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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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Evidence from wide-angle seismic data in the Irish sector of the Rockall Trough suggests that the basin is underlain by thinned continental crust which has undergone differential stretching. The upper crust has been thinned by a stretching factor of 8-10 while the middle and lower crust (and probably also the lithospheric mantle) was stretched by a factor of 2-3. The latter figure is suggested as being representative of the overall lithospheric stretching. Crustal modelling fails to demonstrate any significant effect of the Iceland plume on the development of the basin. The Rockall Trough contains up to 6 km of sedimentary strata. Well and seismic data from adjacent basins indicate that the succession in the basin is likely to be of Late Palaeozoic to Recent age. The pre-Cretaceous facies are suggested to be broadly similar to other basins of the Atlantic borderlands. Cretaceous and Tertiary strata show progressive clastic starvation as thermal subsidence outstripped sedimentation in the basin. Sandy facies are likely to be concentrated towards the faulted basin margins and the re-entrant regions where the basin margin changes orientation. Basin modelling, based on normal incidence and wide-angle seismic profiles from a number of areas in the Irish sector of the basin, demonstrates that the observed seismic geometries and subsidence patterns cannot be explained by a single rift episode in the Cretaceous. The best fit of possible models suggests that the basin developed in response to discrete rift episodes in the Triassic, Late Jurassic and Early Cretaceous.

A wide‐angle seismic profile across the western peninsulas of SW Ireland was performed. This region corresponds to the northernmost Variscan thrust and fold deformation. The dense set of 13 shots and 109 stations along the 120 km long profile provides a detailed velocity model of the crust.  The seismic velocity model, obtained by forward and inverse modelling, defines a five‐layer crust. A sedimentary layer, 5–8 km thick, is underlain by an upper‐crustal layer of variable thickness, with a base generally at a depth of 10–12 km. Two mid‐crustal layers are defined, and a lower‐crustal layer below 22 km. The Moho lies at a depth of 30–32 km. A low‐velocity zone, which coincides with a well‐defined gravity low, is observed in the central part of the region and is modelled as a Caledonian granite which intruded upper‐crustal basement. The granite may have acted as a buffer to northward‐directed Variscan thrusting. The Dingle–Dungarvan Line (DDL) marks a major change in sedimentary and crustal velocity and structure. It lies immediately to the north of the velocity and gravity low, and shows thickness and velocity differences in many of the underlying crustal layers and even in the Moho. This suggests a deep, pre‐Variscan control of the structural development of this area. The model is compatible with thin‐skinned tectonics, which terminated at the DDL and which incorporated thrusts involving the sedimentary and upper‐crustal layers.

Marineg ravity andmagnetic surveys aroundI reland, togetherw ithsatelliteg ravity dataf rom the deepero cean,a reused to investigatethe large-scalec rustals tructureofthe PorcupineB asin,offshorewest of Ireland.The structureofthe syn-rifttopost-riftsedimentary successions,derived from verticalincidence seismic reflection data, isused to isolatethe gravity andmagnetic responsesofthe crust andmantle.The crustalstructure derived from wide-angleseismic datainthe region isused to control the interpretation andmodelling.The results ofgravity modellinginthe southern part ofthe PorcupineBasinsuggest acrustalthickeninginthe centreofthe basin. Comparison withthe Rockall Basin,wherea xialcrustalt hickeningi ss eismicallydefined, suggests a similarp attern ofc rustalextension. The amount ofe xtension isl ess across the narrowern orthern sector ofthe PorcupineB asin,whereanorth-south-trendinga xialgravity high isp resent resultingf rom anomalous density variations int he lowercrust/upperm antle.Anewm odelfor the large-scalestructuraldevelopment ofthe PorcupineB asinisp resented thatexplains the along-axisv ariationsincrustals tructurei nt erms ofc hangesin the kinematicsofcrustalextension across NW-SEtransferzones. Thismodelinvolvestectonic unroofingofthe mantlelithosphereandserpentinization ofthe exhumed mantleperidotites.

Gravity gradients derived from an updated Bouguer anomaly map of Ireland reveal large-scale lineaments, many of which can be related to geological structures and tectonic development. First horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions, while the second derivative helps resolve near surface structure. The dominant NE-SW Caledonian-trending lineaments are interpreted as due to fault boundaries and folds in pre-Carboniferous basement and Caledonian fault reactivation during the Variscan Orogeny. Subsidiary NNE-SSWtrending lineaments are present, as well as weaker NW-SE-trending lineaments. The largest negative anomalies are produced by late Caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed. Both exposed and inferred granites appear to be spatially related to intersections of NE-SW-with NNE-SSW-and NW-SE-trending lineaments. The orientation of the dominant gravity trend changes from NE-SW towards ENE-WSW going westwards across Ireland and forms part of a more regional change in the 'Caledonian' gravity fabric recognised throughout the northeast Atlantic region. 'Variscan' E-W gravity trends in the south of Ireland overlay the 'Caledonian' trends. Relationships between gravity lineaments and anomalies support a 'thin-skinned' model for the Variscan cover.