When seawater, that penetrates the lithosphere through faults and fractures at mid-ocean ridges, gets in contact with mantle rocks serpentine may form. Serpentine bearing rocks are considerably weaker than their source rock thereby causing a drastic change in the rheological strength of the affected lithosphere. Serpentinization is limited by temperature and the availability of active fluid bearing faults. Its maximum depth was previously considered not to exceed 4 km beneath the sea floor. Yield strength envelopes (YSE) represent vertical profiles that predict the maximum stress supported by the lithosphere as a function of depth. We calculated YSEs for the axial lithosphere at an amagmatic Southwest Indian Ridge segment for different geotherms, serpentinization depths and mineralogical compositions in the ductile regime. Assuming the earthquake distribution is somehow linked to the rheological strength profile we then interpreted those YSEs that best correlate with the depth frequency distribution of local earthquakes. By doing so we could constrain the thermals structure, the mineralogical compositions and the deformation mode in the lithosphere. The YSEs show a thick mechanical lithosphere (30–35 km) at the ridge axis that is weakened in its uppermost 8-13 km due to serpentinization. Incorporating the axial morphology we propose a distinct mode of deformation that may also be applicable to other magma starved ultraslow spreading mid ocean ridge segments. Here, deformation and lithospheric accretion are essentially governed by deep reaching boundary faults that are well lubricated and hence aseismic due to extensive, deep-reaching serpentinization.