Southwest U.S. Region - PowerPoint Presentation

Slide1

Southwest U.S. Region Mountain-Building Geology

Mountains Distant from Subducting Plate Boundaries

Mueller State Park

Version 2.0

October 8, 2015Slide2

North America

In moving west, the

North American Plate

slid over several ancient plates, pushing up the

North American Cordillera

The Cordillera is the great belt of mountains from Alaska to Panama

The most important overridden plate is the

Farallon Plate

It is the largest seabed oceanic plate to plunge under the North American Plate

It extended from todays Juan de Fuca Plate on the north through the Cocos Plate on the south

Its subduction built most of the North American Cordillera, bunching up the west end of the North American plate like folds

It is also the origin of the Sierra Nevada Batholith via subduction under North America

The Farallon Plate was separated from the Pacific Plate by the

Farallon-Pacific Ridge

, a spreading zone in Earth’s crust

North American Plate migration carried it across this ridge

Migrating west across the ridge, it made contact with the Pacific Plate migrating northwest, creating the transform San Andreas Fault that grew north and south

Due to their similar directions of movement, the result of the collision was not subduction but side-to-side movement

Continental stretching of North America was also created by the migration, the Basin and Range Province sitting today over what was the Farallon-Pacific Ridge spreading zone

This has resulted in North America consisting of another plate, the Sierran Plate, from the San Andreus Fault on the west to the Walker Lane Sheer Zone on the east (east edge of the Sierra Nevada)

Two remnants of the Farallon Plate, where the continent has not overridden the ridge, still today subduct, the Juan de Fuca Plate and the Cocos PlateSlide3

North American Cordillera

Adapted from Meldahl 2013 (p.185)Slide4

Farallon Plate and Farallon-Pacific Ridge

Adapted from Meldahl 2013 (p.95)Slide5

Sierran Plate

Adapted from Meldahl 2013 (p.70)Slide6

The Foreland Ranges

The Southern Rockies of southern Montana, Wyoming, Colorado, and northern New Mexico

600 to 1000 miles inland from the west edge of the North American Plate

They occur unusually deep in the continental interior, so they are not the result of normal oceanic/continental plate subduction

Ancient continental and terrane collisions do not explain these mountains

They started to rise in the Larimide Orogeny after terrane collisions ceased

Their uplifted cores are made of

basement rock – basement-cored uplift

Deep ancient crystalline rock that forms the continental foundations

The lowest oldest rocks of the continent normally found 5+ miles down

Younger rocks lean in great hogbacks against the uplifted basement cores

The Foreland Ranges are large wedges of basement rock squeezed up by colossal sideways pressure

They rose along ramp-like thrust faults, thrust up and over rocks belowSlide7

The Foreland Ranges

Adapted from Meldahl 2013 (p.186)Slide8

The Foreland Ranges (cont.)

Colorado and Wyoming were squeezed to 4/5ths their original widths

The ranges and faults line up mostly north-to-south and northwest-to-southeast

The pressure was perpendicular to this alignment – mostly from the west southwest

This massive sideways pressure compressed the continental basement and squeezed up the southern Rocky Mountains

The Farallon Plate probably

subducted flat

in a 500-mile wide zone – southern Montana to northern New Mexico

This wide zone was probably an oceanic plateau of vast quantities of lighter basaltic volcanic rock vented onto the seabed up to 20 miles thick

The buoyancy of the plateau material floats the plate upward in the mantle so it slides flat beneath the continent as a great arch and may even tear away from the plate like a large flap

Only 10 extra miles of plateau top layer basalt/gabbro is needed to float the plate

The flat-sliding plate applied the pressure as it scraped the plate aboveSlide9

Farallon Plate Flat Subduction

Adapted from Meldahl 2013 (p.92)Slide10

The Foreland Ranges (cont.)

The Colorado Plateau buckled and warped

Elsewhere, the Farallon Plate subducted normally producing signature features of subduction mountain building

After the Larimide Orogeny, the Farallon Plate bent back to normal subduction as the oceanic plateau moved through

It possibly hinged back down into the mantle southwest of the Great Basin

Volcanoes re-awakened with a vengeance

Flat subduction may have been to blame for such violent volcanism

As the Farallon Plate slid flat, great pressures may have squeezed quantities of seawater into the lower continental crust above

When the Farallon Plate angled back down, inflowing soft mantle above the plate began to melt by depressurization and, also, water lowers the melting point in the mantle

As fresh magma entered the hydrated crust above, it could have exploded up to 750 times its volume

Water-saturated magma is one of Earth’s most violent forcesSlide11

Normal Farallon Plate Subduction

Adapted from Meldahl 2013 (p.93)Slide12

Exhumation of the Southern Rockies

The digging out of the Southern Rocky Mountains from the deep Miocene burial happened next

After the Larimide Orogeny, erosion began to tear down the Foreland Ranges and violent volcanic activity further buried them

The rivers began to dig down, removing the covering debris

The soaring mountains may owe as much to exhumation as to original uplift

Factors causing exhumation

Passive uplift

As rivers excavate, the land floats upward, steepening and promoting further excavation

Since the Earth’s crust floats buoyantly on the denser rock of the mantle, removing weight in a region will cause the region to float higher

Active Uplift

H

ot buoyant mantle beneath the Southern Rockies could have raised the region

The USArray today allows examining the mantle with unprecedented clarity

It is a grid of 400 mobile seismometers spread from Canada to Mexico being moved from west-to-east in steps to build a picture of Earth’s interiorSlide13

Exhumation (cont.)

From the west coast in 2004, the grid mapping has now reached to the Gulf of Mexico

It

shows a giant dome of hot mantle welling up under the

Basin

and Range

Province and

a pocket under Colorado/Wyoming causing

uplift

Regional uplift of the Southern Rockies during the last 5my has resulted in a half mile of uplift, twice than that from passive erosion

Climate Change

The American west has become cooler and more arid in the last 5my

There have been more-focused and higher intensity storms

Mountain valley glaciers have formed and melted, indicating more snow and leading to massive glacial erosion

This has all resulted in more erosion power for the rivers in the region

The

Death of the Farallon Plate

The USArray has shown that the Farallon Plate itself is largely gone today

The destruction of the Farallon Plate has several causes

The North American Plate overtopped the Farallon-Pacific Ridge from which the Farallon Plate grew and slid east, eliminating the plate’s source and energy

The westward movement of the North American Plate over

the now dead Farallon Plate

has broken it upSlide14

Exhumation (cont.)

Subduction and plate movement ended

When the

detached dead plate cracked apart into several

pieces, it sank

into the mantle opening gaps through which hot mantle rocks and magma

rose

The

N

orth American continental crust, tethered to the Pacific Plate at the San Andreas Fault, was then stretched and thinned on the west end, now stretched about 250 miles, to create the Basin and Range Province

As the west gap opened more during the last 5my, more hot mantle rock welled up under the Southern Rocky Mountains to push them up and trigger the Exhumation

Today with USArray, geologists can see the remnants of the ripped-apart Farallon Plate in the mantle

S

everal fragments into which it has been split are clearly visible

To the west of the fragments, the hot mantle upwelling under the Basin and Range Province and the upwelling under the Southern Rocky Mountains are visible

The high oceanic plateau fragment 300 miles under the Great Plains that looks like the flat subducting portion of the plate that slid underneath the continent, to squeeze up the Southern Rockies, is clearly identifiable

A severely tilted fragment under the Southern Rockies that probably was the cause of the tensional forces causing the crust to split into the Rio Grande Rift, with its associated horst uplifts, and depressurization to create the magma necessary for the Front Range Uplift and other regional volcanism is present under the Southern Rocky Mountain region

The Southern Rockies today are still stretching east-to-west along the Rio Grande Rift caused by the pressure of driving Farallon Plate fragments and upwelling hot mantleSlide15

Death of the Farallon Plate

Adapted from Meldahl 2013 (p.212)

Adapted from Meldahl 2013 (

p.213)Slide16

References

Chronic,

Halka

and Williams,

Felicie

.

Roadside Geology of Colorado

, Second Edition. Missoula, MT: Mountain Press, 2002, 2010

Lutgens

, Frederick K. and Edward J.

Tarbuck

.

Essentials of Geology, Eighth Edition

. Upper Saddle River, NJ: Prentice Hall, 2003, 2000

, 1998

, 1995

Matthews, Vincent, Katie

KellerLynn

and Betty Fox.

Messages in Stone: Colorado’s Colorful Geography

. Denver: Colorado Geological Survey,

2003

Meldahl, Keith

Heyer

.

Rough-Hewn Land

. Berkeley and Los Angeles, CA: University of California Press, 2011, 2013

Reed, Jack and Ellis, Gene.

Rocks Above the Clouds

. Golden, Colorado: Colorado Mountain Club Press, 2009