Mid-Atlantic Ridge between 22° and 23° north latitude and the tectonics of mid-ocean rises

Abstract

A detailed geological and geophysical study of the crest and upper western flank of the mid-Atlantic ridge between 22° and 23°N and 44°30′ to 46°20′W reveals that: (1) Several markedly different physiographic zones parallel the ridge axis, but only the crest shows strong topographic linearity. (2) Metamorphosed basalts exposed on the flanks of the median valley indicate regional metamorphism of the lower part of a gradually thickening crust, followed by uplift and tectonic displacement of the overburden. (3) The distribution of sediments and sediment ages suggest that the relief of the upper flank is late Miocene and the crestal relief is Quaternary. (4) The small free-air gravity anomalies over the ridge indicate that the ridge is nearly in isostatic equilibrium. Topography accounts for most of these anomalies. The slope of the regional Bouguer anomaly curve gradually increases toward the ridge crest, indicating a decreasing depth of compensation or an increasing slope of the boundary of the material providing the mass deficit or both toward the crest. (5) Magnetic profiles can be interpreted by the sea-floor spreading hypothesis, and they give a spreading rate of 1.4 cm/year for the crestal part of the ridge. The mid-Atlantic ridge and some other rises are characterized by high relief with a well developed median valley, a thick crust, exposures of greenstones and peridotites, and a slow spreading rate. Others, most notably the East Pacific rise, have a fairly smooth relief and a thin crust, lack a median valley and greenstone and peridotite exposures, and spread much more rapidly. The large topographic differences probably indicate behavioral differences of only the shallow levels of the crust and upper mantle. The thickness of the material that deforms by brittle fracture may determine the behavior of the surficial rocks and hence the topography. Material that deforms by brittle fracture may be thin at sites of fast spreading because higher rock temperatures might occur at shallower depths. This may account for the correlation of rifting and greater topography where spreading rates are less than 2.0 to 2.5 cm/yr and the occurrence of less extensive surficial structural rearrangement where spreading rates exceed these values.