tripple junction (2)

8/2/2019 Tripple Junction (2)

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GSC307Introduction to Global Geophysics

10/10/11 Cal Poly Pomona

Todays Class

Lecture:Triple Junctions StabilityHot Spots







Triple Junctions

Points where three plateboundaries meet are called

triple junctions.

Triple junctions may move(migrate) and change the

tectonic environment.



Triple Junctions

R= Ridge F = transform Fault T = subduction zone

(Trench)

A triple junction is stable if

the relative motions of

three plates theirgeometry do not change

over time.


Example: RTF Junction

In a circuit about atriple junction an

observer must return

to the starting point

the vector diagram ofthe interplatevelocities is a closedtriangle

AVB+BVC+CVA= 0



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Triple Junction Stability

Stability of a TJ is assessedby considering how it canmove along any of its (three)

plate boundaries

A TJ lies at the intersectionof the three velocity lines for

the three plate boundaries.

If these three lines intersect,the TJ is stable. Ifnot, it is

unstable and will evolve into

a different plate geometry.A

B



Velocity Line for a Trench

Consider marbles rolling along a trench at different velocities They will plot on a velocity diagramon a line passing

through the velocity of the upper plate (because the trench

moves with the overthrust plate) and trending in the samedirection as (so: parallel to) the trench

does not require that convergence velocity be perpendicular to trench



Velocity Line for a Transform Fault

Consider marbles rolling along transform at different velocities They will plot on a velocity diagram on a line passing through

both plates (because the transform does not move with respect

to either plate) and trending in the same direction as (so:parallel to) the transform



Velocity Line for a Ridge

Consider marbles rolling along ridge at different velocities They will plot on a velocity diagram on a linetrending in the

same direction as (so: parallel to) the ridge and (if the

spreading is symmetrical and perpendicular to the ridge) thatis the perpendicular bisectorof the velocity vector between the

two plates

A

B



Triple

Junction

Stability

A TJ lies at the intersection of the three lines: ab, bc and ac for thethree plate boundaries.

If these three lines intersect, the TJ is stable. Ifnot, it is unstable

and will evolve into a different plate geometry.



Triple

Junction

Stability





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Triple

Junction

Stability






Another Example

Draw the velocity space diagram and determinewhether this is a stable triple junction, or not. Keep

C fixed.



Volcanism Within Plates: Hot Spots

Mantle plumes and hotspots Linear chains of volcanoes or volcanic islands Systematic variation of age



Hot Spots: Hawaii

Hotspots arestationary, deeplyanchored columns ofhot material

Lithospheric platemoves over thehotspot

Creates a row ofvolcanoesprogressively oldertoward one end



Formation of Hot Spot Volcanoes



Big Island

Kauai


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Plate

Velocities

Dating of basalt samples helps determine platevelocities and directions



Hot Spot

Reference Frame

Results from otherhotspots suggest

they form a

stationary network




Movement on a Spherical Earth

Eulers theorem:Every displacement of a plate from one position to another on

the Earth's surface can be regarded as a rotation of that plateabout a suitably chosen axis, which passes through the centreof the Earth and cuts through the Earths surface at the Eulerpole or pole of rotation.

The Euler pole does not have to be the

North Pole!


Geometry of constructive boundary at which two plates areseparating.

Note that whilst rotation angle remains same along length of split,width of gap increases with angular distance from pole of rotation.

What effect does this have on local spreading rates along length ofplate boundary?10/10/11 Cal Poly Pomona



Rotation VectorsThe relative velocity v between two plates is:

v = R sin

And thus is 0 at the rotation poles and maximum ( R) at90 from the poles.

Given rotation vectors, you can calculate the relative motionat a plate boundary.




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Relative Plate MotionsWhat does the change in spreading rate along the mid Atlantic

ridge tell you about location of the pole of rotation?



Determination of Euler

Poles

The motion of plate 1 relative to plate 2must be a rotation about an Euler pole.

The strike of a transform fault is parallel to

the relative motion between the twoplates.

=> All transform faults on the plate

boundary must be small circles about

that pole.

=> Orientation of transform faults can be

used to locate the Euler pole



Determination

of Euler Poles

Lines (great circles) at right angles totransform faults will pass through the poleof rotation.

Using this method, the pole of rotation of theSouth American Plate can be shown to belocated in the North Atlantic, south ofGreenland.

tripple junction (2)

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