SAFOD - Serpentinite Clasts in Fault Gouge

This photo shows the most informative part of the SAFOD cores. The core is marked with the depth in feet along the drill string. It consists of gouge created by the grinding action of the San Andreas fault at a depth of about 2700 meters. But in addition to the brown weathered clays typical of fault gouge, it also contains clasts of fresh rock from the walls of the fault zone. The greenish particles are serpentinite, and the others are sandstone.

Serpentinite is a soft rock made of serpentine minerals. They are in the process of being stretched and absorbed and broken down in the fault zone. Not only is serpentine a soft, slippery ingredient of the material in this setting, but it is also prone to being altered by deep fluids rich in silica, yielding the exceptionally soft mineral talc. Minerals like these serve to lubricate the fault, enabling it to move in an earthquake even when the stresses across the fault are relatively small. This may explain a longstanding puzzle of the San Andreas fault—why it has such energetic earthquakes even though the ambient stresses on it are low. The presence of fresh serpentinite shows that the fault zone is constantly renewing its lubricating properties rather than growing hard and freezing up, or relying on high fluid pressures or other postulated mechanisms to trigger quakes.