Control of mush complex viscosity on mid-ocean ridge topography: A fluid–structure model analysis

Abstract

This article exploits the interaction dynamics of the elastic oceanic crust with the underlying mush complexes (MC) to constrain the axial topography of mid-ocean ridges (MORs). The effective viscosity (μeff) of MC beneath MORs is recognized as the crucial factor in modulating their axial high vs flat topography. Based on a two-step viscosity calculation (suspension and solid-melt mixture rheology), we provide a theoretical estimate of μeff as a function of melt suspension characteristics (crystal content, polymodality, polydispersity, and strain rate) and its volume fraction in the MC region. We then develop a numerical model to show the control of μeff on the axial topography. Using an enthalpy-porosity-based fluid formulation of uppermost mantle, the model implements a one-way fluid–structure interaction that transmits viscous forces of the MC region to the overlying upper crust. The limiting non-rifted topographic elevations (−0.06–1.27 km) of model MORs are found to occur in the viscosity range of μeff = 1012–1014 Pa s. The higher end (1013–1014) Pa s of this spectrum produces axial highs, which are replaced by flat or slightly negative topography as μeff≤5×1012 Pa s. We discuss a number of major natural MORs to validate the model findings.