March 6, 1999



Near the eastern slopes of the Rudhee Valley, near Silicon Valley, California, we have a relatively rare geological feature. We have the confluence of 3 plates that operate, on occasion, as pairs of plates. Using the diagram at the left, we have a situation where plates A and C will move as a single plate in transformational mode, then plates B and C will do the same, and A and C again.

When this happens, a very characteristic and easy to see geological feature is left behind. It's as if plate C where lifted and lowered. Plate C usually does not quite get lowered to the same spot as before it was lifted, making it easy to see the three plates.


This picture shows the remarkable result of that movement. The triangular wedge that opens the the right is the north end of plate C, and the bottom portion of the picture is plate A. Plate B is on the other side of plate C and cannot be seen in this view.

Plate C has been lifted by a coupled movement of plates B and plate C together, and then lowered by a coupled movement of plates A and plate C. The bright line shows where plate C did not completely lower to its starting position. You can also see the stress cracks near the tip of plate C caused by uneven movement as plates A and B more at different rates relative to plates A amd C. The stress cracks seen in plate A below are cause when plates B and C move, causing plate C to act as a wedge into plate A.

As the Silicon Valley area has virtually constant earthquake activity, many geological events which are not visible due to their very long repeat rates are much more visible here. This picture shows a tree whose trunk is stradling plates A and C. The trunk has split up the center, with one fork following plate C and the other following plate A. The tree is less than one hundred years old, and yet the plate movements have been frequent enough, with enough transformation displacement, to cause this visible phenomenon.
Part of the standard field kit for members of the Society for the Prevention of Plate Tectonics is the graduated suspender set. The suspenders have a graduated scale printed on them. Here, Michael is making one of the yearly measurements of plate C's displacement relative to plate A.

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