Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty Fracture Zone (Mid-Atlantic Ridge, 14°–16.5°N)

Abstract

We analyze geological and geophysical data near the Fifteen-Twenty Fracture Zone to constrain the processes of magmatic accretion and tectonic extension in the area, characterized by extensive outcrops of ultramafic rocks at the seafloor. Close to the fracture zone (<60 km) thin crust is associated with irregular, oblique faults and extensive outcrops of serpentinized peridotite, suggesting a highly heterogeneous lithosphere both compositionally and mechanically. Immediately under the transform valley the crust is thick, probably as a result of enhanced fracturing and serpentinization of the lithosphere along the transform fault. Away from the fracture zone (>60 km) the crust is thicker, associated with circular or elongated gravity lows (bull's eyes), axis-parallel faults and abyssal hills and no peridotite outcrops, consistent with a more homogeneous and magmatic crust. Simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates, and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry. We explain these discrepancies primarily (a) by the entrapment of melt in the lithosphere at deep levels, particularly near the transform, (b) by along-axis melt migration away from the transform, and (c) by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite. The along-axis variations in melt supply, lithospheric thickness, and alteration of the lithosphere can result in a change of rheological structure, and therefore in fault pattern as observed along the ridge axis. Identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor. These detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere. The origin and evolution of these structures is yet unconstrained, but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere, and that once formed they localize strain due to the presence of weak serpentinites along the fault planes.