The mechanisms of frictional sliding in faulted Westerly granite were studied in two ways. Firstly, the experimental activation energy was measured from 300 to 700 °C at 2.5 kbar (2.5 × 105 kPa) pressure and sliding rates from 10−5 to 10−2 cm/s. Secondly, fault samples were examined with an optical and a transmission electron microscope. Below 500 °C the activation energy was about 30 kcal/mol (1.3 × 105 J/mol). The fault gouge was porous and consisted of angular randomly oriented grains. The quartz and the feldspars were unstrained, similar to the grains in a room-temperature fault. Above 500 °C the activation energy increased to about 85 kcal/mol (3.6 × 105 J/mol). Plasticity in quartz about 500 °C was observed optically by the presence of highly strained gouge grains and with the transmission electron microscope by a marked increase in dislocation density from 3 × 108 cm−2 initially to greater than 1011 cm−2 at 700 °C. The quartz grains away from the fault were strain-hardened with inhomogeneously distributed, dense tangles of dislocations. In contrast, the small grains (<10 μm) in the gouge contained a low density of dislocations. The feldspars showed no sign of plasticity up to 700 °C. Biotite and muscovite were plastic at all temperatures, forming thin ribbons along slip surfaces in the fault zone. Glass was not identified in any of the faulted samples studied.

You do not currently have access to this article.