Fifteen-Twenty Fracture Zone
The Fifteen-Twenty or 15°20' Fracture Zone (FTFZ), also known as the Cabo Verde Fracture Zone, is s fracture zone located on the Mid-Atlantic Ridge (MAR) in the central Atlantic Ocean between 14–16°N. It is the current location of the migrating triple junction marking the boundaries between the North American, South American, and Nubian plates. 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. Offsetting the MAR by some 175 km, the FTFZ is located on one of the slowest portions of the MAR where the full spreading rate is 25 km/Myr.
North and south of the FTFZ the axis of the MAR is near-perpendicular to the spreading direction and the spreading rate is 2.6 mm/yr. The axial valley south of the FTFZ is composed of short axial volcanic ridges separated by 8–18 km-long en echelon deeps, while north of the FTFZ the axial ridges are much longer and more linear.
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. Associated with the transition between these two types of terrains (at about 15°50'N and 14°30'N respectively) are V-shaped, south-propagating structures. These transitional structures disappear away from the ridge. Within the rugged terrain serpentinized peridotite and gabbro are capped with a thin layer of extrusive basalt. In the smooth areas the lithosphere is more magmatic in composition.
The FTFZ is flanked by two negative gravity anomalies associated with the accretion of igneous crust. The anomaly south of the FTFZ is twice as large as the northern one. There are also geochemical variations across the fracture zone. On the southern side basalts are enriched MORB (mid-ocean ridge basalt) but on the northern side basalts change from enriched to depleted away from the FTFZ. Peridotites collected from south of the FTFZ have an uncommon composition ascribed to a H2O-rich or hot mantle source.
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