Thermal structure and stress pattern of the oceanic transform fault: insights form 3D numerical modelling

Oceanic transform faults (OTFs) are one of three classes of plate boundaries representing the most seismogenic part of the global mid-ocean ridge (MOR) system. Their cumulate length represents more than 40% of the global MOR system. In a classical view, OTFs are perpendicular to mid-ocean ridges and considered as pure strike-slip zones where one plate moves past another and no material is added or destroyed. Recent studies show that OTFs are oblique boundaries where extensional tectonics and a two-phase crustal grow, which challenges a major concept of plate tectonics. However, thermal structure and stress pattern that are key to explore geodynamics processes at OTFs remain poorly understood.

We conducted 3D numerical simulations of plate separation and dike injection at a ridge-transform-ridge system by using the geodynamic code LaMEM (Lithosphere and Mantle Evolution Model). Our results reveal three key findings. First, OTFs are always deeper and warmer than fracture zones for all models, which could be well explained by focused brittle deformation that locally reduces viscosity and strength of OTFs, allowing the far-field tectonic stretching to be preferentially partitioned into the transform domain. Mantle upwelling beneath rheologically weaken OTFs is therefore locally enhanced. Second, plate boundaries of ridge-transform intersections (RTIs) at depth are oblique, which is structurally different from its seafloor expressions. Its obliqueness increases with depth and reduced dike injection rate to the inside corner of ridge segments. Third, we found in all models, that strike-slip faulting, which is thought to be a main feature of OTFs only occurs at distances away from the RTIs. Approaching the RTIs, maximal horizontal stress is oblique to OTFs by more than 45, indicating transform-normal extension at the inside corner. These results provide a first-order constraint on thermal and mechanical behaviour of OTFs and are in line with recent bathymetry, gravity and micro-earthquake evidence.