Oblique mantle dynamics, fluid flow and serpentinization at oceanic transform faults

Oceanic transform faults offset mid-ocean ridges by up to several hundred kilometers. In the theory of plate tectonics, they are defined as conservative, pure strike-slip boundaries. Consequently, the oceanic crust produced at and near the ridge-transform intersection (RTI) is assumed to be identical to that produced elsewhere on the ridge and formed by similar processes. Transform faults and fracture zones have further been hypothesized to be sites of enhanced fluid flow, mantle serpentinization and hence biogeochemical exchange. We have integrated geodynamic models with geophysical datasets to assess mantle and melt movement as well as mantle serpentinization at oceanic transform faults. Our three-dimensional simulations show oblique mantle flow patterns emerging in simulations assuming a non-linear stress-dependent rheology. Instead of the classic symmetric corner flow-type mantle flow with a sharp strike-slip boundary along the transform fault, a strong asymmetry in mantle upwelling develops beneath the ridge-transform intersections (RTI). This flow is directed from beneath the inside towards the outside corner of the RTI and results in the transfer of mantle material to the opposite side of the ridge. A likely implication of this is that melt migration will also be asymmetric with melt focusing towards the outside corner, possibly explaining why transform faults are so deep and so rarely show clear signs of intra-transform magmatism. In addition, we have explored the interrelations between deformation, fluid flow, and mantle serpentinization and how they depend on fault geometry and slip rate. These insights, based on observations and theoretical considerations, suggest that crust formed near the RTI will have a complex geological history and that crustal accretion is likely to be asymmetric. Our findings further point to transform faults as sights as potential sites of intense fluid-rock interaction and mantle serpentinization.