The thermal history of the western Irish onshore

We present here a low-temperature thermochronological study that combines the apatite fission-track and (U + Th)/He dating methods with a pseudo-vertical sampling approach to generate continuous and well-constrained temperature–time histories from the onshore Irish Atlantic margin. The apatite fission-track and (U + Th)/He ages range from the Late Jurassic to Early Cretaceous and the mean track lengths are relatively short. Thermal histories derived from inverse modelling show that following post-orogenic exhumation the sample profiles cooled to <em>c</em>. 75 °C. A rapid cooling event to surface temperatures occurred during the Late Jurassic to Early Cretaceous and was diachronous from north to south. It was most probably caused by <em>c</em>. 2.5 km of rift-shoulder related exhumation and can be temporally linked to the main stage of Mesozoic rifting in the offshore basins. A slow phase of reheating during the Late Cretaceous and Early Cenozoic is attributed to the deposition of a thick sedimentary sequence that resulted in <em>c</em>. 1.5 km of burial. Our data imply a final pulse of exhumation in Neogene times, probably related to compression of the margin. However, it is possible that an Early Cenozoic cooling event, compatible with our data but not seen in our inverse models, accounts for part of the Cenozoic exhumation.

本研究开展一项低温热年代学（low-temperature thermochronological）研究，将磷灰石裂变径迹（apatite fission-track）与(U + Th)/He测年法相结合，并采用伪垂直采样方法（pseudo-vertical sampling approach），从爱尔兰大西洋沿岸陆上陆缘获取了连续且约束严谨的温度-时间热历史序列。本次测试得到的磷灰石裂变径迹与(U + Th)/He测年年龄跨度为晚侏罗世（Late Jurassic）至早白垩世（Early Cretaceous），且平均径迹长度相对偏短。通过反演模型（inverse modelling）得到的热历史结果显示，造山后剥蚀（post-orogenic exhumation）作用发生后，采样剖面的温度冷却至约75℃。晚侏罗世至早白垩世期间发生了一次快速冷却事件，使岩层温度降至地表温度，且该事件在空间上自北向南呈现穿时性（diachronous）。该冷却事件极有可能由约2.5km的裂谷肩相关剥蚀作用（rift-shoulder related exhumation）引发，且其发生时间可与近海盆地中生代裂谷作用（Mesozoic rifting）的主阶段相对应。晚白垩世至早新生代期间存在一期缓慢的增温过程，该现象可归因于厚层沉积序列的沉积作用，该沉积作用造成了约1.5km的地层埋藏。本次研究的数据表明，新近纪（Neogene times）时期存在最后一期剥蚀脉冲事件，该事件大概率与该陆缘的挤压作用相关。不过，存在一种可能性：早新生代冷却事件虽未在本次反演模型中体现，但与我们的实测数据相符，该事件或许是新生代剥蚀作用的部分成因。