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.

北东-南西向左旋走滑变形（NE–SW-trending sinistral strike-slip deformation）遍布苏格兰高地（Highlands of Scotland）与爱尔兰西北部（NW Ireland），其成因被认为是伊阿珀托斯洋（Iapetus）俯冲及大陆碰撞的构造响应。大格伦断层带（Great Glen Fault zone）的西南延伸段在爱尔兰西北部以莱南恩断层（Leannan Fault）为标识。对塔拉角花岗岩（Tullagh Point Granite）的野外调查证据显示，该岩体于韧性左旋剪切作用期间沿该断层侵位。通过U-Th-Pb锆石年代学（U–Th–Pb zircon geochronology）测试，该花岗岩的结晶年龄为422 ± 2 Ma。对马林角（Malin Head）区域内破裂模式的详细分析表明，左旋破裂（F₁）与主断层束呈锐角发育。这类破裂表现出钾长石的斜向内部定向排列，并被产状相近的张性石英充填节理叠加改造。这些节理的产状与σ₁轴相对F₁发生约90°旋转的特征相一致，该旋转由年龄为412 ± 3 Ma的马林角花岗岩（Malin Head Granite）侵位所引发。在花岗岩侵位相关的应力消散后，后续的左旋破裂开始发育。这类破裂体现了区域σ₁轴的逆时针旋转，且其活动处于与F₁不同的应力张量环境中。据此认为，莱南恩断层分别于约422 Ma和约412 Ma之后发生左旋活动，且花岗岩侵位对断层运动学具有显著影响。