Oceanic transform faults revisited with models and data

Abstract

Plate tectonics describes oceanic transform faults as conservative strike-slip boundaries, where lithosphere is neither created nor destroyed. Seafloor accreted close to ridge-transform intersections (RTI) has therefore been expected to follow a similar subsidence trend with age as lithosphere that forms away from RTIs. Our recent combined analysis of high-resolution bathymetric data, satellite gravity, and three-dimensional numerical models from transform faults segmenting mid-ocean ridges across the entire spectrum of spreading rates challenges this concept.  One striking observation is that transform faults are systematically deeper than their adjacent fracture zones. Gravity data suggests that the underlying reason may be changes in crustal thickness, with transform valleys having thin and fracture zones ‘normal’ crustal thicknesses. Another observation is that outside corner crust often shows symmetric abyssal hills with intact flat top volcanoes, while the inside corner regions show intense and oblique tectonic deformation. Furthermore, so-called J-shaped ridges, volcanic ridges that bend towards the active transform, show that magmatic accretion occurs predominantly along the spreading axis, ‘feeling’ the rotating stress field only in the direct vicinity of the RTI. While these observations do show some dependence on spreading rate, they can be identified across a wide range of opening rates, suggesting that they are expressions of processes inherent to transform faulting.In this contribution, we will review these observations before presenting numerical 3-D thermo-tectono-magmatic models designed to elucidate the underlying processes. These models use a dilation term to mimic magmatic accretion and resolve visco-elasto-plastic deformation. The simulations show that the tectonic deformation axis, the axis of plate separation, becomes oblique at depth resulting in extension and crustal thinning within the transform deformation zones. Complementing simulations that account for magmatic accretion and hydrothermal cooling show that a skew can develop between this oblique deformation axis and the axis of magmatic accretion, implying a possible disconnect between the main diking direction and the direction of tectonic deformation. Taken all evidence together, oceanic transform faulting appears to be much more complex than pure strike-slip motion. It shows a surprisingly complex pattern of tectonic faulting and hints at spill-over magmatism at the RTI.  Crustal accretion at ridge transform intersections may therefore be fundamentally different to accretions elsewhere along mid-ocean ridges.