Generation of Pseudotachylyte by Ancient Seismic Faulting

Abstract

Summary 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, where d and a are measured in centimetres. Work-energy calculations based on this empirical relationship suggest that the pre-failure shear stress on the microfaults must have been as high as 1.6 kbar to overcome the initial frictional resistance (τf), which decreases with increasing slip during a single movement according to the relationship, which may arise solely from the viscous shear resistance of the melt layer. Delineation of palaeoseismic zones by the recognition of those cataclastic rocks which are necessarily the products of earthquake faulting, may assist in the determination of ancient plate boundaries.