CHARACTERISTICS AND EVOLUTION OF THE SEGMENTATION OF THE MID-ATLANTIC RIDGE BETWEEN 20-DEGREES-N AND 24-DEGREES-N DURING THE LAST 10-MILLION YEARS

High-resolution bathymetry and geophysical data collected along the slow-spreading axis and flanks of the Mid-Atlantic Ridge between 20 degrees N and 24 degrees N reveal the characteristics and history of different wavelengths of segmentation during the last 10 m.y. The bathymetric data exhibit a morphotectonic pattern dominated by ridge-normal and oblique bathymetric lows that partition the ridge flanks into rhomb-shaped areas of relatively high elevation. At least four different types of oblique bathymetric lows have been identified which represent the off-axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge-axis segmentation. One group of oblique structures is represented by two deep ridge-normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis. These oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis, while maintaining a constant offset of the latter. Two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge-parallel bathymetric lows symmetric about the ridge axis. Both types of oblique structures frequently change their orientation (from normal to subparallel to the ridge axis) and appear to merge and diverge off-axis. These oblique depressions are characterized by positive filtered mantle Bouguer anomalies, high magnetizations, complex magnetic anomaly patterns, and possible exposure of mantle lithologies. The ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis, with propagation rates varying from 0 to 25 km m.y.(-1). The last and smallest discontinuities observed in this area correspond to small ridge-axis offsets and off-axis traces identified by alignments of the terminations of abutting abyssal hills. The ridge-flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt. The centres of the rhomb-shaped areas correspond to maximum crust production and thin lithosphere, and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites. We propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years.