Download - 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?