Pseudotachylyte forms during coseismic slip and thus records individual fault movements. Constraining P-T conditions of pseudotachylyte formation gives knowledge of the fault environment during seismicity. In order to quantitatively determine the temperature during pseudotachylyte formation, we have applied two different types of geothermometers to microlitic (hornblende ± andesine-labradorite+magnetite) pseudotachylytes from the Outer Hebrides Fault Zone. These pseudotachylytes have average bulk chemistries that are broadly andesitic in composition.

The first geothermometer (O'Hara 2001) measures the thermodynamic melting efficiency using clast concentration, which reflects brittle wear processes during frictional melting. It uses an equation relating wear to melt ratios (W/M), ambient crustal temperature, and melt temperature. The second geothermometer is based on amphibole-quartz-plagioclase equilibria, using the equations from Blundy and Holland (1990).

Applying GIS digitizing methodology to thin section photomicrographs from seven samples, we measured the total area of clasts (n=8776) and the area of the pseudotachylyte matrix. W/M ratios range from 0.10 to 0.40, averaging 0.20 ± 0.08 (1s). Assuming a melt temperature of ~820°C, ambient country rock temperatures are 581 ± 89°C, corresponding to a maximum of 23 ± 3.6 km for the depth of formation using 25°C/km. Using electron microprobe analyses we applied the amphibole-plagioclase geothermometer to adjacent primary microlites in the pseudotachylyte matrix. Using 23 km depth yields an average crystallization temperature of 708 ± 84°C.

These data are substantiated by the presence of plastically deformed pseudotachylytes and pseudotachylyte veins crosscut by mylonites indicating moderate crustal levels of formation. Our results show a relatively small temperature contrast between the ambient crustal and crystallization temperatures; this explains in part the presence of large microlites (up to 400 mm) and the absence of glass and vesicles. Our results show the Outer Hebrides Fault Zone was at least intermittently seismically active at 23 ± 3.6 km depth.