7/05/2009

Corona Heights Fault

Emily found this fault by searching the internet for local slickenline photos and landing at Andrew's site, geology.about.com. What did we do before the internet? The fault outcrops in the Castro Area (Corona Heights) where a post-1906 aggregate quarry has been repurposed for a playground and pet cemetery. The relatively recent exposure of the fault allows excellent preservation of the slip surface itself.

The fault is a thin breccia zone (less than a meter) with an anastomosing network of highly polished, slickenlined surfaces within the breccia zone.



Nils admiring his reflection in the fault surface. Seriously it is so so shiny. I've never seen anything like it. Twice now I have gone out to the Marin Headlands Terrane (same cherts) to look for fault surfaces and see if I can find anything similar. Can't. Went the second time because it was too foggy the first time and I worried I might have missed it.
Emily and Amir worked on a big normal fault in southern Oregon which had a similar fault rock structure - thin, superfine-grained polished slicked core, thin breccia zone with pinch-swell structures, rapidly dropping off to undeformed bedrock on either side of the fault. That thin breccia zone probably takes up a lot of deformation and accomodates the roughness on the polished slip surface. (See Sagy, Brodsky and Axen (2007) in Geology; it's available here.)

Here's Emily getting a grip on "Layer II", that granular layer which must break or flow in order for slip on these uneven, anastomozing polished surfaces to occur. Her hand is on that ~ 40cm layer of breccia in between two slip surfaces.

The texture of the polished surface is truly remarkable. You can see some rounded fragments of chert - which are green, while the local rock is all red chert. They are embedded in a translucent super fine-grained silica matrix. You can see through it. It is positively glassy (in a descriptive sense; I have not examined it for molecular structure). It has beautiful tensile cracks which are generally perpendicular to local slickenlines - and rotate as the slicks rotate and the fault surface undulates. Truly amazing.

OK what you can almost see in this photo are tiny round white spots in the superfine silica layer - they are lensoid fractures in the silica. I have some theories about what this stuff is and what the tiny fractures mean. But I have apparently been too far out on a limb lately (at least for some reviewers) so I will just ask you, on the off chance that anybody is still reading this blog...
1. What is the shiny, transluscent silica and how did it form?
2. Why does it have tiny lens-shaped fractures inside?

Probably the way I worded those questions leads toward my ideas about the answers... but ... anyway...











6 comments:

Kim said...

Don't know about the silica or the fractures, but I once found that outcrop the old-fashioned way: wandering through San Francisco and stopping to look at pretty rocks. I think it's my favorite outcrop in the Bay Area.

Erik said...

I also stumbled across this spot by accident while trying to find my way to the top of the hill. I was trying to figure out how they got away with such a steep cut through that chert, then I looked up close and was like :O

Trifarina said...

Shiny!

andrew said...

I'm very happy my site was useful for Dr. Brodsky, whose paper on that Oregon site was so impressive.

Beats me what those teeny white bits are. I look at that stuff and think of toffee and caramel; I'm not much of a geologist.

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