basic premise of plate tectonics - glg310--structural geology

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GLG310 Structural Geology


Plate tectonics

• Evidence and evolution of thinking about it

– Wegener and continental drift

– Magnetic reversals and sea floor spreading

– Distributions of volcanoes, earthquakes, measured plate motions, mountain belts

• Basic premise of plate tectonics

• Plate boundaries

• History of plate motions

– Global

– Western North America

• The seafloor

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http://emvc.geol.ucsb.edu/index.htm

-Essential paradigm for

understanding significance of

geologic structures.

-Plate interactions create rock-

forming environments

-Plate motions generate stresses that

drive deformation

The importance of Plate Tectonics


Plate tectonics








– Global


• The seafloor

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1915 – Alfred Wegener’s

Observations

1. Pieces fit together like a puzzle

2. Past glaciations a) Evidence of glaciers found at low latitudes

b) Striations (scratches carved in underlying rock by an advancing glacier)

c) Glacial till – mud, sand and pebbles left behind by receding glacier

d) South America, Southern Africa, South India, Australia, Antarctica – which are now widely separated & many lie at low latitudes

e) But, put the puzzle back together and see what you get


Evidence

Paleoclimate evidence

Ice sheets covered big areas of southern hemisphere

~ 220-300 million years ago

(ancient)


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Paleoclimate evidence

Glacial

striations

Ice sheets covered big areas of southern hemisphere

~ 220-300 million years ago

Evidence



Observations 3. Equatorial Climate distribution

– a. North America, Southern Europe, NW Africa

– b. Deposits of low latitudes

i. Coal

ii. Reefs

iii. Deserts

4. Fossil Distribution

– a. Several species were found to live in these regions

that are now separated by vast ocean


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Evidence


Evidence


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Observations

5. Matching geologic units

– a. Same rocks occurred (same age, same

formation) on East coast of South America and

West coast of Africa

– b. Appalachians of the eastern US and

Canada match those of Greenland, UK and

Scandanavia


Continental Drift

1915

Alfred Wegener published

hypothesis of

continental drift

He hypothesized:

existence of single

“super-continent”

Pangaea (“pan - GEE - uh”)

~ 200 million years ago Pangaea broke into

smaller pieces, & “drifted” to present positions


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Continental Drift

1924

Wegener’s book translated to English

& met with hostile criticism

Main objection: no way to explain continental drift.

No one believed his explanation (Continental

Drift) because he did not come up with a

mechanism for driving the motion, nor did he

explain a few minor problems (ie Do the

continents just plough through the sea floor?).



Plate tectonics








– Global


• The seafloor

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Revisiting the Earth’s magnetic

field

Why does the Earth have a magnetic field?

Why does the polarity shift every 10,000

years or so?

How might magnetic minerals on the sea

floor be affected by this magnetic field?


p.42-43bc

Earth’s magnetic field


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Earth’s magnetic Field and paleomagnetism

Wegener’s idea died until 1950’s.

Renewed interest from rock magnetism

Paleomagnetism: ancient magnetic field of Earth recorded

and frozen into rocks

How? Need to know about 2 things:

- Earth’s magnetic field

- Magnetism in rocks



Rock magnetism

•Certain minerals are magnetic (e.g., magnetite)

•Magnetic grains align w/ Earth’s magnetic field

•When cooled below Curie point (580oC for iron), grain alignment

frozen in


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Rock magnetism

•Certain minerals are magnetic (e.g.,

magnetite)

•Magnetic grains align w/ Earth’s

magnetic field

•When cooled below Curie point

(580oC for iron), grain alignment

frozen in



Sea floor spreading

Observations of the Sea Floor •Magnetic reversal record in sea floor basalts

• symmetric

•age of sea floor increases with distance from ridge

ocean ridges are above mantle upwellings, which cause

seafloor to spread, like a conveyor belt

magma replaces seafloor as it moves away, becoming new

oceanic crust

deep ocean trenches are locations where oceanic crust dives

back into planet

Mechanism to explain observations: Harry Hess & Bob Dietz

in the early 1960’s proposed: Sea Floor Spreading


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Magnetic Reversals

The polarity of Earth's magnetic field reverses with time. The main figure demonstrates how sea-floor anomalies, also known as magnetic stripes, develop during sea-floor spreading. The inset image records the reversal of Earth's dipole.


Sea Floor Spreading This animation shows progressive stages in the

opening of the Atlantic Ocean.

The youngest rocks (in red) clearly outline the mid-ocean ridge system, complete with transform faults.

The oldest ocean crust (in blue), is confined to offshore regions adjacent to the United States, Canada and western Africa.

This distribution demonstrates that the North Atlantic began to open before the South Atlantic.


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Plate tectonics








– Global


• The seafloor

Volcano Distribution


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Earthquake Distribution


Measured motions of benchmarks


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Mountain Belts


http://geology.about.com/library/bl/maps/n_map_GSHAP1500.htm


http://geology.about.com/library/bl/maps/n_map_GSHAP1500.htm

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Earth’s Plates (& evidence of their existence)

• The Lithosphere is fractured into 12 major plates + (8 to 20 microplates)

• What evidence led to the identification of these plates?

• Earthquake Belts – that’s where all the action is happening – movement leads to Earthquakes

• Volcanic Belts

• Mountain Belts

• Measured benchmark motions

• Geologic evidence

• Most deformation occurs at the plate boundaries – relatively little at the interior of plates

• Likewise, Plate interiors are relatively seismically free regions – stronger, so they don’t accommodate the movement.


Earth’s Major Plates


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Basic Premise of Plate

Tectonics

• Earth’s lithosphere is divided into plates

• Some plates are oceanic lithosphere and some are continental lithosphere

• They float on and move relative to the underlying asthenosphere

• They move relative to one another

• Anywhere from 1 to 15 cm per year

• Most of the plate remains undeformed except at the plate boundaries


• Asthenosphere – can flow (higher temp + different composition), therefore it convects

– Oceanic lithosphere – high density

– Continental lithosphere – low density

– New Oceanic crust –lower density (because it’s hot)

– Older Oceanic crust – higher density (because it’s cold)

• Earthquakes occur to depths of 670 km – no earthquakes below this (probably because material is too hot to fracture

• Lithosphere – crust + upper mantle (cooler so viscosity goes up) – can’t flow

– Avg continental – 150 km (up to 300 km)

– Avg oceanianic - 100 km (old) down to 10 km at mid-ocean ridges

Revisit Earth’s Layers

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Fig.4.26

Driving mechanisms??? PLATE TECTONICS

http://mcnamara.asu.edu/content/educational/main/CitcomCU_plumes/akm_perspective1.mpg

W. W. Norton

Continental Margins -- 2 types

• Active continental Margins (Plate

boundary)

• Passive continental Margins (no plate

boundary)

• Some plates are either just ocean or just

continent or a mix of both


http://mcnamara.asu.edu/content/educational/main/CitcomCU_plumes/akm_perspective1.mpg

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Earth’s Major Plates


The Grand Unifying Theory

Insert revised figure 3.16 here

Plate boundary movies: 4.1(Transforms), 4.2 (Divergent), 4.3 (Convergent)


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Plate tectonics








– Global


• The seafloor

Many cool animations:

http://emvc.geol.ucsb.edu/

Northern Pacific and North American Plates

Fate of the Farallon Plate and western

North America




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https://geoinfo.nmt.edu/tour/federal/mon

uments/gila_cliff_dwellings/home.html

http://www.geopoem.com/2013/04/tectonic-congestion-on-road-from.html modified from

http://www.sciencedirect.com/science/article/pii/S0012821X02009858

Fate of the Cenozoic Farallon slab

FAR = Farallon; NAM = North America

Data Model

https://geoinfo.nmt.edu/tour/federal/monuments/gila_cliff_dwellings/home.html

https://geoinfo.nmt.edu/tour/federal/monuments/gila_cliff_dwellings/home.html

http://www.geopoem.com/2013/04/tectonic-congestion-on-road-from.html









http://www.sciencedirect.com/science/article/pii/S0012821X02009858

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http://bulletin.geoscienceworld.org/content/12

0/3-4/451/F6.expansion

Baja California


Global Paleogeography

http://cpgeosystems.com

http://bulletin.geoscienceworld.org/content/120/3-4/451/F6.expansion




http://cpgeosystems.com/

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GLG310 Structural Geology http://cpgeosystems.com




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http://cpgeosystems.com GLG310 Structural Geology



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http://www.transantarcticmountains.com/

Colloquium: Wednesday, September 4, 2013, ISTB4 185 @ 4:00 PM

Reception: ISTB4 first floor lobby @ 3:30 PM

Speaker: Prof. Ed Stump

Title: 40 Years Rambling in the TAM Abstract: When the supercontinent of Rodinia split in the Neoproterozoic and Antarctica and

North America parted company, the rifted margin of Antarctica was destined to become the

Transantarctic Mountains (TAM). A full blown cycle of mountain building occurred along this

margin, commencing in the Neoproterozoic and ending in the Early Ordovician, the so-called

Ross orogeny. The present-day mountains are a spectacular rift shoulder uplifted in the Cenozoic

that essentially tracks the axis of the Ross belt. Following the Ross orogeny, a period of erosion

leveled the land. From Devonian to Triassic, sediments, primarily of terrestrial origin (Beacon

Supergroup), blanketed the region. In the Jurassic, synchronous with the breakup of Pangaea,

basalts intruded the Beacon in vast sills and flooded the land. Today these rocks comprise the

highest reaches of the mountains.

Highlighting geological research accomplished during my 40-year career in Antarctica, my talk

will emphasize the evolution of the mountains during the Ross cycle, summarizing the regional

similarities and differences of this diverse orogenic belt, with particular emphasis on the Byrd

Glacier area. Byrd Glacier marks a major discontinuity with rocks of highly different character

on opposite sides, crystalline rocks to the north and primarily sedimentary rocks to the south.

My premise is that the discontinuity is inherited from an offset in the original rift during the

break up of Rodinia, and that the subsequent geological evolution of the region was built upon

that configuration.

http://www.transantarcticmountains.com/

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Rodinia — Russian for

“homeland”

Sometime between 1.2 and 1

billion years ago, the

supercontinent Rodinia formed.

It was surrounded by an ocean

called Mirovia. Three or four

existing continents collided

in a large mountain-building

episode, called the Grenville

Orogeny. When two continents

collide, neither will sink (as they

are both low-density continental

crust, like corks bobbing in the

ocean). Instead, they thrust over

one another near the surface and

fold at depth (where rocks are

hotter) to form fold-and-thrust

mountains. Rocks trapped

in this collision or suture zone

are highly metamorphosed by

high temperatures and

pressures.

Modern example = India-Eurasia

collision and the Himalaya

http://www.ccsf.edu/Departments/History_of_Time_and_Life/PDFs/Rodinia36x36.pd

f

Rodinia begins to break up

at about 800 Ma. This

causes thinning of Western

North America and also a

failed rift in the middle of

North America

Modern

Analog: East

Africa

http://www.ccsf.edu/Departments/History_of_Time_and_Life/PDFs/Rodinia36x36.pdf




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http://cpgeosystems.com GLG310 Structural Geology



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http://topex.ucsd.edu/marine_topo/mar_topo.html

The seafloor holds much of the key to understanding plate tectonics


http://topex.ucsd.edu/marine_topo/mar_topo.html

basic premise of plate tectonics - glg310--structural geology

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