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GeoSnaps - Image of the Day

Geologic snapshots capturing Arizona’s geologic setting and mining history with a picture of the day from 1 January through 31 December 2013.

Check out previous GeoSnaps/Images of the Day.

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Monday, December 16, 2013
Valley of the Sun, metamorphic rocks, mylonite

November 7, 2013 | South Mountain | Maricopa County, AZ

The South Mountains are located five miles south of downtown Phoenix, AZ and are part of the largest municipal park in the United States at over 15,000 acres. Also known as the Salt River Mountains, their domed profile belies their geologic origin as a metamorphic core complex. Beginning in the mid-Tertiary about 30 Ma, the crust in central Arizona was stretched horizontally and as blocks of rock were dislocated along low angle detachment faults, the underlying crystalline rocks bulged upwards. Mylonitic gneiss developed on the upper part of this crystalline mass giving the range its present-day profile. The upper plate rocks (the Papago Buttes in this instance and now lying 10 miles to the northeast of South Mountain) were faulted as they moved away off of the core. (Photo by Wayne Ranney)