RHEOLOGICAL EVOLUTION OF THE OCEAN CRUST - A MICROSTRUCTURAL VIEW

Rheological studies of the oceanic lithosphere have used direct observ ations of rocks and inferences based on thermal, mechanical, and exper imental models. Modeling studies help to constrain an average crust an d mantle rheology and the deep-seated processes that control regional- scale strength and stress field variations. The smoothing of fine-scal e variations in such models can obscure many of the geological process es that influence strain localization and deformation partitioning in the ocean crust. Examination of deformation mechanisms and histories i n ocean crust rocks provides a complementary approach to modeling. Exa mples of structures and deformation histories in diabases from Deep Se a Drilling Project/Ocean Drilling Program site 504B, the Hayes and Atl antis fracture zones, (Mid-Atlantic Ridge), and the Troodos ophiolite, (Cyprus), are presented in conjunction with a synthesis of microstruc tural studies of the ocean crust over the last 25 years. A survey of b rittle, quasi-plastic, and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural charac teristics and variable magmatic and hydrothermal histories on deformat ion mechanisms and strain localization in the ocean crust. Geological evidence indicates that hydrothermal fluids strongly influence the nat ure of deformation and that effective stresses may be low due to fluid overpressures. Melt distribution will strongly influence strain local ization at the base of the crust and synkinematic hydration during cry stal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies. A schematic distribution of failure me chanisms in the ocean crust is used to discuss the controls on variati ons in lateral and vertical strength profiles and their possible relat ion to spreading rates. Although microstructural studies of ocean crus t are still in their infancy, they provide valuable constraints for rh eological models and further insights to explain the distribution of s eismicity at spreading centers and acoustic signatures.