Javier Escartín

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) the crust is thicker, associated with circular or elongated gravity lows (bulls 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.

Focused volcanism and growth of a slow spreading segment (Mid-Atlantic Ridge, 35‡N)

Abstract Using off axis bathymetry, gravity and magnetic data, we studied the formation of a prominent seamount chain across segment OH1 (Mid-Atlantic Ridge, 35°N), and its relation to the past segmentation of the area. We also studied the size and shape of the seamounts to understand the processes leading to their formation. The chain is elongated in the spreading direction, and extends from the present day segment center to ∼6 Ma on both flanks. It coincides with a pronounced low in the residual mantle Bouguer gravity anomaly, suggesting thicker crust and thus more abundant magmatism than in surrounding areas. Magnetic anomalies are well defined over the seamount chain, consistent with formation on or near the axis. The seamounts within the chain are larger on average than those from other areas of the Mid-Atlantic Ridge, reflecting higher magma volumes and fluxes during eruptions. The distribution of seamounts suggests a focused magmatic source, located beneath the eastern side of the ridge axis, at a constant distance (∼45 km) from the Oceanographer transform fault. A V-shaped trend defines the southern end of OH1 and indicates that the segment propagated rapidly southwards, increasing in length from 50 to 90 km. The onset of propagation at ∼6 Ma coincided with the initiation of the volcanic chain, suggesting that magma supply at that time was focused at the end of the segment rather than at its center, as is typical for Mid-Atlantic Ridge segments. We propose that this unusual configuration is a consequence of the cold edge effect of the Oceanographer fracture zone. We also propose that enhanced and focused magmatism beneath the seamount chain may have caused the rapid southward propagation of OH1 over the past ∼6 Ma.