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...

mini field trip to the Rodeo Cove Thrust

Calling all Franciscan lovers out there - where are you? Are we going extinct? I am busy recruiting for our cult.

The Franciscan is the accretionary complex along the coast of California which preserves records of Mesozoic subduction: the wedge edge of the more famous Sierra Batholith-generating subduction zone. The Farallon Plate subducted under North America for over 100 MY until its tail edge hit the coast (with the Pacific Plate behind it) and the San Andreas Fault was born. While it was active the Farallon Trench subducted, offscraped, underthrusted and underplated a huge amount of marine sediment and a bit of igneous oceanic crust, mostly in discrete fault-bounded packets = terranes.
In the Marin Headlands, just north of San Francisco's Golden Gate Bridge, you can see the contact between two terranes (white teeth) where an ocean-island seamount (Bonita Terrane) was thrust northeastward over a stack of greenstone-chert-sandstone nappes (Marin Headlands Terrane). Within those nappes of the Headlands Terrane, one major fault outcrops on the beach (black teeth): the Rodeo Cove Thrust. Also, it's frickin beautiful out there on Cronkite Beach and there's a really sweet hostel out there. Field trip planners take note.



The "Cron" is probably the most accessible, adequate surf spot in southern marin. The beach is pebbly and composed almost entirely of chert/ jasper in many colors of red, green, black, gray and orange, aka carnelian (apparently orange chert gains some kind of metaphysical significance?)

The beach divides the sea from the Rodeo Lagoon - happy birds. The bridge out to the beach makes it feel like you're leaving Marin behind.
My dear friend Francesca wrote an amazing paper about the Rodeo Cove Thrust*. Sadly she was not able to guide us on this field trip because she was needed in Italy to make soap out of olive oil. I am not making this up. Anyway, Francesca described the intense veined zone in the thrust:


The kiwi delegation checks out the veined zone, comparing it to kiwi veined zones:
At the top of the veined zone, a small, fault-bounded slab of pillows. VT observes that pillows are all nearly the same size, except for some rhyolitic pillows she has seen. I experience the dual sensations of the elation at realization that there is a such thing as a rhyolitic pillow and disappointment that my Geolutions list for 2009 is already full. Where are these rhyolitic pillows of unusual size?

A close-up look at the perlitic texture on the surface of the pillows. This texture forms when volcanic glass slowly hydrates and develops nano, then micro crystals over time. Glass is an uncomfortable state of being for cations and anions. This is evidence that the outside of the pillows were once glassy.

South of the pillows: greenstone cataclasite. Yummy. It's right about here that this old Mesozoic thrust fault is cross cut by a steep, NW-striking smectity gougey fault zone. The San Andreas is right off the beach. I think this young fault is part of the SAF. But this cataclasite... old or new? A bit of both maybe.

Coming next: Ring Mountain.

*Meneghini, F. and Moore, J. C. (2007) Deformation and hydrofracture at seismogenic depths: The Rodeo Cove thrust zone, Marin Headlands, California. Geological Society of America Bulletin 119 1-2, 174-183, DOI: 10.1130/B25807.1