Interplay between magmatic accretion, spreading asymmetry and detachment faulting at a segment end: Crustal structure south of the Ascension Fracture Zone

- Magmatism, detachment faulting and changing symmetry of crustal accretion at a ridge segment end.
- PmPs from an OCC argue for constant magma production during detachment faulting.
- Refraction seismic modelling and PmP events reveal very thin (4 km) oceanic crust at a segment end.

A wide-angle seismic section across the Mid-Atlantic Ridge just south of the Ascension transform system reveals laterally varying crustal thickness, and to the east a strongly distorted Moho that appears to result from slip along a large-offset normal fault, termed an oceanic detachment fault. Gravity modelling supports the inferred crustal structure. We investigate the interplay between magmatism, detachment faulting and the changing asymmetry of crustal accretion, and consider several possible scenarios. The one that appears most likely is remarkably simple: an episode of detachment faulting which accommodates all plate divergence and results in the westward migration of the ridge axis, is interspersed with dominantly magmatic and moderately asymmetric (most on the western side) spreading which moves the spreading axis back towards the east. Following the runaway weakening of a normal fault and its development into an oceanic detachment fault, magma both intrudes the footwall to the fault, producing a layer of gabbro (subsequently partially exhumed).