plate motions

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EPS 122: Lecture 4 – Plate motions

Plate motions Reading: Fowler Ch 2

EPS 122: Lecture 4 – Plate motions

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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EPS 122: Lecture 4 – Plate motions

NUVEL-1A Comparison with GPS observations

EPS 122: Lecture 4 – Plate motions

Global plate motion modeling Seismicity

Seismicity Plate geometry

…problems

Global distributions of continuous GPS

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EPS 122: Lecture 4 – Plate motions

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 = =

EPS 122: Lecture 4 – Plate motions

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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EPS 122: Lecture 4 – Plate motions

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

EPS 122: Lecture 4 – Plate motions

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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EPS 122: Lecture 4 – Plate motions

Changing plate boundaries

What happen to the triple junction?

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

EPS 122: Lecture 4 – Plate motions

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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EPS 122: Lecture 4 – Plate motions

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

EPS 122: Lecture 4 – Plate motions

Triple junction stability

What are the conditions for stability?

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EPS 122: Lecture 4 – Plate motions

Triple junction stability

What are the conditions for stability?

EPS 122: Lecture 4 – Plate motions

Triple junction stability

What are the conditions for stability?

Mendocino triple junction

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EPS 122: Lecture 4 – Plate motions

Triple junction stability

What are the conditions for stability?

Mendocino triple junction

EPS 122: Lecture 4 – Plate motions

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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EPS 122: Lecture 4 – Plate motions

Hotspot tracks

Hawaii

Easter

MacDonald

Galapagos

EPS 122: Lecture 4 – Plate motions

HS3-NUVEL-1A

Where is the pole for the Pacific plate?

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EPS 122: Lecture 4 – Plate motions

HS3-NUVEL-1A

Absolute

Eurasia fixed

EPS 122: Lecture 4 – Plate motions

World stress map

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EPS 122: Lecture 4 – Plate motions

World stress map

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

EPS 122: Lecture 4 – Plate motions

World stress map

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