1%
10%/m.y.
Island arc assemblage
S
USA
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
The Cordilleran Orogen of western North America
is part of the circum-Pacific orogenic belt.
USA
Rift
now
wUS_Intro.ppt
USA
Mercator map projection about pole at 25° N 15 °
Oceanic
subduction
complex
Present
The western U.S. Cordilleran Orogen is defined by broad zones of uplift, deformation and magmatism
Cordilleran uplift
The area of uplift (which is unusually wide in the western U.S.)
4 km
3
2
gPE and resulting stress
is where the Farallon slab is thought to have been flat against
North America lithosphere during the
100-175 km S-wave velocity
Laramide orogeny.
1
0
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.
tension
compression
± 5.0%
Cordilleran interior
deformation
Surface Velocity
mm/yr
48
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 rightlateral shearing.
12
1
1/2
0
Velocity of points relative to stable
North America. Projection s such that
the Pacific plate moves toward the top
of the page.
Plate interactions
Yellow arrows
show plate interaction
forces
Stress field created by combination of gPE and
plate interaction. Light gray areas are weak.
Figure to left shows stresses caused by
plate interaction alone; figure at top right
shows stresses created by gPE alone.
Summary
> Plate interaction
> Grav. PE
> Plate weakness
} Inheritance
Lund (2008)
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
end of the Precambrian that rifting split away the
western continent. The continent margin (west of the
gray line at above) formed an Atlantic-type
“miogeoclonal’ margin, with a normal sedimentary
shelf (see cross section to right).
Precambrian (pink and orange in top map) forms stable continental cores .
Simplified Geologic Maps.
Precambrian
Paleozoic Passive Margin
Dickinson (2006)
and the Miogeocline
Main Geologic Events of the Southwest U.S.
(for the area shown in the Blakey figure at bottom left)
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).
Tectonic
Event
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.
Laramide
Sevier
This state of affairs continued
through much of the Paleozoic.
Nevadan
Sonoma
Ancestral
Rockies
Antler
QuickTime™ and a
decompressor
are needed to see this picture.
Rift
Rift
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 paleoPacific. Deposits were thin on the continental platform east of
the hingeline, and thicken rapidly to the west of the hingeline.
Dickinson (2006)
Paleozoic
and Mesozoic
Dickinson (2002)
magmatic arc accretion and growth
Dickinson
(2002)
55-30 Ma
Northern
core complexes
Southern core
complexes
30 -17.5 Ma
Siletzia accretion and the Ignimbrite flareup
ignimbrite flareup. (SMO, Paleogene
Post-Laramide
Laramide
Cenozoic