Geology: The creeping San Andreas
Scientists have discovered talc in rock samples drilled through the San Andreas fault, which could explain why this segment of the tectonic boundary experiences such a high rate of creep. The central Californian part of the San Andreas fault very slowly moves or ‘creeps’ along at a rate of up to 28 millimetres a year and is thought to be the weakest zone of the 1,300-kilometre plate boundary between North America and the Pacific. This weakness was proposed to be due to the presence of the metamorphic rock serpentinite, but recent pressure and temperature measurements have shown that serpentinite is actually too strong and unstable to produce the observed conditions.
Diane Moore and Michael Rymer report the discovery of talc in cuttings collected during drilling of the fault zone. This three-kilometre-deep drill hole — part of the San Andreas Fault Observatory at Depth (SAFOD) — is located near Parkfield in central California. Instruments have been installed across the fault to record data near the source of earthquakes, and rock and fluid samples have been collected to help scientists evaluate the geological properties that control the seismological behaviour of the fault in the region.
The authors examined the mineral composition of grains of serpentinite collected at three-metre intervals. They found that talc of recent origin had replaced the serpentinite along veins and foliations. The presence of talc in the active trace of the San Andreas fault is significant because talc has a very low shear strength in the temperature range found in the fault. They conclude that talc may therefore provide the connection between serpentinite and creep in the San Andreas fault.

CONTACT
Diane Moore (US Geological Survey, Menlo Park, CA, USA)
Tel: +1 650 329 4825; E-mail: dmoore@usgs.gov

Christopher Wibberley (Universite de Nice - Sophia Antipolis, Valbonne, France) N&V author
Tel: +33 4 92 94 26 32; E-mail: wibbs@geoazur.unice.fr