11_PlateTectonics_2016

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Processes in Structural Geology & TectonicsBen van der Pluijm

© WW Norton+Authors, unless noted otherwise

3/29/2016 2:04 PM

Whole Earth Structure

and Plate Tectonics


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We Discuss …

• Earth’s topography

• Earth’s layers• The crust

• The mantle

• Tenets of plate tectonics

• Today’s plates

•Plate boundaries

• Kinematic of plate tectonics

• Linear and angular

velocities

• Absolute and relative

motions

• Mechanics of plate tectonics• Tectonic cycles

• Wilson Cycle

• Supercontinent Cycle

Tectonics ©PSG&T 2


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Tectonics ©PSG&T 7

Earth’s Surface - 3D Topography


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Tectonics ©PSG&T 8

Hypsometric (=cumulative frequency) Elevation Curve


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Tectonics ©PSG&T 9

Earth’s Seismologic and Rheologic Layering

strong

weak


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Crustal Section and Characteristic Rock Types

Tectonics ©PSG&T 10

Gabbro

Granite


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Crustal Thickness

http://earthquake.usgs.gov/data/crust/



The Crust and Geologic Provinces


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Age of the Crust




Oceanic vs. Continental Crust

Composition Continental crust has a mean composition that is less mafic

than that of oceanic crust.Formation mode Continentalcrust isanamalgamationof rock that originally

formedatvolcanicarcs orhotspots,and then subsequentlypasses through the rock cycle. Mountain building, erosion

and sedimentation, andcontinuedvolcanismadd toor

changecontinentalcrust.Oceaniccrustallformsatmid-ocean

ridges by the process of seafloor spreading.

Thickness Continentalcrustrangesbetween25kmand70kmin

thickness.Most oceaniccrust isbetween 6 kmand10 km

thick.Thus, continentalcrust is thicker thanoceaniccrust.

Heterogeneity Oceanic crust can all be subdivided into the same distinct

layers, worldwide. Continental crust is very heterogeneous,

reflecting its complex history and the fact that different

regions of continental crust formed in different ways.

Age Continental crust is buoyant relative to the upper mantle,

and thus cannot be subducted. Thus, portions of the

continental crust are very old (the oldest known crust is

about 4 Ga). Most oceanic crust, gets carried back into

the mantle during subduction, so there is no oceanic crust

on Earth older than about 200 Ma, with the exception of

the oceanic crust in ophiolites that have been emplaced

and preserved on continents.

Moho The Moho at the base of the oceanic crust is very sharp,

suggesting that theboundarybetweencrust andmantleis

sharp.ThecontinentalMohotendsto be lessdistinct.

Gabbro

Granite



The Mantle - Tomography and PlatesSW California SE North Carolina

P-waves

S-waves

Grand et al., 1997

Redis slow, is hotBlue is fast, is cold

Ritsema


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http://denali.gsfc.nasa.gov/dtam/

The Tenets of Plate Tectonics




Types of Plate Boundaries



Divergent: Red Sea

Convergent: Japan

Transform: New Zealand

Examples of Plate Boundaries


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Insights from Earthquakes




The Kinematics of Plate Tectonics

Relative velocity

Mantle hotspots as “fixed”

reference frame

Absolute velocity



Plate Kinematics on a Sphere

Rotation on Euler pole

(=rotation axis):

a) Displacement follows

small circles

b) Transforms parallel

small circle segments

c) Same angular

velocity (w) between

plates

Different linear

velocity (v) as

function of distance

from Euler pole



Extra: Triple Junctions and Evolution

(a) Stable ridge-trench-transform triple junction.

(b) With time ridge-trench-transform triple junction

location changes (from T to T’), but the geometry

stays same.



Today’s Plate Motions (Absolute and Relative Velocities)


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Absolute Motions and

Speed Limit?


Zahirovic et al., 2015



Gravitational forces (colored):

• Ridge push: topographic spreading

• Slab pull: negative buoyancy of slab

Resistive forces (black) are relatively

small

Mechanics of Plate Tectonics - Driving Forces


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Plate Tectonic Cycles:

The Wilson Cycle


a-b) Continent rifts, such

that crust stretches, faults

and subsides.

c) Seafloor spreading

begins, forming a new

ocean basin.

d) The ocean widens and

flanked by passive margins.

e) Subduction of oceanic

lithosphere begins on one

passive margins, closing

ocean basin.(f-g) Ocean basin is

destroyed by continental

collision.


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Plate Tectonics Cycles: The Supercontinent Cycle

(a) Continents gradually aggregate over a

mantle downwelling zone.(b) While supercontinent exists, large-scale

convection in the mantle reorganizes.

(c) Upwelling begins beneath

supercontinent and weakens it, leading

to breakup.


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1 b.y. of Plate Motion (late Precambrian-Future)

Scotese, 2004

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