Ultramafic-Mafic Plutonic Rock Suites Exposed Along the Mid-Atlantic Ridge (10°N-30°N). Symmetrical-Asymmetrical Distribution and Implications for Seafloor Spreading Processes.

Abstract

Along specific portions of the axis of the Mid-Atlantic Ridge (MAR), the spreading of the lithosphere appears to occur without abundant magmatism. Horizontal extension of the ocean floor is then accommodated mostly by tectonic stretching. Such tectonically governed processes lead to the creation of an ocean basement composed of mantle-derived ultramafics and associated mafic plutonic rocks. In this paper, we present a review of the geological and tectonic settings of the most intensively surveyed areas where such mantle and deep crustal rocks have been recovered along the central Atlantic Ridge between 15°N and 35°30'N. The regions where detailed geological observations have been made are: 1. the intersection high north of Oceanographer fracture zone; 2. two intersection highs near small-offset axial discontinuities north of Hayes fracture zone; 3. the region south of the Kane fracture zone near 23°N (MARK area); 4. the northern and southern intersections of the 15°20'N fracture zone; and 5. off-axis oblique depressions representing the trace of migrating segment discontinuities in the MARK area. Mantle-derived serpentinites and associated gabbros are commonly exposed along only one side of the axial valley wall, that is, asymmetrically with respect to the spreading axis, while basaltic formations are found on the opposite wall. However, we emphasize that such an asymmetrical distribution in the topography, structure, and geology of the axial valley is not observed all along the studied areas of the MAR, for example in the 15°N region where serpentinized peridotites are documented across wide areas on both sides of the axis. Two major points are discussed. (1) The processes leading to the unroofing of mantle ultramafics and associated rocks are not necessarily linked to a stage of amagmatic extension of a previously constructed thick crust; some appear to occur in areas of permanent low magma budget, at segment ends or within wider regions of starved magma production. Such regions may retain this character over millions of years as shown by the geology of the off-axis traces of axial discontinuities. (2) Since geological settings with a symmetrical distribution of deep crustal and mantle rocks on the valley walls are found, a model of asymmetrical extension and denudation along a single major detachment fault cannot be applied all along the ridge axis. This implies that more attention must be paid to the degree of across-axis asymmetry or symmetry of tectonic processes responsible for the uplift and exposure of mantle-derived and lower crustal rocks.