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cataclastic metamorphism    The grinding of rocks as they shift along an active fault pulverizes the rocks; hence the term cataclastic (cata- for catastrophic and -clastic for fragmentation) metamorphism.    Rocks that experience cataclastic metamorphism are subjected to sudden high-pressure and low-temperature conditions, resulting mainly in a change in texture of the original rock.   Repeated movement of rocks along a fault can crush mineral grains into a fine powder, called fault gouge.   Since cataclastic metamorphism occurs where rocks grind past each other along a fault, it is a common feature along most plate boundaries, where plates are interacting with each other generating tremendous stresses and resulting movements of the crust.

  These are views of the Punchbowl Fault, which runs just south and east of Devils Punchbowl County Park, southern California.   The first image shows the fault cutting through the San Gabriel Mountains, from the middle-right to the upper left of the image.    The second image is a closeup-view of the fault.   Note the fragmentation of the granite as it has shifted and fractured due to movement along this active fault.   See the black camera-lenscap (two inches in diameter) for scale in the second image.

Fault gouge developed due to long-term movement along the Elsinore Fault.   This exposure is within a roadcut on Montezuma Road near Anza Borrego State Park, southern California.

A closer view of Steve and the fault gouge from the image above.    Steve could easily dig his fingers into the crushed rock, whereas the gneiss on each side of the fault is still very solid and hard.

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