The Cordilleran Orogen of western North America

1
now
wUS_Intro.ppt
At any time, geology is made over only a small
fraction of the Earths 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
The western U.S. Cordilleran Orogen is defined by
broad zones of uplift, deformation and magmatism
Present
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
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.
Cordilleran uplift
tension
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.
deformation
Cordilleran interior
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
Yellow arrows show plate interaction forces
3
Precambrian
Simplified Geologic Maps. Precambrian (pink
and orange in top map) forms stable continental
cores . 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
wasnt 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).
4
Paleozoic
Passive Margin and the
Miogeocline
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.
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
Dickinson (2002)
and magmatic arc accretion
and growth
Mesozoic
Paleozoic
6
Post-Laramide
Laramide
Cenozoic
30 -17.5 Ma
Southern core complexes
Northern core complexes
55-30 Ma
Dickinson (2002)