28 December 2015

Corona Heights Fault, San Francisco

Posted by Callan

On my final day at the AGU Fall meeting, I made a pilgrimage to a place I’d long wanted to visit: Corona Heights, where there is a subvertical exposure of a fault, bedecked with both fault breccia and the most amazing set of slickensides I’ve ever seen. I was able to find it easily thanks to the awesome map that Christie Rowe made for AGU visitors to get a taste of local geology.

From the plunging orientation of these lines, raking across the subplanar fault surface, we can deduce the kinematics (motion) of the fault. In this case, it’s an extraordinary example of an oblique slip fault, cutting cherts of the Franciscan complex.

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“Slickensides” is a general term for linear features on a fault surface. The lines may be grooves that result from gouging, in which case we can call them slickenlines, or they may be mineral crystals that grow in small extensional spaces along the fault surface, in which case we might refer to crystal fiber lineations. At Corona Heights, it’s slickenlines, and they are profoundly well developed, well exposed, and well preserved.

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They are also coated in many places with a silica gel, making the fault mirror-like in its reflectivity when viewed from certain angles. Here is the perspective looking up the cliff at the same spot as the previous photo, showing the reflectivity of the fault surface:

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The exposure is steep (note the rowhouse roof line in the background as a horizontal reference):

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Swiveling our perspective to the right, here is an animated GIF to show the shiny surface of the fault:

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At one place, the fault surface (and its slickensides) have broken off, revealing a look deeper into the footwall. There, you can see a fold in the chert layers:

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Here, the slicks plunge into the ground, neighbored by a zone of fault breccia, upon which the pencil lies:

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Close-ups of the breccia:

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I brought the GigaPan:

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Here are the resulting explorable GigaPan images, the result of about two hours spend on-site. Can you find the scale pencil? Can you find exposures of breccia? Enjoy exploring them for fun details.

Link Image by Callan Bentley

Link Image by Callan Bentley

Link Image by Callan Bentley

This is a world-class site. If you have any interest in faults, it should be your top stop when next in San Francisco.