Second-order segmentation; the relationship between volcanism and tectonism at the MAR, 38°N¯35°40N

Deep-tow sidescan sonar data acquired along 240 km of the Mid-Atlantic Ridge (MAR) between 35°40ʹN and 38°N have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second-order segments and their bounding non-transform offsets (NTOs). The segments show a range of volcanic and tectonic types, but in general the northernmost segments (i.e. those with greater influence from the Azorean hotspot) are shallower and more volcanically robust than those to the south, with hydrothermal activity in segment centres (for example, Menez Gwen and Lucky Strike). Nonetheless, this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment. The NTOs are broad right-stepping discontinuities, locally up to 25 km wide, and accommodate offsets between 10 and 50 km. The discontinuities are mostly sediment-floored, and link the spreading segment tips with a range of structures. These include locally dense arrays of en echelon extensional normal faults, short lengths of linear strike¯slip fault strands occasionally cutting basement blocks apparently stranded within the offsets. Basement blocks within the offsets are cross-cut by complexes of intersecting faults, suggesting that deformation is distributed across the zone. The pervasive faulting taking place at the NTOs favours fluid circulation and associated hydrothermal activity, as at the Rainbow Site at 36°17ʹN.