Seismic studies have established that large-offset transforms along the slow spreading Mid-Atlantic Ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extremely heterogeneous in both thickness and internal structure. It is frequently quite thin (<1--2 km thick) and is characterized by low compresssional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilometers from these fractures zones. Anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the Mid-Atlantic Ridge. The geological nature of the seismically anomalous crust found within Atlantic fracture zones, and show this crust forms, are still controversial. One interpretation consistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places.

Variations in apparent seismic crust thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three-dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large-scale detachment faults that form preferentially in the cold, brittle lithosphere near the ends of segments.