FUNDAMENTALS OF EARTH SCIENCE I

FALL SEMESTER 2018CONTINENTSSTRUCTURE AND HISTORY

https://www.researchgate.net/figure/Paleogeographic-view-of-the-Earth-at-the-K-T-transition-Courtesy-Ron-Blakey-Department_fig33_251124882

✶ Review: Geologic time scale✶ Review: Earth’s layered structure1234

1 CRUST (0-~75 km)

3 OUTER CORE (~3000-5000 km)

2 MANTLE

4 INNER CORE (~5000-6000 km)

✶ Review: Crust, Mantle, Lithosphere and asthenosphere

CONTINENTAL
CRUSTCONTINENTAL CRUST~7 km~70 km LITHOSPHERE(Brittle)

OCEANIC CRUST~30-50 km~ 100-200? km LITHOSPHERE(Brittle)
ASTHENOSPHERE(Ductile)
Weak, ductile layer of the mantle that
deforms to accommodate the motion oftectonic plates.

Broken into tectonic plates moving
relative to one another

MANTLE✶ Review: Plate boundaries

Understanding Earth 6th Ed.

Mid-Ocean Ridge

Continental Rift

DIVERGENT
BOUNDARIES

Ocean-Ocean Subduction

Ocean-Continent Subduction

CONVERGENTBOUNDARIESContinent-Continent Collision

TRANSFORM
FAULTS✶ How to reconstruct the history of the Earth’s crust?
540 Myr
PrecambrianUnderstanding Earth●The Earth is 4.6 billion years old.

●The oldest oceanic crust is “only” 200 million years old.

●The oldest uncontroversial age of a rock from the continental crust is
4 billion years (the Acasta Gneiss from northwestern Canada).

!

Information on the distant past of our planet is locked
in rocks composing the continents✶ Review: Supercontinents
310 MaLate Carboniferous

PANGAEA
Cocks and Torsvisk (2006)

Understanding Earth 6th Ed.

✶ The Wilson Cycle

Understanding Earth 6th Ed.

470-270 million years
Formation of Pangaea

Pangaea<180 million years
Opening of the Atlantic
PacificN. Am.

Atlantic

Eurasia

Pacific

Rocky Mountains

Appalachians

1234

Pacific

344

Atlantic

Active margin

Passive margin

56KeelFAULT

Precambrian rocks(>540x106 years)

Undeformed during
the Phanerozoic
Deformed during
Phanerozoic orogenies

1
Phanerozoic rocks(<540x106 years)

2

Mountain chain(“active”)

Mountain chain(“inactive”)

3 Precambrian Shield
Undeformed
4 Platform
Deformed

Sediments deposited
in inland seas, lakes,
wetlands… (2-5 km thick)3 + 4= craton

Passive margin
Edge of continent

25678
Carbonate reef(tropical)
Submarine fan

Region of extended crust (normal faults)

5 Coastal Plain
Sediments transported by rivers anddeposited in the coastal area.

7 Continental slope

6 Continental Shelf
Region near the edge of continents
below sea level that is subsiding
and where thick piles of sediments
can accumulate.

8 Abyssal PlainRift basin(created ascontinental
lithosphere isstretchedand breaksup)

Ocean basin

Thermalsubsidencebasin(created as thetwo plates arepulled apart andnew oceanic
lithosphere isproduced)(created asthelithospherecools andcontracts)
Understanding Earth 6th Ed.

✶ Tectonic provinces of the continents

Understanding Earth 6th Ed.

1000 kmPlatform

Mountain BeltCraton
Precambrian Shield

Physical Geology (9th ed.)★ Shield● Uplifted Precambrian rocks exposed to thesurface.
● Undeformed during the Phanerozoic.

★ Platform● Flat-lying sediments covering thePrecambrian basement rocks.
● Undeformed

Shield + Platform = Craton
A craton is the oldest and most stable part of continents(! relatively unaffected by Phanerozoic orogenies)
Understanding Earth 6th Ed.

Examples: Canadian Shield, Australian Shield
Metamorphosed igneous
intrusion (granite) (Archean)

Metamorphosed sedimentary and extrusive
igneous (basaltic) rocks (Archean)~ 10 km★ Intracratonic BASINS and DOMESDOMEHeating !

THERMAL SUBSIDENCE BASIN
upliftCooling !

subsidence

Lithosphere
Asthenosphere

Upwelling

Physical Geology (9th ed.)Continental Basin● Region of the continental crust that has

subsided and formed a depression wheresediments can accumulate.
● Sedimentary layers dip toward the center ofthe basin.
Example: Michigan Basin (USA)J.L. Ahern (Univ. of Oklahoma)

Continental Dome
● Region of the continental crustthat has been uplifted, forming astructural dome.
● Sedimentary layers dip away fromthe center of the basin.
Example: Cincinnati Arch (USA)YoungerOlderIntracratonic basin with a more
complex history:
Example: Amadeus Basin (Australia)

Intracratonic basins: important reservoirs of coal, oil, and natural gas!

Wikipedia

http://peakenergy.blogspot.com/2013/09/manufacturing-gas-crisis.html 2020/3/6

The Richat dome: a structural dome related to a magmatic intrusion

Matton et al. (2005)

★ Phanerozoic orogens (= mountain belts)● Mountain belts are formed by episodes ofcompressive deformation where two tectonicplates converge.
● Orogeny refers to the process of mountainbuilding.
Examples: Appalachian Mountains (USA)
Alpine-Himalayan orogenic belt (Eurasia)~ 13 km

The Appalachian mountains are marked by two distinct successive episodes ofcompressive deformation (orogenies):●●Caledonian orogeny resulting from thecollision of the continents Laurentia(part of N America, Greenland), Baltica(N-E Europe), and Avalonia (part of N-E
America, part of W Europe) 450-400 Maago.

460 MaMiddle Ordovician

LaurentiaAvalonia

The collision between Laurentia, Baltica,and Avalonia resulted in the formation of
the continent Laurussia.
400 MaVariscan* orogeny resulting from theEarly Devonian
collision of the continents Laurussia andGondwana (S America, Africa,Madagascar, India, Antarctica, andLaurussia
Australasia) 400-300 Ma ago.
The collision between Laurussia andGondwana resulted in the formation of
the supercontinent Pangaea.
*also referred to as the Hercynian orogeny

Baltica

Gondwana

Gondwana

Appalachians-USA!

WikipedEifel-Germany!

Wikiped!

Wikipedi

Ardennes-Belgium

460 Ma
Middle Ordovician

Baltica

440 MaLate Ordovician

420 MaLate Silurian

370 MaLate Devonian

340 Ma
Early Carboniferous

Laurentia

Avalonia
Gondwana

400 Ma
Early Devonian

Laurussia

Gondwana

310 Ma
Late Carboniferous

PANGAEA

Paleogeographic reconstructions based on:● Mapping rock types and geologic structures● Fossil distribution● Rock paleomagnetism

When lava cools, iron-rich minerals
become magnetized in the direction ofEarth’s magnetic field. By measuringrock magnetization, it is possible todetermine the paleolatitude at which
the rock formed.

Cocks and Torsvisk (2006)

Henriksen et al. (2008)

The magnetic field of iron-bearing minerals aligns
with Earth magnetic field in a cooling magma. The
resulting solid rock preserves the originalorientation of the magnetic field.

Rock magnetized at
this latitude has amagnetic field with adifferent inclination.

Trends of Earth’s magnetic
field lines of force !
LINES OF FORCE AT
DIFFERENT LATITUDESHAVE DIFFERENTINCLINATIONS!

Rock magnetized at
the North Pole bears amagnetic fieldoriented vertically.

EARTH

http://www.nature.nps.gov/geology/usgsnps/noca/sb10paleomag.html

Rock paleomagnetism constrains the paleo-latitude of a rockformation but not its paleo-longitude.

http://people.eku.edu/ritchisong/nav_orient.htm

Alpine-Himalayan orogenic belt (still active)

Chains of mountains resulting from thecollision between the African, Arabian,and Indian plates with the Eurasian plates.

Understanding Earth 6th Ed.

★ Extended crust● Region of the crust where the most recent

deformation has involved large-scale crustal
extension.
e.g. Atlantic coastal plain
Basin and Range Province (USA)Extension resulting from theopening of the Atlantic Oceanbeginning 180 million years ago.

Extension resulting from upwelling
convection currents in the mantle.
Physical Geology (9th ed.)✶ Tectonic provinces vs. tectonic ages

Understanding Earth 6th Ed.

Tectonic age = time of the last major episode of crustal deformation

550 Myr-0
Phanerozoic

2.5-0.55 Gyr

2.5-4 Gyr

Understanding Earth 6th Ed.

✶ The growth of continents★ Magmatic addition●Direct addition of magma in subduction zones(water-induced melting, various igneous rocks produced)

Wakita (2012)

★ Accretion●Tectonic plates carry crustal material that can be incorporated toexisting continental masses.

Understanding Earth 6th Ed.

Pieces of buoyant, lightmaterial plastered onto
other continental masses
are called accreted
terranes (10s to 100s ofkm in width).
Akiyoshi terrane
Examples of accretedmaterial: islands,underwater volcanoes,ocean floor sediments.

Wakita (2012)

Examples of accreted material from the subducting plate in Japan

http://www.pref.yamaguchi.lg.jp
http://www.geol.sci.hiroshima-u.ac.jp

http://www.geocaching.com

Fossil atoll reefs (Akiyoshidai, Carboniferous-Permian)

University of Edinburgh

Geological Survey of JapanRadiolarian chert

Wrangellia terrane moved
5000 km north before
reaching its presentposition.

The west coast of North America
is mainly composed of “small”
crustal blocks incorporated to thecontinental mass over the past
200 million years.

http://www.dnr.wa.gov/

Understanding Earth 6th Ed.

✶ Epeirogeny(expansion of litho.)(Mantle materialpushes the lithosphere)
Heating → lower density → lithosphere “floats”
Expansion
higher on asthenosphere

Understanding Earth 6th Ed.

Example of glacial rebound: raised beaches resulting from the melting of the ice cap
after the last glacial maximum 20,000 years ago. The land that was once covered with
ice is progressively raised after the ice melted (ice unloading)Land raised
Relative dropof water level!

Shores of Point Lake (Canada)

Understanding Earth (6th ed.)✶ What explains the long-term stability of cratons?

Understanding Earth (6th ed.)

Diatreme
1.

2.

Kimberlite

3.

4
.

Understanding Earth 6th Ed.

Acasta Gneiss(Canadian Shield, 4.0 Ga)

John Grotzinger & Thomas H. Jordan (2010) Understanding earth 6th edition W H Freeman & Co

Amphibole-bearing rock(Canadian Shield, 4.28 Ga)