Continental Margin settings

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Continental Margin settings
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400-250 Ma Japanese-type Offshore subduction

• The ocean offshore widened and aged until it
  became unstable to subduction, which initiated
  sometime in the early Paleozoic under an offshore
  arc, with a steadily closing marginal sea
  attached to the North American plate.

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3
Devonian (370 Ma). Carbonate deposits continue to
dominate the continental shelf. Significant
changes in sea level have incised canyons into
the shelf
4
View to west of previous slide. The Antler arc
approaches SW North America, plowing into
slope-rise deposits off the continent. As the arc
progresses eastward, it will thrust these deep
water deposits onto the continental shelf
5
400-250 Ma Japanese-type Offshore subduction

• In the late Devonian, this offshore arc ran up
  against the North American margin in the Antler
  orogeny

The arc terrane ended up accreted to the
continent, overlying the miogeocline across the
Roberts Mountain Thrust Fault
The orogeny ends when the arc terrane is
transferred to the North American plate and a new
subduction boundary is initiated offshore.
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Antler Orogeny
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Mississippian (340 Ma). The colliding Antler arc
shoved strata deposited in deep water over the
continental shelf. The weight of the thickened
crust caused eastern Nevada and western Utah to
subside and thick Upper Devonian and Lower
Mississippian strata formed there. Farther east a
broad carbonate shelf blanketed much of the
Western Interior.
8
View to west of previous slide. Some parts of the
Antler arc possibly collided with the off shore
continental blocks. Here subduction continued but
the polarity of the subduction (direction of dip
of subduction zone) reversed so that subduction
now dipped eastward. One or more of these
portions of the reversed Antler arc may have been
the precursor of the McCloud arc, a Late
Paleozoic tectonic element of western North
America. Some geologists interpret the McCloud
arc to be exotic to North America
9
Pennsylvanian (310 Ma). Huge changes in
paleotectonics affected western North America.
The Ancestral Rockies rose across the southern
western interior. The Antler Mountains were being
worn down by erosion and began subsiding in some
areas. A marine basin covered most of western
Utah and eastern Nevada.
10
View to west of previous slide. In this
interpretation, the Stikine fragment of the
southern McCloud arc migrated westward while the
Quesnell fragment of the Northern McCloud arc
remained near western North America. The
transform fault between the two may also have
truncated SW North America during the Late
Pennsylvanian and Permian. Follow this
interpretation and watch for a contrasting one in
the Permian and Mesozoic
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Permian (280-270 Ma). Parts of the Ancestral
Rockies remain high and shed sediments into
adjacent basins. The Havallah back arc basin
expands over the site of the previous Antler
Mountains
12
View to west of previous slide. The Stikine
fragment has moved NW to a position west of the
Quesnell fragment. In other words, the McCloud
arc is a double arc the western portion was
transported along a transform fault
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400-250 Ma Japanese-type Offshore subduction

• After a couple of arc-polarity reversals, the
  same thing happened again, more or less, in the
  Early Triassic Sonoma orogeny, bringing a new
  sequence of oceanic rocks on top of the
  miogeocline and the Antler rocks.
• The island arc so accreted forms the basement for
  the western Sierra Nevada batholith

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Sonoma Orogeny
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Latest Permian (250 Ma). The Western Interior,
like most continental regions around the Earth,
was well above sea level at this time
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View to west of previous slide. The Stikine arc
collapses against the Quesnell arc. The Havallah
basin is thrust eastward over the continental
shelf and the Sonoman orogeny occurs. Oceanic
plateaus, derived from the Tethyian Ocean
half-way around the globe, are caught against and
between the various subduction zones. This is how
this interpretation resolves "exotic elements"
within arcs not exotic to North America
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Permian Exotic Paleogeography (270 Ma). In this
interpretation, the McCloud arc is a single arc
separated from North America by a broad back arc
basin, the Havallah basin. The Stikine arc is out
of view to the west and interpreted as exotic to
North America
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Latest Permian Exotic Paleogeography (250 Ma).
The McCloud arc collapses against North America
and the Sonoma orogeny forms. Tethyian oceanic
plateau fragments are accreted along the Cache
Creek forearc basin. The plateau fragments
preceded the arrival of exotic Stikine arc, still
out of view to the west
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Early Triassic (240 Ma). A broad fluvial and
shallow marine depositional system (the Moenkopi
Formation) covers much of the southern western
interior. To the west, the Stikine and Quesnell
elements of the McCloud arc collide and in turn
collapse the Havallah back arc basin as the
Sonoman orogeny continues. The Cache Creek
forearc complex is trapped between the two arc
elements
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View to west of previous slide. Oceanic plateau
fragments continue to accrete to western North
America and the ocean plate subducts below the
continent. As subduction continued, the
Cordilleran arc became established
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Klamath Mountain Province
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Trinity Alps in Klamath Mountains
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Eastern Klamath Belt

• Rocks are Cambrian to Triassic in age
• Consist of metamorphosed andesitic volcanic rocks
  from an oceanic arc as well as cherts
• Emplaced during Antler orogeny
• Contains Trinity ophiolite
• Similar to Shoo Fly terrane in northern Sierra
  Nevada
• Old island arc and ophiolite is the ocean crust
  on which it stood

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Mt. Eddy and ultramafic boulders (eastern belt)
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Castle Crags (Klamath Mountains)
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Ophiolites
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Pillow Basalts - KM
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Central Metamorphic Belt

• Lies on the Trinity thrust fault
• Rocks composed of schists, marbles, and amphibole
  gneisses
• Rocks were subjected to higher heat and pressure
  as they were dragged beneath the Eastern Klamath
  belt during Antler event

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Western Paleozoic and Triassic Belt

• Separated into several smaller terranes
• Rattlesnake Creek contains late Triassic to
  early Jurassic metavolcanics and chert
• Hayfork/Sawyers Bar contains Jurassic oceanic
  metavolcanics and metachert
• North Fork/Stuart Fork contains late Triassic
  metamorphic rocks associated with high pressure
  and low temperature (subduction zone)
• Stuart Fork terrane is similar to Feather River
  terrane in northern Sierra Nevada

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Western Klamath Belt

• Josephine ophiolite is the centerpiece one of
  oldest and most complete pieces of oceanic crust,
  it contains a complete section through oceanic
  crust
• Overlain by Jurassic metasedimentary rocks of
  Galice Formation

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Josephine ophiolite location map
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Josephine ophiolite
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Josephine Ophiolite wehrlites (lower crust)
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Wehrlitic and gabbro sills
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Franciscan Complex

• Separated from the Western Klamath Belt by the
  South Fork Mountain Thrust
• Considered a northern extension of the Coast
  Ranges