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


Lockwood said...

Excellent rocks, and a number of interesting points. Can rhyolite be fluid enough to form pillows? That would be fascinating to see. Perlitic texture on glassy rind of basaltic pillows- I have seen so many pillows I hardly look at them anymore. I'll bet if I start looking I can find this texture. I love the messy confusion of Franciscan-style geology. The southern Oregon coast isn't -called- Franciscan, but that's basically what it is, so I've seen a lot of this style of geology. One nice aspect is that it's so complex, I don't feel very compelled to try to make sense out of it most of the time... it's just big pile of very cool rocks! ;-)

Silver Fox said...

Great field trip! I've never seen rhyolitic pillows, either, and am wondering where one can find some. Any ideas?

Btw, my verification word is "shist" - appropriate, if spelled incorrectly.

Anonymous said...

RPoUS? I dont believe in them -kit-O

Andrew Alden, Oakland Geology blog said...

Now I must go back to this locality!