2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 154-9
Presentation Time: 11:00 AM-11:15 AM

SEISMICITY AND THE STRANGE RUBBING BOULDERS OF THE ATACAMA DESERT, NORTHERN CHILE

QUADE, Jay1, REINERS, Peter W.2, PLACZEK, Christa J.3, MATMON, Ari4, MURRAY, Kendra, and PEPPER, Martin1, (1) Geosciences, University of Arizona, Tucson, AZ 85721, quadej@email.arizona.edu, (2) Geosciences, University of Arizona, PO Box 210077, Tucson, AZ 85721, (3) School of Earth and Environmental Sciences, James Cook University, MS-J514, Townsville, QLD4811, Australia, (4) The Fredy and Nadine Herrmann Institute of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, 91904, Israel

We encountered large fields of 0.5 to 8 metric ton boulders in the Atacama Desert of northern Chile worn to smoothness around their mid-sections. These smoothed surfaces exhibit varying degrees of desert varnish, from very dark to fresh and white where currently in contact with adjacent boulders. We suggest that the boulder smoothing is the cumulative result of millions of years of rubbing between boulders during earthquakes. 10Be cosmogenic ages of boulder tops from these fields average about 1-2 x 106 years, unsurprisingly old given the hyperaridity of the Atacama. Quite by chance, during our second field visit to one major boulder site we experienced an earthquake which rocked but did not tip the boulders, causing them to rub against each other for about a minute. The earthquake measured 5.3 and was centered about seventy miles east of the site. In the seismically active Atacama earthquakes of this energy occur about once every four months, suggesting that the average boulder experiences ~50-100 kilohours of abrasion. This remarkable evidence underscores the largely unrecognized role that seismicity probably plays in hillslope sediment transport in the nearly rainless Atacama Desert and perhaps on other seismically active but dry planets like Mars.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 154
Defining Rates of Landscape Evolution in Drylands Using Numerical Dating Methods
Minneapolis Convention Center: Room L100DE
8:00 AM-12:00 PM, Tuesday, 11 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 385

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