Tectonics control alteration-induced rheological heterogeneities in magma-poor ultraslow-spread oceanic lithospheres

At ultraslow, magma-poor spreading ridges, plate divergence is controlled by tectonics, leading to the formation of detachment faults. These faults cut through thick, brittle lithosphere (up to 15 km) and accommodate tens of kilometers of displacement, exposing heterogeneous, altered rocks. Among the alteration reactions, serpentinization has garnered significant attention for its role in sustaining chemosynthetic microbial life and influencing the spatial distribution of earthquakes within the lithosphere. Although the influence of serpentinization on seismicity is largely recognized in ultraslow-spread lithospheres, the nature and extent of alteration remain poorly constrained.

To address this, we use a 2D visco-elasto-plastic model with thermodynamic calculations to simulate lithospheric alteration during ultraslow seafloor spreading under a low magma budget. By coupling water availability and lithospheric hydration progress with active deformation, we reveal: (i) a tectonically controlled vertical extent of alteration along detachment faults; (ii) the preservation of amphibole-facies in exhumed serpentinized footwalls, forming kilometer-scale asperity-like features; and (iii) significant lithospheric-scale rheological heterogeneities resulting from tectonically induced spatial variations in alteration mineral assemblage equilibria across the lithosphere. The largest rheological changes occur along the deep hydration front near the brittle-ductile transition zone, where the alteration of exhumed fresh mantle begins to form high-temperature amphibole-bearing assemblages.

By comparing our model results with seismic data from two magma-poor segments—the easternmost Southwest Indian Ridge and the Knipovich Ridge—we observe that sparsely seismically active regions correlate with highly serpentinized domains in the shallow lithosphere, while deeper seismically active zones correspond to areas with low alteration degrees and the presence of amphibole, talc, and chlorite in amphibole-bearing assemblages. These findings support a conceptual model suggesting that tectonics controls the formation of alteration-induced rheological heterogeneities, which play a key role in controlling earthquake depth distribution at mid-ocean ridges and associated transform faults, and also have implications for seismogenesis in subduction zones.