RHEOLOGICAL EVOLUTION OF THE OCEAN CRUST - A MICROSTRUCTURAL VIEW

Rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal, mechanical, and experimental models. Modeling studies help to constrain an average crust and mantle rheology and the deep-seated processes that control regional-scale strength and stress field variations. The smoothing of fine-scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust. Examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling. Examples of structures and deformation histories in diabases from Deep Sea Drilling Project/Ocean Drilling Program site 504B, the Hayes and Atlantis fracture zones, (Mid-Atlantic Ridge), and the Troodos ophiolite, (Cyprus), are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years. A survey of brittle, quasi-plastic, and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust. Geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures. Melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies. A schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates. Although microstructural studies of ocean crust are still in their infancy, they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures.