San Andreas Fault in the Salton Trough
June 05, 2007
Near the southern end of the San Andreas Fault in Imperial County, California, several classic features of a transform fault are beautifully revealed. The fault runs diagonally across this GoogleMap picture from upper left (northwest) to lower right (southeast). The fault motion carries the Pacific Plate (southwest, lower left) northwest relative to the North American Plate (northeast, upper right). Movement along the fault has juxtaposed one kind of rock to the northeast against a different formation to the southwest. The different colored rocks across the fault trace are obvious.
Water flows southwest toward the Salton Sea, in extreme southern California. The upper parts of the streams are missing, “beheaded” in geological terms, having been sheared off and carried southeast along the fault. The stream channels seem to come out of nowhere, starting at or very close to the San Andreas Fault.
The other interesting aspect of this picture is the vegetation. Northeast of the fault (upstream, to the upper right), there's sparse but evident vegetation. Below the fault (downstream to the lower left), there's virtually no vegetation. This happens because the fault has ground up rock into fine, water-impervious powder called gouge. The gouge prevents underground water from crossing the fault. Below the fault, the water table is much lower, and plant roots have almost no chance of reaching enough water to grow. See also the Earth Science Picture of the Day for Oct 26, 2005.