SEGMENTATION AND MORPHOTECTONIC VARIATIONS ALONG A SLOW-SPREADING CENTER - THE MID-ATLANTIC RIDGE (24-DEGREES-00'N-30-DEGREES-40'N)

Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 2 4-degrees 00'N and 30-degrees 40'N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis T ransform, there are no transform offsets along this 800-km-long portio n of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10-100 km by non-transform discontinuities, usually locat ed at local depth maxima along the rift valley. At these discontinuiti es, the horizontal shear between offset ridge segments is not accommod ated by a narrow, sustained transform-zone. Non-transform discontinuit ies along the MAR can be classified according to their morphology, whi ch is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-tran sform discontinuities are associated with off-axis basins which integr ate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal low s which flank the neovolcanic zone at the ends of each segment. The of f-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuitie s along the spreading center. The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segme nt morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis ba thymetric high between the Kane and Atlantis Transforms. These segment s are associated with circular mantle Bouguer anomalies which are cons istent with focused mantle upwelling beneath the segment mid-points. W ide, U-shaped segments in cross-section are preferentially found in th e deep part of the long-wavelength, along-axis depth profile. These se gments do not appear to be associated with circular mantle Bouguer ano malies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distributi on of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range o f observed, first-order variations in segment morphology reflects diff erences in the flow pattern, volume and temporal continuity of magmati c upwelling at the segment scale. However, despite large first-order d ifferences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represent s differences in the relative importance of volcanism and tectonism al ong strike away from a zone of enhanced magma upwelling within each se gment. The contribution of volcanism to the morphology will be more im portant near the shallowest portion of the rift valley within each seg ment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget a long the strike of a segment will result in along-axis variations in c rustal structure. Segment mid-points may coincide with regions of high est melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesi s is consistent with available seismic and gravity data. The rift vall ey of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away fr om the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ra mp. These observations suggest that half-grabens, rather than full-gra bens, are the fundamental building blocks of the rift valley. They als o indicate that the pattern of faulting varies along strike at the seg ment scale, and may be a consequence of the three-dimensional, thermo- mechanical structure of segments associated with enhanced mantle upwel ling beneath their mid-points.