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 c. 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 c. 422 Ma and after c. 412 Ma, and that granite emplacement had pronounced effects on the fault kinematics.

北西-南东走向的左旋走滑变形在苏格兰高地及西北爱尔兰地区广泛分布，并被解释为对亚特兰蒂斯板块俯冲和大陆碰撞的响应。大 Glen 断层带的西南延伸部分在西北爱尔兰以 Leannan 断层为标志。Tullagh Point 花岗岩中获得的野外证据表明，其在韧性左旋剪切过程中沿断层侵入。U-Th-Pb 锆石年代学分析为该花岗岩提供了 422 ± 2 Ma 的结晶年龄。对 Malin Head 地区断裂模式的详细分析表明，左旋断裂（F1）以锐角相对于主断层带发育。这些断裂显示出斜向的 K-长石的内部排列，并被以相似方向出现的扩张性石英充填的节理所叠加。这些节理的趋势与 Malin Head 花岗岩（412 ± 3 Ma）侵入导致的相对于 F1 的约 90° 的 σ1 轴旋转一致。随后的左旋断裂在花岗岩相关力的消散后形成，它们定义了区域 σ1 的逆时针旋转，并受到与 F1 不同的应力张量作用。据此得出结论，Leannan 断层在约 422 Ma 时开始左旋运动，并在约 412 Ma 之后，花岗岩的侵入对断层动力学产生了显著影响。