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Pseudotachylyte occurs as vein material infilling highly brittle shear and extensional fractures developed along the western margin of the late Caledonian, Outer Hebrides Thrust zone in NW Scotland. Vein geometries and textures show clearly that the pseudotachylyte has been through a melt phase. From the composition of the pseudotachylyte matrix which is close to that of a basaltic andesite, probable melt temperatures of around 1100 "C are inferred. Field and theoretical studies demonstrate that the pseudotachylyte was generated by relatively high stress seismic faulting in crystalline sialic crust devoid of an intergranular fluid, most probably at the time of thrust inception and at a depth of around 4-5 km. A study of pseudotachylyte-bearing ' single-jerk ' microfaults shows that the slip (d)is related to the thickness of the pseudotachylyte layer (a) by the equation,

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