The post-Variscan evolution of Ireland – a case study in constraining shallow crustal exhumation and deformation

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• David Chew, Trinity College Dublin
• Tuesday 12 November 2024, 12:00-13:00
• Department of Earth Sciences, Tilley Lecture Theatre.

If you have a question about this talk, please contact Dr Rachael Rhodes.

The Cenozoic uplift and exhumation history of the British Isles has been attributed to epeirogenic exhumation driven by the proto-Iceland plume, or multiple phases of Cenozoic compression driven by far-field stresses. This talk focusses on the post-Variscan exhumation and shallow crustal deformation history of Ireland and western Britain, which in Ireland is particularly challenging to constrain given the near-complete absence of post-Variscan sediments.

High-resolution thermal history models (from inverse modelling of apatite fission track and (U–Th–Sm)/He data from vertical bedrock profiles) clearly show rapid Paleocene exhumation (1 – 2.5 km) focused on the Irish Sea. Paleocene exhumation is greatest in the north of the Irish Sea and decreases to the south and west. Its spatial distribution agrees with the extent of magmatic underplating inferred from receiver function data and lithospheric thickness estimates derived from surface wave tomography. Earlier (Mesozoic) exhumation onshore is linked to hinterland exhumation during the complex and long-lived rifting history of neighbouring offshore basins. The extent of Neogene exhumation is difficult to constrain due to the poor sensitivity of the thermochronometers at low temperatures but appears most signifcant on the western Atlantic coast. The Cenozoic topographic evolution of a significant portion of British Isles thus appears to be the result of plume-driven uplift and exhumation, with inversion playing a secondary role.

Even so, Cenozoic shortening has affected onshore Ireland. The Carboniferous North Dublin Basin (150 km north of the supposed Variscan “front”) exhibits tight chevron folds and kinematically linked calcite vein sets, along with bedding-parallel veins with slickenfibres. Late Eocene LA-ICP-MS U-Pb ages were obtained from many calcite veins, including fold hinge breccias and bedding-parallel slickenfibre veins clearly associated with N-S shortening (flexural slip). This late Eocene fold reactivation is attributed to far-field, N-directed shortening associated with the Alpine/Pyrenean orogenies, but was not of sufficient magnitiude to manifest in the low-temperature thermochronology record.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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