VARNET-96: influence of the Variscan and Caledonian orogenies on crustal structure in SW Ireland

Abstract

VARNET, an international multidisciplinary project, was designed to examine the ‘Variscan Front’ in the southwest of Ireland. As part of the seismic experiment a 200 km long wide-angle seismic profile (Line A) was recorded from the Old Head of Kinsale in the south of Ireland to Galway Bay on the west coast. Along Line A, 170 seismic stations were deployed at approximately 1 km intervals. Results from ray trace modelling indicate a multilayered crust. The upper crust, extending to a depth of about 14 km, is laterally variable. South of the Shannon Estuary sedimentary basins alternate with uplifted basement (5.8-6.0 km s−1). This correlates well with the surface geology. The larger sedimentary basin in the south reveals a shallowly south-dipping structural feature that coincides with the Killarney-Mallow Fault Zone. North of the Shannon Estuary surprisingly high velocities (6.4 km s−1) at shallow depths were observed. Between the upper and middle crust there is a transition layer about 2-4 km thick. The middle crust thickens towards the Shannon Estuary. These structural changes coincide with the proposed location of the Iapetus Suture Zone. The middle crust is underlain by a lower crust with a variable velocity structure, and the total crustal thickness varies from 29–32 km. Detailed traveltime investigations have also revealed a reflector in the upper mantle at depths between 39 and 44 km. The seismic model shows that there are no deep crustal variations beneath the Killarney-Mallow Fault Zone, indicating a thin-skinned mechanism of deformation.