Tectonics control seismic velocity anomalies in magma-poor ultraslow-spread oceanic lithospheres

At ultraslow-spreading mid-ocean ridges (MORs, spreading rate <20 mm/yr), limited magma supply often results in tectonic extension and the formation of oceanic detachment faults. These faults cut through thick brittle lithosphere (up to 15 km), accommodating tens of kilometers of displacement and exposing heterogeneous rocks altered by seawater-rock interactions. Among these reactions, serpentinization has drawn significant attention for its role in sustaining chemosynthetic microbial life and modulating geological carbon budgets. However, quantitatively determining the extent and distribution of serpentinization within the lithosphere remains challenging, as large-scale estimates rely primarily on seismic observations that struggle to differentiate between serpentinized mantle, gabbro, and fresh mantle at depth. Despite advances in seismic resolution, key uncertainties persist regarding how magmatic, tectonic, and alteration processes shape velocity anomalies in newly formed oceanic lithosphere. Here, we address lithospheric alteration during magma-poor seafloor spreading by coupling a geodynamic model with thermodynamic calculations of alteration reactions and seismic properties as a function of pressure-temperature and mineral assemblages. We focus on the well-documented magma-poor ridge at 64°30′E on the Southwest Indian Ridge, where recent seismic surveys have been conducted. Our model reproduces the “smooth-smooth” seafloor morphology shaped by alternating flip-flop detachments. By coupling water availability and lithosphere alteration with active deformation, we reveal: (i) vertically controlled alteration along detachments, including deep alteration beyond serpentine stability; and (ii) tectonically-induced lateral velocity anomalies caused by variations in alteration mineral assemblages in the detachment footwall. Comparing our thermodynamically-constrained velocity model with seismic observations from 64°30′E SWIR suggests that the imaged alteration boundary along detachment faults likely represents a peak in serpentinization, rather than the traditionally interpreted serpentinization front.