Understanding fault zone evolution is crucial to appreciating how deformation mechanisms may change in time and space. This is of particular importance when considering seismic slip, with its potential for human hazard. Histories of fault evolution and reactivation are recorded as overprinted structures in ancient fault zones, now exhumed from seismogenic depths. Recognition of ancient seismicity is aided by the occurrence of pseudotachylyte, a solidified frictional melt generated at seismic slip speeds along faults. Because pseudotachylytes form on similar timescales to the duration of seismic slip, they capture a snapshot of earthquake parameters such as temperature, depth, strength, magnitude and stress drop.
The Outer Hebrides Fault Zone, UK, was repeatedly reactivated during long-lived collision and bears widespread pseudotachylyte. It is used in this thesis as a case study in which to constrain the seismic history. Slip directions on pseudotachylyte faults are identified using field observations supported by microstructural evidence. The depth and temperature of faulting, and the coseismic temperature rise, are studied using the composition and microstructures in the pseudotachylyte veins, whilst experimentally produced melts further understanding of the control of lithology on coseismic fault strength. Finally, the static strength and the dynamic weakening are derived from further field observations.
Seismicity occurred at ≥ 10 km, somewhat deeper than has been previously thought, and was initially scattered diffusely around the fault zone on small, strong faults. Magnitudes are recorded up to MW 6.3 and static stress drops from 1.5-8.8 MPa. The lithology hosting the fault is shown to control the coseismic strength of the pseudotachylyte-bearing fault both through the melt composition and through the development of fault roughness. Overall, results show that seismicity in the Outer Hebrides occurred throughout a long convergence history because fault weakening, slip localisation and fluid influx were heterogeneously distributed around the fault zone.
