Evolution of Overlapping Spreading Centers: A Sea MARC II Investigation.

摘要

The long term goal is to further the understanding of mid-ocean ridge processes and their relation to seafloor morphology. With recent advances in high resolution sonar swath mapping technology we have been able to detect discontinuities currently along the spread axis and to trace related features which have been rafted off onto the ridge flanks. We can investigate the temporal and spatial interactions between the processes of volcanism and tectonism which shape the seafloor. Study of these ridge segmentation processes should tell us a great deal about the timing and spacing of major melting events in the upper mantle and their relation to axial magma chamber. During a major Sea Marc II expedition in May-June 1987 we mapped all the overlapping spreading centers (OSC's) north of the equator between 8 deg. and 18 deg. N and all their related off-axis scars. The main results include: We find that OSC's leave abandoned ridge tips and fossil overlap basins as distinctive scars on the flanks of the EPR, disrupting a significant amount (more than 20%) of the normal seafloor fabric; Traces of the discordant zones indicate that OSC's have migrated both to the north and to the south, not necessarily following pole of opening or hot spot traces; Large OSC's can persist for 0.5 to 2.0 my whereas small offset OSC's are transient, persisting less than 10,000 years. The character of the axial summit graben can be used to predict the presence and size of the underlying crustal magma chamber; and Zones of highly magnetized rocks are found in the proximity of several OSC ridge tips, due to fractionated basalts rich in Fe-Ti content. (EDC)