Role of mush complex viscosity in modulating axial topography in mid-oceanic ridges

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 ($\mu_{eff}$) of MC beneath MORs is recognized as the crucial factor in modulating their axial high versus flat topography. Based on a two-step viscosity calculation (suspension and solid-melt mixture rheology), we provide a theoretical estimate of $\mu_{eff}$ 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 $\mu_{eff}$ on the axial topography. Using an enthalpy-porosity-based fluid-formulation of uppermost mantle the model implements a one-way fluid-structure interaction (FSI) that transmits viscous forces of the MC region to the overlying upper crust. The limiting non-rifted topographic elevations (-0.06 km to 1.27 km) of model MORs are found to occur in the viscosity range: $\mu_{eff}$ = $10^{12}$ to $10^{14}$ Pa s. The higher-end ($10^{13}$ to $10^{14}$) Pa s of this spectrum produce axial highs, which are replaced by flat or slightly negative topography as $\mu_{eff} \leq 5\times 10^{12}$ Pa s. We discuss a number of major natural MORs to validate the model findings.