Fifteen-Twenty fracture zone

[4][5] The FTFZ is roughly parallel to the North and South America—Africa spreading direction and has a broad axial valley produced over the last ten million years by the northward-migrating triple junction.

[8] North and south of the FTFZ the ocean floor is relatively smooth with long abyssal hills, probably detachment faults, aligned near-parallel to the ridge axis.

In contrast, close to the FTFZ the terrain is more rugged and adorned with short, oblique fault scarps.

Within the rugged terrain serpentinized peridotite and gabbro are capped with a thin layer of extrusive basalt.

Peridotites collected from south of the FTFZ have an uncommon composition ascribed to a H2O-rich or hot mantle source.

[8] These structures and ultramafic rocks outcropping on either side of the MAR (in contrast to other parts of the ridge) indicate considerably reduced magma supply near the FTFZ.

[10] The North American–South American–African triple junction is associated with the initial opening of the Atlantic Ocean and has had a complex tectonic history.

Location of the Fifteen-Twenty fracture zone and the triple junction between the North American, South American, and African (Nubian) plates. The Azores–Gibraltar Transform Fault is in the top right corner.