The geological record of oceanic crustal accretion and tectonism at slow-spreading ridges

Abstract

The objective of this Thesis was to interpret the structural development of slowspreading ridge segments by: 1) delineating the nature, magnitude, and relative importance of primary tectonic and volcanic processes that control crustal morphology, 2) investigating the spatial and temporal variability of these processes, and 3) examining how rheological variations in the lithosphere control its structural configuration. To that end, this Thesis provides detailed documentation of faults and volcanoes (seamounts) at the Mid-Atlantic Ridge from 25°25'N to 27°10'N and extending from zero-age crust at the ridge axis to -29 Ma crust on the ridge flank. This information was used to analyze the evolution of ocean crust from initial formation in the rift valley to degradation by aging processes on the ridge flank. Accumulation of sediments affects the seafloor morphological expression of ocean crustal structure, and sediment thicknesses were also mapped to facilitate study of the morphological record of crustal accretion and tectonism. In addition, deformation conditions in the lithosphere were analyzed by study of microstructure and geothermometry of abyssal peridotite mylonites recovered from fault zones at slow-spreading ridges.