How to make detachment faults at melt-starved mid-ocean ridges that have a very thick axial lithosphere?

Slow-spreading mid-ocean ridges develop large offset normal faults, or detachments, that exhume mantle-derived peridotites to the seafloor. Successive detachments that flip polarity (flip-flop detachments) have been documented in corridors of nearly amagmatic spreading at the eastern Southwest Indian Ridge (SWIR; Sauter et al., Nature Geosci., 2013). The depth of earthquakes' hypocenter (Schlindwein and Schmid, Nature, 2016) supports the existence of a brittle lid thicker than 20 km in these corridors. A very thick axial lithosphere is also consistent with our petrological observations on dredged peridotites from the area (plagioclase-free, spinel-bearing dynamically recrystallized assemblages in shear zones and deviatoric stresses > 200 MPa from geopiezometers for dynamically recrystallized olivine).

Thermo-mechanical models using a strain dependent cohesion weakening mechanism do not produce detachments for a brittle lithosphere thicker than 20 km (Lavier and Buck, JGR, 2002). Here, we explore two other weakening mechanisms observed in situ, and for which we have temperature constraints from rock sampling: serpentinization (at T < 350°C) and grain size reduction (at T 800-1000°C). Serpentinization and more generally hydrous alteration minerals have been shown to localize strain at detachment-dominated slow-spreading ridges locations. Grain size reduction due to dynamic recrystallization of the peridotites under high stress is visible in several samples from the eastern SWIR. Our models explore the effect of implementing these weakening mechanisms separately or together, and of varying the threshold conditions for their activation. We show that serpentinization alone does not produce detachments, while grain size reduction alone produces detachments with unrealistically high relief. Combining the two, we obtain several modes of axial deformation: horst mode, successive faults mode, long detachments and flip-flop detachments. The transition between these modes appears to depend on the amount and location of serpentine that has been produced in the previous stages of faulting. The flip-flop mode, once initiated, can develop in steady-state, with fault topography and offsets that are consistent with geological observations in the eastern SWIR nearly amagmatic study area.