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Stop 31, Franciscan Nappes: The Setting

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Stop 31 is at the top of Schmidt Lane in El Cerrito, a former quarry on the west side of the Berkeley Hills facing San Francisco Bay. (more below)
A suburban setting
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
This locality displays the contact zone between two nappes (thrust sheets) of Franciscan rocks. The upper nappe is a former sandstone that was buried about 30 kilometers deep during subduction. Under the microscope it can be seen as consisting of high-grade metamorphic minerals of the blueschist (jadeite) facies. But the rock is not obviously a blue schist, just a tan quartzite. Beneath this nappe is a thick layer of bluish mélange that is of lower metamorphic grade. The mélange consists of blocks of sandstone, basalt and chert (the typical Franciscan mix) tumbled in a shaly matrix. Clearly the high-grade rocks have been thrusted over the lower-grade rocks, probably a vertical distance of about 15 kilometers. The motion on the thrust probably occurred around 70 million years ago, while the rocks themselves probably date from about 150 Ma.

This is a problematic site, fenced off and visible to the people using the recycling depot below. It requires care and discretion in visiting. Also, the rocks have become overgrown with vegetation and are crumbling from exposure to the weather.

Day 1, Coast Range: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Day 2, Sierra Nevada: 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18

Day 3, Sierra Nevada: 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28

Day 4, Coast Range: 29 | 30 | 31 | 32 | 33

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