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

High-resolution bathymetry and geophysical data collected along the sl ow-spreading axis and flanks of the Mid-Atlantic Ridge between 20 degr ees N and 24 degrees N reveal the characteristics and history of diffe rent wavelengths of segmentation during the last 10 m.y. The bathymetr ic data exhibit a morphotectonic pattern dominated by ridge-normal and oblique bathymetric lows that partition the ridge flanks into rhomb-s haped areas of relatively high elevation. At least four different type s of oblique bathymetric lows have been identified which represent the off-axis traces of axial discontinuities and suggest a complex and on going evolution of ridge-axis segmentation. One group of oblique struc tures is represented by two deep ridge-normal depressions with typical fracture zone characteristics that are connected to the present activ e transform by oblique depressions near the ridge axis. These oblique traces correspond to the southward shift of axial discontinuities asso ciated with the propagation of the ridge axis, while maintaining a con stant offset of the latter. Two other types of oblique structures corr espond to elongate bathymetric lows and oblique alignments of ridge-pa rallel 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-ax is. These oblique depressions are characterized by positive filtered m antle Bouguer anomalies, high magnetizations, complex magnetic anomaly patterns, and possible exposure of mantle lithologies. The ridge segm ents defined by these oblique depressions lengthen or shorten along th e ridge axis, with propagation rates varying from 0 to 25 km m.y.(-1). The last and smallest discontinuities observed in this area correspon d to small ridge-axis offsets and off-axis traces identified by alignm ents of the terminations of abutting abyssal hills. The ridge-flank mo rphotectonic patterns produced by the evolution of these elementary se gments of accretion may represent temporally variable upwelling volume s of melt. The centres of the rhomb-shaped areas correspond to maximum crust production and thin lithosphere, and the discontinuities corres pond 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 accreti on over periods of several million years.