Bathymetry, gravity and magnetic data study between 14° and 16°N, Mid-Atlantic ridge. Evidence for large-scale mechanical extension

The completed research targets interpretation of the bathymetric, gravity and magnetic data collected during the Shinkai 6500/Yokosuka cruise during the summer of 1998. The data cover a large offset fracture zone at 15° 20' on the Mid-Atlantic Ridge as well as the spreading segments north and south of the fracture zone.; The extensional models along the Mid-Atlantic ridge (e.g., Karson, 1998) propose two modes of spreading---with magmatic infilling and by mechanical extension. This research objective is to determine evidences for these two extensional modes in the area by processing and interpretation of bathymetry, gravity, magnetic and petrologic data The change in mode of extension is presumably dependent on the availability of basaltic magma in sufficient quantities to allow throughgoing, magma-filled fractures to penetrate the lithosphere. As long as a sufficient rate of magma supply is maintained, initiation of seafloor spreading is likely to terminate an extensive mechanical extension and formation of low-angle detachment faults. However, reduction in the magma supply rate may result in the mode of extension reverting to faulting mechanisms. If this is the case, detachments and strongly rotated tilted blocks may occur in the parts of the oceanic lithosphere. Tectonic core complexes (also known as megamullions) along slow spreading centers have now been recognized in many places where mechanical extension dominates. The study area contains a number of large core complexes, the products of low-angle detachment faulting. Geomorphological, geological, and geophysical characteristics of them are presented.; The dredging and diving results have shown that mantle peridotites are exposed along both flanks of the Mid-Atlantic ridge over a wide area. On the other hand, the geophysical surveys along areas of the ridge that have abundant seafloor outcrops of residual ultramafics yield significant crustal thickness. As serpentinization of peridotites decreases the seismic velocity and density, partial serpentinization can produce rocks with seismic velocities and densities identical to upper crustal rocks. The forward gravity modeling constrained by petrological information and velocity model along the ridge has been conducted to address this issue. Additional forward gravity models have been calculated in the area. The regional crustal thickness estimates have been derived from the gravity data. They show regional thinning of the crust in the areas of the megamullion formations. Crustal age estimates are done based on magnetization linear patterns derived from magnetic anomaly.