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

TL;DR

This study investigates how the effective viscosity of mush complexes beneath mid-ocean ridges influences their axial topography, combining theoretical estimates with numerical modeling to explain observed ridge features.

Contribution

It introduces a new theoretical and numerical framework linking mush complex viscosity to axial topography in mid-ocean ridges, validated with natural examples.

Findings

Viscosity range of 10^{12} to 10^{14} Pa s controls topography.

Higher viscosity correlates with axial highs.

Lower viscosity results in flat or negative topography.

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 ($\mu_{eff}$) 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.