The brittle evolution of a major strike-slip fault associated with granite emplacement: a case study of the Leannan Fault, NW Ireland

NE–SW-trending sinistral strike-slip deformation is widespread throughout the Highlands of Scotland and NW Ireland and has been interpreted as a response to the subduction of Iapetus and continental collision. The southwestern continuation of the Great Glen Fault zone is marked in NW Ireland by the Leannan Fault. Field evidence derived from the Tullagh Point Granite indicates its intrusion along the fault during ductile sinistral shearing. U–Th–Pb zircon geochronology yields a crystallization age of 422 ± 2 Ma for this granite. Detailed analysis of fracture patterns throughout the Malin Head region demonstrates that sinistral fractures (F₁) developed at an acute angle to the main fault strands. These fractures display oblique internal alignment of K-feldspar and are overprinted by dilatational quartz-filled joints at a similar orientation. The trend of these joints is consistent with <em>c</em>. 90° rotation of the σ₁ axes relative to F₁ as a result of intrusion of the Malin Head Granite dated at 412 ± 3 Ma. Subsequent sinistral fractures developed after dissipation of granite-related forces. They define an anticlockwise rotation of the regional σ₁ and moved under a different stress tensor from F₁. It is concluded that the Leannan Fault moved sinistrally at <em>c</em>. 422 Ma and after <em>c</em>. 412 Ma, and that granite emplacement had pronounced effects on the fault kinematics.