• We present a thermo-gravity modeling framework for a nearly-amagmatic ultraslow-spreading ridge at Southwest Indian Ridge 13–14°E. • Best-fit lithospheric thickness is 14–16 km with hydrothermal circulation confined to depths above 6–9 km. • Hydrothermal cooling contributes only 3–5 km to lithospheric thickening. • Spreading rate, rather than hydrothermal penetration, controls lithospheric thickness at nearly-amagmatic ultraslow-spreading ridges. The thermal dynamics of mid-ocean ridges (MORs) are driven by heating from asthenosphere upwelling and melt emplacement, and cooling via hydrothermal circulation. Nearly-amagmatic ultraslow-spreading ridges, like the Southwest Indian Ridge (SWIR) at 13–14°E, are expected to have the thickest and coldest axial lithosphere among global MORs,yet the extent to which hydrothermal circulation contributes to lithospheric thickening remains poorly constrained. Here we present a thermo-gravity modeling framework that integrates 2D hydrothermal convection simulations with gravity anomaly analysis, providing a quantitative constraint on lithospheric thickness in amagmatic settings. By systematically varying hydrothermal penetration depth (0–21 km) and permeability, we simulate a wide range of thermal structures, which are extended along spreading flowlines and tested against gravity anomalies. The best-fit lithospheric thickness beneath the SWIR 13–14°E is 14–16 km. It is based on the brittle–ductile transition (BDT) depth defined by the 650 °C isotherm, and consistent with global spreading-rate trends but much thinner than some seismic interpretations of >25 km. Within this framework, hydrothermal cooling accounts for no >5 km of lithospheric thickening, with circulation confined to depths above 6–9 km. These findings directly challenge the prevailing deep hydrothermal hypothesis: hydrothermal cooling is insufficient to generate an extremely thick lithosphere at a nearly-amagmatic ultraslow-spreading ridges. Instead, spreading rate emerges as the primary control. More broadly, our framework provides the first quantitative thermo-gravity constraint on hydrothermal cooling, reconciling seismicity, isostatic deviation, and gravity perspectives on lithosphere accretion, offering new insights into lithosphere accretion in magma-poor ridge environments.
Tan et al. (Tue,) studied this question.