Mechanisms of ridge inflation in slow spreading back-arc basins

Slow spreading ridges typically show deep axial valleys due to low magma supply from depth. However, inflated ridges exhibiting distinct geophysical and geochemical patterns are also documented along some segments of mid-ocean ridges and back-arc basins. The mechanisms governing the formation and evolution of such ridges remain poorly understood due to a lack of data on their along-axis internal structure. The inflated Marsili seamount spreading ridge, associated with the formation of the Tyrrhenian Sea back-arc, shows central volcanoes at the terminations of its main axis and exhibits inverse relationship between spreading rate (from 3 cm/yr to about 1.8 cm/yr) and the volume of erupted products (from 2.08 × 10 -3 km 3 yr⁻ 1 to 4.04 × 10 -3 km 3 yr⁻ 1 ) over the last 1.77 Ma. Potential field and geochemical data, combined with thermal modeling results, demonstrate that ridge inflation is driven by cyclic magma input-cooling processes, which may persist also after the decline and possible cessation of spreading. Along-axis thermal variations reflect changes in the degree of mantle melting yielding distinct constructive styles between the ridge edges and its central sector. The outer ridge margins cool more rapidly promoting the growth of vertically developed intrusive bodies, whereas slower cooling and higher degrees of mantle melting sustain continuous magma upwelling beneath the central sector favoring local inflation and fissural volcanism. The proposed model demonstrates that renewed, active magma upwelling can occur even after back-arc spreading has ceased, providing a comprehensive framework to explain ridge inflation and along-axis structural and morphological variations in slow-spreading back-arc systems.