Morphology and tectonics of the southern mid-Atlantic ridge: Implications for accretion of oceanic lithosphere

In the following investigations we have used high-resolution Sea Beam bathymetry and surface-ship magnetic data to study the morphology, structure and plate boundary evolution of two portions (25$\sp\circ$-27$\sp\circ$30$\sp\prime$S and 31$\sp\circ$-38$\sp\circ$S) of the southern Mid-Atlantic Ridge (MAR) to determine how the processes of plate accretion interact in time and space along this medium (35 mm/yr full rate) spreading ridge system. Continuous along-axis Sea Beam coverage of the southern MAR in the two survey areas shows that the ridge axis is segmented by both transform and non-transform discontinuities. The variable morphotectonic geometries associated with non-transform discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike-slip tectonism and that the geometries are unstable in time. Analysis of the bathymetric and magnetic data collected over three non-transform discontinuities and their off-axis terrain out to 5-7 m.y. documents that these small-offset discontinuities can evolve from large-offset transform fault boundaries through prolonged differential asymmetric spreading. Between and within ridge segments that are opening at uniform rates, the morphology of the ridge axis is observed to vary from a well-defined rift valley to a gently rifted axial swell. Three-dimensional inversion and forward modeling of magnetic data suggest that anomalously low magnetization distributions associated with axial highs are attributed to demagnetization effects of crustal thermal anomalies. A series of distinct axial magnetization highs that persist for up to 2 m.y. off-axis are observed at several ridge-discontinuity intersections, and are probably the product of highly fractionated, strongly magnetized FeTi basalts. Taken together the bathymetric data and magnetic modeling document strong along-axis gradients in crustal and upper mantle properties and indicate that ridge accretionary processes along this portion of the mid-ocean ridge system are three-dimensional. The finite element technique is employed to examine the lithospheric stresses at non-transform offsets of the MAR. The results of this modeling suggest that the structure and evolution of these discontinuities is a function of the age offset and corresponding lithospheric thickness across the discontinuity, as well as the ratio of ridge normal extensional stresses to transform shear stresses in the vicinity of the offset