Can grain size reduction initiate transform faults? - Insights from a 3D numerical study


Abstract Oceanic transform faults formed at mid-ocean ridges are intrinsic features of modern plate tectonics. Nevertheless, numerical mantle convection models typically struggle to reproduce the strike-slip movement observed along transform faults on Earth. Instead, mantle convection models tend to produce mostly convergent and divergent plate boundaries. Based on regional visco-(elasto)-plastic thermomechanical models it has been demonstrated that a strong strain-induced weakening of rocks has to be assumed in order to initiate and stabilise the characteristic orthogonal ridge-transform spreading patterns. However, the physical origin of such intense rheological weakening remains unclear. Considering that in nature oceanic transform faults show a strongly reduced grain size, a potentially strong influence of grain size-reduction processes on the rheological strength of these structures can be assumed. Employing 3D thermomechanical visco-plastic models we study the effect of grain size reduction on oceanic transform fault initiation. Our results show that ductile weakening induced by grain size reduction indeed results in sufficient localisation to initiate a transform fault. Without any additional weakening mechanisms, transform faults in our models remain stable up to 2 Myr. We identify parameters that affect stability and longevity of the transform fault during the initiation phase, such as the grain damage formulation and grain growth prefactor. In our models, transform faults initiate in the brittle crust and propagate downwards, thus indicating a top-down initiated localisation process. The observed grain size, rheology and strain rate inside the shear zone of our models agree well with observations in nature; however the longevity of natural examples cannot be reached.