Mid-Atlantic Ridge crest topography between 12° and 15°N

Abstract

A clay model of the Mid-Atlantic Ridge between 12° and 15°N, based on a survey with 8-km line spacing, reveals the irregular character of the medium-scale ridge topography. The median valley is far from constant in cross-section, ranging from an asymmetric V-shaped valley to shallower and more symmetric forms. Strict linearity and parallelism are not obeyed. This configuration is explained by assuming that the function of the inner valley as the actual spreading center is taken over every 500,000 years or so by a newly developing near-parallel rent at several kilometers distance from the existing spreading center. In addition the inner valley is not continuous. This gives rise to en echelon structures. The ragged new ocean floor is subsequently transported uphill over the walls of the median valley and down again. Besides two orthogonal fracture zones of the “classical” type, a twist is found in the median valley, that offsets the axis at 13°45′N over 16 km. The fossil expression of this oblique fracture zone (or leaky transform; cf. the Kurchatov fracture zone) can be followed over 100 km on either side of the ridge crest and to the limits of the coverage as a zigzagging zone of very irregular topography. This type of fracture zone occurs if the offset is so small that the asthenospheric conduit remains uninterrupted, the critical offset presumably being approximately 20 km. Orthogonal spreading prevails in areas where orthogonal and oblique fracture zones occur simultaneously. It is suggested that this conclusion also applies to the FAMOUS area, where the present spreading direction then is 107° as delineated by the Oceanographer fracture zone. Fracture zones A, B and C would be of the oblique type.