1. The Cordilleran Orogen of western North America
Mercator map projection about pole at 25° N 15 ° E
USA
Island arc assemblage
Oceanic
subduction
complex
Rift S
now
wUS_Intro.ppt 1%
10%/m.y.
At any time, geology is made over only a small fraction of the Earth’s surface. The colored area in the map above show current-day strain rate, i.e., where geology currently is happening. This occurs at spreading centers, subduction zones, and in areas of continental orogeny. The western U.S. is such a place.
The Cordilleran Orogen of western North America
is part of the circum-Pacific orogenic belt.

2. } Present I’m assuming a flat slab Here are some things it did…
The western U.S. Cordilleran Orogen is defined by broad zones of uplift, deformation and magmatism
The area of uplift (which is unusually wide in the western U.S.) is where the Farallon slab is thought to have been flat against
gPE and resulting stress 1 2 3
4 km
North America lithosphere during the Laramide orogeny.
Currently, the upper mantle is very slow to seismic waves beneath the uplifted area (the mantle is hot or partially molten).
Also, the uplifted area has high amounts
of gPE (gravitational potential energy), which acts to drive extension. km
S-wave velocity
± 5.0%
Cordilleran uplift
tension
Mexico
Canada
Pacific Plate
North American Plate
Surface Velocity
1/2 1 12 48
mm/yr
compression
The area of active deformation (which is unusually wide in the western U.S.) occurs both at the plate margin and where intense magmatism occurred recently (in the Basin and Range). Only the western portion of the high gPE area is extending, which argues that the deforming area is relatively weak.
This is consistent with its history of recent, intense magmatism.
Interior deformation
is mostly extension, whereas near the transform plate margin deformation is dominated by right-lateral shearing.
Cordilleran interior
deformation
I’m assuming a flat slab
Here are some things it did…
But how did it do it?
Stress field created by combination of gPE and plate interaction. Light gray areas are weak.
Velocity of points relative to stable North America. Projection s such that the Pacific plate moves toward the top of the page.
Figure to left shows stresses caused by plate interaction alone; figure at top right shows stresses created by gPE alone.
Plate interactions
Summary
> Plate interaction
> Grav. PE
> Plate weakness
Inheritance }
Yellow arrows
show plate interaction
forces

3. Precambrian
North America (above). Archean craton blocks were assembled at mobile belts; a series of Proterozoic arcs accreted to the south side of the Archean continent. Proterozoic rifts emplaced intrusions (mid-continent rift, 1.1 Ga) and made basins (diagonal dashes below), but it wasn’t until the
Precambrian (pink and orange in top map) forms stable continental cores .
western continent. The continent margin (west of the
“miogeoclonal’ margin, with a normal sedimentary
end of the Precambrian that rifting split away the
gray line at above) formed an Atlantic-type
shelf (see cross section to right).
Simplified Geologic Maps.
Lund (2008)

4. Main Geologic Events of the Southwest U.S.
Dickinson (2006)
Paleozoic
Passive Margin
and the Miogeocline
Dickinson (2006)
Main Geologic Events of the Southwest U.S.
(for the area shown in the Blakey figure at bottom left)
core complexes
Rift
Nevadan
Laramide
Sonoma
Antler
Sevier
Ancestral
Rockies
Tectonic
Event
Latest Precambrian rifting stretched the margin of North America (the area between the hingeline and the western edge of purple crust in the tilted figure below).
Post-rift lithospheric cooling caused subsidence and creation of a passive “Atlantic-type” margin that received a thick accumulation of sediment in the “miogeocline” west of the hingeline.
This state of affairs continued through much of the Paleozoic.
hingeline
sediment
Precambrian
lithosphere
post-rift lithosphere
miogeocline
ocean crust
crust
Blakey (web site)
Cambrian (~510 Ma) paeogeography. The passive margin setting is established. Carbonate rocks, fluvial and shallow marine sandstone and mudstone grade westward into the paleo-Pacific. Deposits were thin on the continental platform east of the hingeline, and thicken rapidly to the west of the hingeline.

5. and magmatic arc accretion and growth
Dickinson (2002)
and magmatic arc accretion and growth
Mesozoic
Paleozoic

6. Siletzia accretion and the Ignimbrite flareup21 28 34 27 37 43
Cascade Arc
magmatism
Challis-trend Ma
Siletzia accretion and the Ignimbrite flareup
Cenozoic
Laramide
Post-Laramide
ignimbrite flareup. (SMO, Paleogene
Southern core
complexes Ma
Northern
core complexes
55-30 Ma
Dickinson
(2002)