophiolite of the Eastern Klamath Mountains
is interpreted as such a supra-subduction
zone ophiolite (Fig. 5) on the basis of its rock
compositions (Wallin and Metcalf, 1998).
Terrane Accretion
Terranes are large blocks of lithosphere that
(1) contain rocks record similar geologic histories, and (2) are typically separated from
one another by faults that mark ancient
subduction zones or shear boundaries.
Many of the terranes found in the Klamath
Mountains actually originated far from North
America. Some were formed from fragments
of continental lithosphere continental that
had been rifted away from their parent
landmasses, just as Baja California is being
rifted away from North America today. Others were once offshore island chains, similar
to the modern Japanese archipelago, that
collided with western North America when
Figure 5. Series of schematic cross-sections showing the development of the supra-subduction zone
ophiolite that comprises the Trinity subterrane
between about 440 and 400 Ma. From Wallin and
Metcalf (1998).
the intervening ocean basins were closed
by subduction. These continental fragments
and island arcs were carried across the Pacific Ocean basin as parts of subducting oceanic plates and collided with North America
when subduction consumed the intervening
oceanic lithosphere (Fig. 6).
Rocks in each terrane have been dated using fossils from sedimentary rocks (Fig. 8)
or the radiometric ages of volcanic rocks.
These dates provide maximum ages for the
times when each terrane was accreted. After
a terrane collided, subduction “jumped”
westward and magmas rose from the new
subduction zone and intruded into the
freshly-accreted terrane or into its boundary
with adjacent terranes to form “stitching plutons”. The radiometric dating of these latter
plutons provide lower limits on the ages of
the accretionary events. Table 1 summarizes
the ages of the eight accretionary events
that built the Klamath Mountains based on
mapping and dating by Irwin and Wooden
(1999). The last accretionary event occurred
about 145-140 Ma and corresponds. approximately, to the time of the Nevadan
mountain-building event farther south in
California.
After the accretion of the Pickett Peak terrane about 145-140 Ma, uplift created a
large, gently-dipping fault in the Eastern
Klamath terrane (Cashman and Elder, 2002).
This detachment fault (shallowly-dipping extensional fault) caused the forearc of the old
Trinity subduction zone (Yreka subterrane)
to move northwards and the backarc (Redding subterrane) to move southwards as the
central part (Trinity subterrane) rose (Fig. 9).
KLAMATH GEOLOGIC HISTORY
Although we do not know when the individual subterranes that comprise the Eastern Klamath Terrane were assembled, we
MINERAL DEPOSITS IN THE KLAMATHS
Mineral exploration in the Klamath Mountains began around 1850 when miners real-
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Figure 6. Block diagrams of subduction in the Pacific Northwest showing the approach (a) and accretion (b)
of a terrane to the westrern margin of North America. In part (b) notice that subduction has “jumped” to the
western edge of the first terrane and that a second terrane is approaching outboard of the first. From Orr
and Orr (1992).
Accretionary episode
Central Metamorphic
Fort Jones
North Fork
Eastern Hayfork
Western Hayfork
Rattlesnake Creek
Western Klamath
Pickett Peak
Figure 8. Trilobite fossil from the limestone of the
Devonian-Silurian Gazelle Formation.
Age (Ma)
380-400
240-260
193-198
≈180
168
164
150-152
140-145
Chromite (FeCr2O4) crystallized from basaltic
magmas as they rose through the peridotite
basement of the Trinity subterrane. Because
it is denser than the silicate minerals with
which it is associated, it settled to the bottoms of small intrusions where it formed
layers and segregations. Today it is found as
small bodies within the serpentinite of the
Trinity subterrane.
Table 1. Chronology of accretionary events that
assembled the terranes of the Klamath Mountains
from Irwin and Wooden (1999). Ages are given in
millions of years ago (Ma).
ized that the rocks exposed here are very
similar to those exposed in the Mother Lode
country of the northwestern Sierra Nevada.
Although placer gold mining continued
through the mid-20th century, deposits of
chromite associated with serpentinite of the
Trinity subterrane were only worked during
World War II when foreign chromium sources were unavailable. Mercury was mined at
the Altoona Mine from the late 19th century
through the middle 20th century and used,
in part, to recover gold by amalgamation.
Chromite Deposits
Lode Gold Deposits
Figure 7. Geologic map of the Klamath Mountains
showing the major accreted terranes and the plutons that intrude them or cut across the faults that
separate them. From Irwin and Wooden (1999).
As magmas rose from successive subduction
zones beneath the Klamath Mountains they
accumulated to form plutons in the upper
crust. The cooling of these plutons circulated groundwater that “scavenged” soluble
materials such as gold and silica from the
the plutons’ volcanic and sedimentary host
rocks. The hot solutions cooled as they rose
through fractures in the overlying crust and
deposited the silica as quartz to form veins
that also contain small amounts of gold and
sulfide minerals (Fig. 10). “Lode gold” was
recovered by mining out these veins, crushing the rock, and recovering the small gold
particles by smelting with a flux.
Figure 9. Simplified geologic map of the Trinity detachment fault and schematic cross-sections showing stages in its development. (a) The detachment
fault surface, marked by paired tics, separates the
Yreka subterrane (north) and the Redding subterrane (south) from the underlying Trinity subterrane.
(b) Series of schematic cross-sections along line A-A’
showing how the Yreka and Redding subterrane
rocks have moved as the Trinity subterrane rocks
have risen. From Cashman and Elder (2002).
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Hirt–Eastern Klamath Mountains
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GLOSSARY
Asthenosphere: Layer of Earth’s upper mantle that lies between depths of about 100
and 350 km and is relatively “soft” or weak
because of the presence of a small amount
of melt along the boundaries of mineral
grains within the peridotite.
Figure 11. Dredge spoils from large-scale placer
gold mining near Callahan, California.
Figure 10. Section of a small quartz vein from a
mine near Mugginsville that shows crystals that
grew in from the walls and choked off the flow of
hot water.
Placer Gold Deposits
Much of the gold in quartz veins from
the Klamath Mountains has been freed
by weathering and moved downslope to
streams by erosional processes . Native gold
is much denser than typical crustal rocks and
collects in stream beds at places where water velocities fall off, such as bedrock cracks
and bends in the channel. Such gold can be
recovered by dredging old stream deposits
as was done along the Scott River near Callahan (Fig. 11).
Mercury Deposits
Mercury occurs in the mineral cinnabar
(HgS) in calcite veins at the Altoona Mine in
the upper Trinity River drainage. The calcite + cinnabar veins were deposited from
alkaline solutions that flowed through the
serpentinite of the Trinity subterrane. This
occurrence is very similar to that of the large
mercury deposits at New Idria in the California Coast Range, although what the ultimate
source of the mercury in these deposits is
is not clear. A major cleanup undertaken by
the EPA in 2008 isolated the Altoona Mine
tailings from which mercury had been leaching into the Trinity River.
REFERENCES
Cashman, S.M., and Elder, D.R., 2002, PostNevadan detachment faulting in the Klamath
Mountains, California: Geological Society
of America Bulletin, v. 114, no. 12, p. 15201534.
Irwin, W.P., and Wooden, J.L., 1999, Plutons
and accretionary episodes of the Klamath
Mountains, California and Oregon: U.S. Geological Survey Open-File Report 99-374.
Orr, E.L., Orr, W.N., and Baldwin, E.M., 1992,
Geology of Oregon, 4th ed.: Dubuque, Iowa,
Kendall/Hunt, 254 p.
Simkin, T., Unger, J.D., Tilling, R.I., Vogt, P.R,
and Spall, H., 1994, This Dynamic Planet:
U.S. Geological Survey, scale 1:30,000,000.
Wallin, E.T., and Metcalf, R.V., 1998, Suprasubduction zone ophiolite formed in an
extensional forearc: Trinity terrane, Klamath
Mountains, California: Journal of Geology, v.
106, p. 591-608.
Basalt: Volcanic rock with a low silica content (about 47 to 52 wt. %) that typically
has a fine black groundmass and contains
coarser crystals of olivine, plagioclase, and
pyroxene.
Crust: The uppermost solid part of the Earth.
It consists of a 7 kilometer-thick layer of
mafic igneous rocks (mostly basalt) beneath
the oceans, and a 45 kilometer-thick layer of
mostly felsic igneous rock (granite) beneath
the continents.
Dike: A sheet-like body of igneous rock that
cuts across older rock bodies and is formed
from magma that solidified within a fracture.
Felsic: A family of igneous rocks—including coarse-grained granite and fine-grained
rhyolite—that contain 65-75% silica and
consist mostly of light-colored minerals such
as quartz and feldspar.
Hydrothermal: Literally, “hot water”. Hydrothermal systems in volcanic areas are typically fed by rain or snow melt that percolates
down into the Earth, is heated by hot rock or
magma at a shallow depth, and rises back to
the surface.
Lithosphere: The rigid outer layer of the
Earth, which includes both the crust and the
cool, stiff uppermost mantle. Lithosphere
is up to about 100 km thick beneath the
oceans and 150 km thick beneath the continents.
Lithospheric plate: One of several dozen
independent pieces of the lithosphere that
move about relative to one another across
Earth’s surface. Most seismic and volcanic
activity occurs at the boundaries between
these plates.
Mafic: A family of igneous rocks—including coarse-grained gabbro and fine-grained
basalt—that contain 45-55% silica and
contain significant amounts of dark iron and
magnesium-rich minerals such as olivine and
pyroxene.
Magma: Partially-molten rock; typically a
mixture of melt, mineral crystals, and gas
bubbles.
Mantle: Earth’s middle layer, which lies beneath the crust and above the planet’s ironnickel core. The mantle is 2900 km thick and
makes of most of the planet’s volume.
Peridotite: Coarse-grained igneous rock that
forms Earth’s mantle and consists mostly of
peridotite, augite, and hypersthene.
Ophiolite: A piece of the oceanic lithosphere
that has been thrust up onto a continent
rather than subducted.
Subduction zone: A dipping surface along
which a plate of oceanic lithosphere is overridden by another plate and sinks into the
mantle.
Terrane: A piece of relatively buoyant lithosphere that has been transported by plate
motion from where it formed and accreted
(added) to another terrane or a continent.
Vein: A sheet-like body of minerals, such as
quartz and pyrite, that has been deposited
from hydrothermal solutions as they flowed
along a fracture and cooled.
Volatiles: Chemical elements and compounds, such as H2O, CO2, Cl and SO2 that occur as gases at relatively low temperatures.
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face above adit. Dumps contain vein quartz
stained by goethite formed as pyrite decomposed.
FIELD TRIP ROAD LOG
This road log gives a brief overview of some
of the geologic features we’ll see today
during our field trip through the eastern
Klamath Mountains. The mileages given are
approximate and begin at College of the
Siskiyous in Weed.
0.0 mi. Enter I-5 and drive south.
From the campus loop road turn left onto
Siskiyou Way and then right on South Weed
Boulevard. Continue to the South Weed exit
and enter I-5 going south.
x.x mi. Exit at Castella.
Turn right onto Castle Creek road and continue about 11.3 miles up past Castle Crags
to the overlook just east of Whalen Summit.
Outcrops are of the Bragdon Formation a
marine shale of Carboniferous age that is
part of the Redding subterrane. To the north
are exposures of the Castle Crags pluton
(Fig. 12), a Jurassic (175 Ma) granite that was
intruded into serpentinite of the Trinity subterrane. Retrace route to I-5 and head north.
x.x mi. Exit at Edgewood.
Continue north on on Highway 99 into the
Shasta Valley. On the eastern skyline is a
chain of young shield volcanoes that have
been formed by eruptions of fluid basaltic lavas, and which define the axis of the
modern High Cascade range. The small hills
and swales in the foreground are underlain
by a gigantic debris avalanche deposit that
formed about 300,000 years ago when the
northern flank of ancestral Mount Shasta
collapsed and a tongue of shattered rock
swept across the western Shasta Valley. To
the southeast is Mount Shasta, a stratovolcano that has grown as a result of four
separate eruptive episodes during the past
200,000 years. Two of these episodes have
occurred during the past 10,000 years, and
dating of mapped deposits suggests that
Mount Shasta typically erupts every 600-800
Hirt–Eastern Klamath Mountains
x.x mi. Chromite Prospects.
Figure 12. View of the Castle Crags pluton from
Stop 1 near Whalen Summit.
years.
x.x mi. Turn right on Gazelle-Callahan road.
Drive west out of the Shasta Valley and up
into the Klamath Mountains.
x.x mi. Junction with Macks Gulch Road.
Stay left and continue on the Gazelle-Callahan Road. To the right is “Limekiln Gulch”
which is marked by prominent gray outcrops
of limestone. Despite some recrystallization
these limestones contain fossils of trilobites
(Fig.8) and other organisms which indicate they are a marine deposit of Silurian
age. The limestones are part of the Gazelle
Formation which consists mostly of marine
shales and cherts and may be one of the
source units for gold in this area. Note the
outcrops of slate and chert along the switchbacks as the road climbs the western wall of
the canyon.
x.x mi. Gazelle Mountain Summit.
From this high point, the road drops into
the valley of the East Fork of the Scott River.
The valley follows the approximate contact
between igneous rocks of the Trinity subterrane to the south and the metasedimentary
rocks of the Yreka subterrane to the north.
x.x mi. Richter mine.
Turn left onto dirt road just past summit
and then stay left at Y; proceed to the adit
Richter Mine. Detachment fault exposed in
Backtrack to the Y and turn left. Continue
about y.y miles to dumps on the left side of
the road. This area hosts many small deposits of chromite. Chromite crystals settled
form basaltic magmas that accumulated in
the ancient ocean crust to form deposits
that were last worked on a large scale during
the Second World War. Backtrack to GazelleCallahan Road and turn left. To the north
of the highway are the rocks of the Yreka
subterrane.
x.x mi. Junction with Highway 3.
Continue westward into the town of Callahan. We’ll stop here for lunch. After lunch
continue northward along the mainstem of
the Scott River into the Scott Valley.
x.x mi. Dredge tailings along the Scott River.
Pull off the road to the right and stop near
the bridge. This area is covered by tailings
(reworked river gravels) left behind by a gold
dredge that operated here in the valley until
the early 1950s. Rocks you’ll see in the tailings include: serpentinite, greenstone, and
hornblende gabbro from the Trinity subterrane (oceanic crust) to the south; slates
and quartzites from the Yreka subterrane
(marine sedimentary rocks) to the east; and
diorites and granites from younger plutons
that cut up into the older rocks. Keep a sharp
eye out for white pieces of vein quartz and,
if you’re very lucky, a bit of gold!
x.x mi. Dredge headquarters at Sugar Creek.
Continue northward on Highway 3 towards
Etna. At Sugar Creek note the building on the
right that used to be the headquarters for
the dredge company. As we go farther north
watch for a change from the “moonscape” of
the dredge tailings to an undistrubed, grassy
valley floor. This marks the property line of
rancher who refused to allow the dredge to
operate on his property. After the dredge
had to stop its owners sold it it and it was
disassembled and taken to South America.
x.x mi. Etna, California.
64.9 mi. Turn left at Greenview.
After passing through Etna, continue north
along Highway 3 through the Scott Valley.
The Marble Mountains are on your left, and
Duzel Rock (a klippe of limestone) can be
seen on the skyline to the right. Turn left off
Highway 3 at Greenview and continue northwestward around the southern end of Chaparral Hill. Turn left where the road forks to
go north to Oro Fino and continue west up
and across the southern end of Quartz Hill.
The road winds down through Mugginsville
and past the site of the old stamp mill (Fig.
3). Follow the paved road north along the
western side of Quartz Hill. The Quartz Hill
area was a very rich site of lode gold mining.
Turn right onto the Scott River Road at the
Meamber School and continue west back to
the junction with Highway 3 at Fort Jones.
x.x mi. Turn left back onto Highway 3.
Follow Highway 3 northeast and then east
into the valley of Moffett Creek. Turn left
onto West Moffett Creek Road. Note the
quartz veins that crop out from the phyllite
in the cut on the left-hand side of the road at
Stage Point (x.x mi.). Continue east on West
Moffett Creek Road until it meets Highway
3. Turn left on Highway 3 and continue east
up to Forest Mountain Summit.
x.x m. Robbers Rock.
Pull over to the right side of the road and
park. Walk over to see this fwell-known
block of blueschist from the Yreka subterrane that was the site of a stage robbery
during the 19th century. Follow Highway 3
into Yreka, turn onto I-5 south and return to
the COS campus in Weed.
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x.x mi. College of the Siskiyous, Weed.
Log ends. Last updated 5-Oct-2012.
Hirt–Eastern Klamath Mountains
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