plate motions

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EPS 122: Lecture 4 – Plate motions

Plate motions Reading: Fowler Ch 2


NUVEL-1A Measuring present day plate motions

Combine multiple observations and solve for best-fitting rigid plate motions

• 277 ridge spreading rates – use distance of last magnetic

reversal from ridge

• 121 ocean transform-fault azimuths

• 724 earthquake slip vectors

Eurasia fixed

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NUVEL-1A Comparison with GPS observations


Global plate motion modeling Seismicity

Seismicity Plate geometry

…problems

Global distributions of continuous GPS

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Relative plate motion Calculating

Defining v – need a magnitude and azimuth

magnitude: v = R sin a

azimuth: = 90 + C

We can see that

b = 90 – x

c = 90 – p

also

A = p – x

For spherical triangles:

cos a = cos b cos c + sin b sin c cos A

sin a sin b sin c

sin A sin B sin C = =


Relative plate motion Calculating

Defining v – need a magnitude and azimuth

magnitude: v = R sin a

azimuth: = 90 + C

After substitution:

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Changing plate boundaries What can cause a global plate reorganization?

rotation pole must move – velocities at all points on the surface of the plate change


Changing plate boundaries

What will happen to the triple junction?

These are stable triple junctions – the geometry does not change with time

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Changing plate boundaries

What happen to the triple junction?

(a) is not stable and will evolve with time to (c) which is stable


The importance of triple junctions As they migrate the can cause significant reworking of lithospheric material e.g. west coast of North America

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Describing triple junctions

• 16 possible types of triple junctions

• some always unstable, e.g.

FFF

• some always stable, e.g. RRR

• others depend on the geometry


Triple junction stability

What are the conditions for stability?

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Mendocino triple junction

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Mendocino triple junction


Absolute plate motions

All plate motions thus far have been relative

need stationary reference points on the surface of the earth for absolute plate motions

hotspots

How can we be sure the hotspots are stationary?

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Hotspot tracks

Hawaii

Easter

MacDonald

Galapagos


HS3-NUVEL-1A

Where is the pole for the Pacific plate?

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HS3-NUVEL-1A

Absolute

Eurasia fixed


World stress map

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World stress map

Correlation? …are the forces moving the plates responsible for lithospheric stresses?


World stress map

Correlation? …are the forces moving the plates responsible for lithospheric stresses?

plate motions

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