The Tow Valley Fault Zone in north-east Ireland consists of number of ENE–WSW trending major fault segments and shorter N–S ones with different dips, downthrows and geophysical characteristics. The fault zone is associated with high magnetic gradients and a marked north-westerly decrease in gravity. Modelling of gravity data suggests that near Garvagh (south-west) and Ballycastle (north-east) the major fault plane dips to the north-west at about 20°–25° whereas the central section dips at 56° to the north-west. Thick sediments (c. 2.4km), mainly Permo- Triassic in age, occur beneath the Tertiary basalts north-west of the fault zone. Ground-based magnetic profiles across the fault zone yield a range of signatures. At some localities the fault is characterised by a 1km-wide zone of fracturing in the Tertiary basalts. Seismic data indicate that the fault zone continues offshore to the north-east of Rathlin Island.