The Rheology and Morphology of Oceanic Lithosphere and Mid-Ocean Ridges

Abstract

The rheology of oceanic lithosphere is primarily a function of temperature, the abundance and distribution of lithologies and fluids, and their mechanical properties. Rheology controls the overall strength and mode of deformation. Seafloor morphology is the surface expression of this deformation, modified by additional processes such as volcanism. Rheological models are key to interpreting both naturally deformed rocks as direct indicators of deformation conditions and the resulting morphology. Simple thermo-mechanical models have proven useful to study ridge processes, but are limited by lack of knowledge of lithospheric architecture, composition, and rheology. The mechanical properties of some components (olivine, dolerite, olivine plus melt, serpentinite) are reasonably known, but must be extended to other important materials such as alteration products and include the role of fluids and compositional variations. While the overall composition of oceanic lithosphere is relatively well known, particularly for fast-spreading ridges, the distribution and abundance of melt and alteration products is not. Though sparse, these weak phases can strongly control the overall strength, mode and localization of deformation. Thermo-mechanical models successfully reproduce observed axial relief and general faulting patterns. They provide plausible mechanisms of lithospheric behavior, but cannot constrain actual deformation processes. In particular, they must assume rheology, thermal structure, and composition and distribution of materials, and are non-unique. The most accurate constraints on rheology and deformation processes will come from study of naturally deformed rocks. This will guide the choice of the models used to interpret morphology and infer the detailed thermal structure under ridges.