LARAMIDE OROGENY - University of Colorado Boulder

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Transcript LARAMIDE OROGENY - University of Colorado Boulder

GEOG 3251 summer 2010
term B
Laramide Orogeny
• Major tectonic event that formed the Rocky
• Occurred 70-40 My ago
• Occurred in the interior of a plate
• Occurred 1,000 miles from nearest
subduction zone
Can you explain the Laramide orogeny
using the paradigm of plate tectonics?
Intersection b/w NA plate
and Pacific Plate
Note small plates: Juan de
Fuca, Rivera, Cocos Plates
These small plates are
remnants of the much
larger Farallon Plate
Gulf of California is a
spreading center
• What really caused the Laramide?
• To what extent was the subducting Farallon plate
• How does intracontinental deformation on the scale of the
Laramide even occur?
• The Rockies are as high as ever and still rising. How much
of the cumulative Rocky Mountain uplift can the Laramide
claim, and when did it occur, early or late in the orogeny?
Prior to the Laramide Orogeny
• there were active faults
•they fit the present
day Rockies quite well
How did these faults develop?
Faults developed as part of the
Frontrangia Orogeny 330 My ago
•East Pacific Rise analogous to
today’s Atlantic Rise
•Farallon Plate moving to NE
•Collision with the North
American plate
•Subduction zone off present
day California, similar to the
Andes today
•Hot, new plate material moving
underneath North America
•Floats near the crust to Colorado
•Cools enough to subduct in
•East Pacific Spreading Center comes
closer to the North American Plate
•The Farallon Plate becomes much smaller,
broken into smaller plates
•Little new plate material produced
•Laramide Orogeny shuts off
•Farallon plate begins to sink into the mantle
due to cold slab pull (denser, thicker plate).
•Asthenosphere rises towards surface,
causing San Juan volcanoes
•East Pacific Rise almost completely
overrun by the North American Plate
•East Pacific Spreading Center becomes
the San Andreas Fault around present-day
•Gulf of California begins to form
•Farallon plate underneath western
North American continues to subduct due
to cold slab pull, leaving a “thin” spot
underneath the Basin and Range province.
•Asthenosphere continues to rise to fill void,
causing uplift.
•Still a “thin” spot under continental crust of
the Basin and Range province caused by
subduction of Farallon Plate during
Laramide Orogeny
•Asthenosphere still rises towards surface to
fill that “thin” spot
• Rockies getting taller today
•Explains hot springs in Basin and Range
•Gulf of California may “unzip” California
Laramide Orogeny
80 Ma: normal high-angle
(50 deg.) plate subduction
65 Ma:
• Farallon/Kula plate
formed near NA plate,
• subduction angle decreases
• Farallon plate moves
horizontally to Colorado
40 Ma: Farallon plate dies
Rocky Mtn orogeny -summary• 30 million year period of mountain building
in western North America
• started 70 million years ago, and ended 40
million years ago
• Farallon Plate was sliding under the North
American plate
• the angle of subduction became so low that
no volcanics occurred at subduction zone
Rocky Mtn orogeny -summary• The volcanic arc associated with a subduction
occurred not near the plate edges (as in the
Andes, for example), but far to the east
• Farallon plate dragged along the bottom of
the continental crust of the N.American plate
• folding and faulting of rocks
Consequences of
Laramide Orogeny
Western US
Rocky Mountains
Basin and Range
Basin and Range province
• Earth's crust was
being pulled apart
• thinned 100%
• Pacific plate
moving N relative
to N.Am plate
Basin and Range province
• extends east from the Sierra Nevada all the way
to the Colorado Plateau
• extends S to Baja California
•pattern of linear mountain ranges
and valleys