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Schematic of Subduction


Before we begin, some background on subduction zones, California, and the tour guide that I followed. (see below)
Study this before you start
U.S. Geological Survey images
The upper diagram shows the different parts of the continental-oceanic convergent margin, which I will refer to throughout the tour. Subduction zones are the part where the oceanic plate moves beneath the continental plate, mixing crustal rocks and sediments and fluids into the upper mantle. The lower diagram shows details of that mixing process, sometimes called the subduction factory. It produces high mountains of erupted lava, deep plutonic bodies of granite, and large amounts of sediment that erodes off the mountains. All of those are found in California's subduction-zone rocks. In addition, parts of the incoming oceanic plate get scraped off by the outer wedge of the continental plate. You'll see those on this tour, too.

California is a famous oceanic-continental convergent margin, and its rocks are considered the type example of the products of this tectonic setting. Northern California displays subduction-zone rocks from the last 500 million years. Today much of coastal California is a transform zone, and sideways motion near the coast has dissected the older rocks. That makes things more complicated, but it also reveals more pieces of the puzzle.

This four-day field trip was created by Eldridge Moores, John Wakabayashi, and Jeff Unruh for the 100th Anniversary Earthquake Conference in 2006. Its 30 stops represent all parts of a subduction system. There are also three stops of archaeological interest—stops 9, 21 and 22—contributed by Sharon Waechter of Far Western archaeological consultants.

The field trip was not actually held because not enough people signed up, but it was published in GSA Field Guide 7 (look for copies). I followed its directions on my own. Stops 1 to 9 are on day 1 (Coast Range), stops 10 to 18 on day 2 (Sierra), stops 19 to 28 on day 3 (Sierra), and stops 29 to 33 on day 4 (Coast Range).

Most sites have precise GPS coordinates plus road mileages, but bring a navigator if you can. Some of the published directions have errors, and I've tried to present better information here when I can. Much of the tour is in rugged country, but it's on state highways and little of it is actually remote. There is only one bit of dirt road. Remember your field manners and do not collect or damage rocks in park lands. Roadcuts, the geologist's best friend, are usually fair game.

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