The 3-D geometry of detachment faulting at mid-ocean ridges

Seismic images of Cretaceous slow spreading crust from the eastern Central Atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting. The seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present Mid-Atlantic Ridge. The detachments are consistent with the structure and microearthquakes of the Mid-Atlantic Ridge at 26 degrees N, with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments. Other seismically imaged detachments have similar dimensions and geometry, but are covered by a layer of small fault blocks. The detachment types differ in whether or not the fault locks up in the subsurface, probably controlled by the fault strength, the elastic thickness, and whether the exhumed footwall is partly covered by basalts. Toward the segment middle, decreasing mantle serpentinization, decreasing elastic thickness, and thicker median valley basalts all increase the likelihood that the fault locks up, and a new fault propagates upwards from the still active root zone, transferring a slice of the hanging wall to the footwall, to be rafted with the footwall out of the median valley. As a result an oceanic detachment fault, exhuming the footwall at a segment end to form an oceanic core complex, may disappear laterally beneath rafted blocks; detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping.