The

Cor

dille

ran

Oro

gen

of w

este

rn N

orth

A

mer

ica

is p

art

of t

he c

ircum

-Pac

ific

orog

enic

bel

t.

1%10

%/m

.y.

At a

ny ti

me,

geo

logy

is m

ade

over

onl

y a

smal

l fra

ctio

n of

the

Ear

th’s

sur

face

. T

he c

olor

ed a

rea

in th

e m

ap

abov

e sh

ow c

urre

nt-d

ay s

trai

n ra

te, i

.e.,

whe

re g

eolo

gy c

urre

ntly

is h

appe

ning

. T

his

occu

rs a

t spr

eadi

ng

cent

ers,

sub

duct

ion

zone

s, and in areas of continental orogeny. The western U.S. is

such a place.

Mer

cato

r m

ap p

roje

ctio

n ab

out

pole

at

25°

N 1

5 °

E

no

w

US

AU

SA

US

A

Island arc assemblage

Oceanicsubductioncomplex

Rift

S

wU

S_I

ntr

o.p

pt

Mexico

Canada

Pacific P

l ate

No rth A

meric an P

late

Surface Velocity

1/2

1

12

0

48

mm/yr

100-175 km S-wave velocity

± 5.0%

The western U.S. Cordilleran Orogen is defined by broad zones of uplift, deformation and magmatism

1

23

0

4 km

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

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.Cor

dille

ran

uplif

tC

ordi

llera

n in

terio

r

compression

tension

gPE and resulting stress

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 deformationis mostly extension, whereas near the transform plate margin deformation is dominated by right-lateral shearing.

defo

rmat

ion

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.

Plate interactions

Figure to left shows stresses caused by plate interaction alone; figure at top right shows stresses created by gPE alone.

Yellow arrowsshow plate interactionforces

Summary > Plate interaction > Grav. PE > Plate weakness

Inheritance}

Present

Sim

plif

ied

Geo

log

ic M

aps.

Pre

cam

bria

n (p

ink

and

oran

ge in

top

map

) fo

rms

stab

le c

ontin

enta

l cor

es .

Nor

th A

mer

ica

(abo

ve).

Arc

hean

cra

ton

bloc

ks w

ere

asse

mbl

ed a

t mob

ile b

elts

; a s

erie

s of

Pro

tero

zoic

arc

s ac

cret

ed to

th

e so

uth

side

of t

he A

rche

an c

ontin

ent.

Pro

tero

zoic

rift

s em

plac

ed in

trus

ions

(m

id-c

ontin

ent r

ift, 1

.1 G

a) a

nd m

ade

basi

ns (

diag

onal

das

hes

belo

w),

but

it w

asn’

t unt

il th

een

d of

the

Pre

cam

bria

n th

at r

iftin

g sp

lit a

way

the

wes

tern

con

tinen

t. T

he c

ontin

ent m

argi

n (w

est o

f the

gray

line

at a

bove

) fo

rmed

an

Atla

ntic

-typ

e“m

ioge

oclo

nal’

mar

gin,

with

a n

orm

al s

edim

enta

rysh

elf (

see

cros

s se

ctio

n to

rig

ht).

Pre

cam

bria

n

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Lu

nd (

20

08)

QuickTime™ and a decompressor

are needed to see this picture.

Blakey (web site)

Dickinson (2006)

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.

hingeline

sediment

Precambrian lithosphere

post-rift lithosphere

miogeoclineocean crust

crust

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.

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

AncestralRockies

Rift

Tectonic Event

Paleozoic

Dic

kin

son

(2

00

2)

and

m

agm

atic

arc

acc

retio

n an

d gr

owth

Pal

eozo

icM

esoz

oic

Dic

kin

son

(200

2)

igni

mbr

ite

flar

eup.

(S

MO

, Pal

eoge

ne

55-3

0 M

a30

-17.

5 M

a

La

ram

ide

Po

st-L

ara

mid

e

N

orth

ern

core

com

plex

es S

outh

ern

core

co

mpl

exes

212834 27

37

43

21

Cascade Arc

Cha

llis-tr

end

m

agm

atism

45 - 5

5 M

a

Sile

tzia

acc

retio

n an

d th

e Ig

nim

brite

flar

eup

43Cen

ozoi

c