The Porcupine basin is a deep sedimentary basin on the continental margin west of Ireland. All available geologic and geophysical data have been used to interpret the history of the basin. It contains up to 8 km of Cretaceous and younger sediments, underlain by an unknown thickness of Jurassic and Triassic rock. Gravity and magnetic anomaly models have been used to investigate the nature of the crust beneath the Porcupine basin. These models suggest a thinned continental crust rather than the oceanic crust previously proposed by some authors, although the nature of a narrow zone of crust (< 10 km thick) in the central southern basin cannot be determined conclusively. A high-amplitude elongate gravity high associated with a negative magnetic anomaly in the axis of the n rthern Porcupine Seabight correlates with a volcanic ridge, probably of Early Cretaceous age, seen on seismic reflection profiles.

Regional geologic and geophysical studies show that the Porcupine basin was originally structured in the early Mesozoic as part of a complex northeast to north-northeast-trending intracontinental rift system extending from the Grand Banks of Newfoundland to the present northwest Europe continental shelf. The basin can be divided into a sequence of segments bounded by north-northeast-trending normal faults and northwest-trending strike-slip faults. The degree of extension within the basin decreases northward across each strike-slip fault, accounting for the overall northward narrowing of the basin. The early Mesozoic rift fabric of the Porcupine basin was reactivated during the Early Cretaceous when the basin became caught up in the northwest-trending rift that later evolved into the N rth Atlantic. This second phase of extension led to subsidence and the accumulation of extreme thicknesses of Cretaceous and younger sediments.