17 March 2023

Friday fold: Pinto Gneiss, varnished and un-

Posted by Callan Bentley

Last week, I was lucky enough to visit Pushawalla Canyon in the Indio Hills region of southern California. There, my colleague Kim Blisniuk (San Jose State University) led our students through an exercise mapping alluvial deposits as a way of constraining offsets along the Mission Canyon strand of the San Andreas Fault. I noted that much of the alluvium was sourced to the northeast, to the Little San Bernardino Mountains. Particularly striking to my eye was the Pinto Gneiss, whose foliation is frequently contorted into lovely folds, like this one:

But on older alluvial terraces, desert varnish has accumulated, and the trace of the folded foliation is a bit harder to see:

To me, this comparison/contrast between two similar boulders of gneiss offers a striking visual that qualitatively shows some alluvial deposits to be older, and others fresher. When quantitative methods are applied to constraining the ages of these deposits (10Be exposure ages coupled with U-series soil carbonate ages), rates of offset along the San Andreas can be calculated.

Happy Friday, and Slàinte Mhath!