New wide-angle seismic data were gathered along a 230 km long profile that runs east-west across a deep structural feature in the Porcupine Basin, offshore Ireland, known as the Porcupine Arch. Ocean bottom seismometers were deployed at 3-4 km intervals and seismic sources fired every 120 m along it. Prominent primary and secondary arrivals indicate that the continental crust is extremely thin (locally less than 2 km) across the basin centre. The sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers. The two uppermost layers are interpreted as mostly a post-rift succession of Cretaceous and Cenozoic strata. The lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly Jurassic synrift sediments. A strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced. Crustal thinning is far greater than in the adjacent Rockall Basin and local exhumation of continental mantle lithosphere may have occurred in parts of the Porcupine Basin. Low P n velocities beneath the Porcupine Arch are compatible with larger amounts of mantle serpentinization than in the Rockall Basin.

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Summary 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.