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Andrew Alden

GSA Fresno: One More Day

By May 23, 2013

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Wednesday was the last day of the formal sessions at the GSA Cordilleran Section meeting. The talks I attended were the final segment of the "meeting in a meeting" and centered on the hot new tool for sorting out the history of the highly scrambled western margin of North America: detrital zircons. Many bodies of rock have zircons with a mixture of ages, reflecting zircons inherited from previous rocks and (in the case of igneous rocks) zircons they have created. It's conceptually straightforward to use them like barcodes for tracing the history of rocks and the mountain chains, volcanic arcs and plate interactions that make up that history. Detrital zircons are the ones that go downstream with sediments and end up inside younger sedimentary rocks. We can use them to tell what kind of rocks they came from—what the ancient, long-vanished mountains were made of.

So that was fun. In the afternoon, I took a drive in the Sierra foothills. After sunset, I was pleased to see three planets together low in the western sky: Mercury, Venus and Jupiter. They'll be there all week, folks.

Thursday is a field trip into "hard-to-access outcrops of the Mesozoic metasedimentary framework and gabbroids of the Early Cretaceous Sierra Nevada Batholith." In lay language, the field-trip leaders will take us to rocks on private land that document the earliest phases of the great story behind the Sierra Nevada. Some geologists occasionally treat fences the way Woody Guthrie did: there are no "No Trespassing" signs on the other side of them. More specifically, they answer the old philosophical chestnut in the negative: "no tree fell (or trespass occurred) if no one was there to see it." Another approach is the quantum one: an observer can have a virtual existence on both sides of the fence at once. Be that as it may, the ideal is to visit a location with the owner's full knowledge and permission, and it should be a great time.

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