COSEISMIC DEFORMATION CONDITIONS: EVIDENCE FROM PSEUDOTACHYLYTE IN THE OUTER HEBRIDES FAULT ZONE, NW SCOTLAND

Pseudotachylyte, a rock formed through coseismic frictional melting, provides geological insight into the earthquake mechanics along ancient faults. One such example is the Outer Hebrides Fault Zone in NW Scotland. This Proterozoic crustal-scale fault zone has been reactivated within varying tectonic regimes, and consequently displays a range of complex pseudotachylyte vein geometries and orientations, posing questions as to how the melt reflects the rupture event(s). In order to improve understanding of coseismic deformation processes and the physical conditions that govern them, this study utilises a combination of field observations, petrological and microstructural investigation and experimental rock deformation.

Preliminary results show that vein compositions indicate melt is locally generated by discrete coseismic events, and so the wide distribution of pseudotachylyte implies a distribution of slip across the zone. However, the range of vein orientations displays no systematic relationship towards the orientation of the main fault, which may imply a change in stress field through time or relate to structural heterogeneities. To help constrain the mechanical influence of the differing lithologies and fault rocks seen through the fault zone, rotary shear experiments reveal the frictional behaviour and associated microstructures produced under known conditions.