Rheology of mid-ocean ridge flip-flop detachment fault systems : numerical models and field observations

Flip-flop detachment fault systems characterize magma-starved regions of ultraslow mid-ocean ridges (MOR). They involve the succession of large-offset normal faults that face alternatively to one then to the other diverging plate,accommodate most of the plate divergence and consistently expose mantle-derived serpentinized peridotites on the seafloor. Currently the best documented MOR flip-flop detachment fault system is located in the 64°E region of the Southwest Indian Ridge (SWIR). Here, we report on two recent research projects focused on this region of the SWIR.

One project uses thermo-mechanical models to investigate which overall, plate boundary-scale, strength contrasts between the fault zones and the surrounding lithosphere favor the flip-flop faulting mode. It highlights how relatively modest rheological contrasts (equivalent to a 0.1-0.2 reduction in frictional strength for a cohesion loss of 20-25 MPa) between intact and deformed lithosphere enables large-offset flip-flop faulting in the thick lithosphere of magma-starved and ultraslow MOR regions. To better understand the flip-flop mode, this modelling project also develops an energy minimization analysis of a configuration with two antithetic faults, one older, and fully weakened, and the other new and not yet fully weakened, but cutting through the thinned footwall of the first fault. It shows that the rate of fault weakening in this new fault is a key parameter to determine whether or not it takes over as the new detachment.

The other project is based on studying actual rock samples and submersible dive videos from the exposed fault zone of the presently active SWIR 64°E axial detachment. It shows that deformation in the upper regions of the fault (at temperatures consistent with serpentine stability) is primarily brittle but that the most highly strained horizons are serpentinite gouges that exhibit syn-tectonic chrysotile fiber growth and dissolution-precipitation textures, indicating fluid-assisted semi-brittle deformation. While these gouges probably have extremely low frictional strength, it is their distribution at outcrop to fault zone scales, their thickness, and interconnectedness, along with the availability of hydrous fluid, that likely control the overall strength of these upper, serpentinized, regions of the fault zone. Further, several of these characteristics are likely influenced by prior distributed brittle and semi-brittle deformation in the deeper, hotter and non-serpentinized regions of the fault.

The study of natural samples therefore indicates that the strength of the axial lithosphere in the nearly amagmatic 64°E SWIR region is controlled by complex interactions between brittle failure, ductile deformation, fluid percolation and hydrous mineralogical transformations in and around fault zones and across a range of depths and temperatures. Numerical models suggest that, overall, these processes result in a moderate integrated rheological contrast between intact rocks and strain weakened fault zones. Yet it is likely that they also cause spatial and temporal variations of fault weakening rates, with consequences on whether and when new antithetic faults successfully take over.