Oceanic transform faults and fracture zones “in modern dress”

Transform faults represent one of three classes of plate boundaries and show strike-slip tectonism where one plate moves past another and are considered to be conservative plate margins where crust is neither formed nor destructed. The other two types of plate boundaries are divergent and hence extensional plate boundaries where new crust is formed (mid-ocean ridges, MOR) and convergent or destructive plate margins (subduction zones) where crust is recycled back into Earth’s interior. Researcher focused their attention on MOR and subduction zones, but transform faults got rather little attention over the last decades, especially in ocean basins where they outline the direction of plate motion supporting the Wilson cycle.

Oceanic transform faults (OTF) are gigantic features – up to 900 km long – and without oceans masking the seafloor, they would be among the most prominent features on Earth, offsetting mid-ocean ridges, forming tens of kilometres wide and up to 7 km deep valleys on the ocean floor. Yet, they are defined as simple strike-slip faults, but how can a transcurrent plate boundary, generating magnitude 7+ strike-slip earthquakes, promote extension forming the deep and wide valleys? Interestingly, for over half a century, researchers failed to appreciate that OTF are always deeper than adjacent oceanic features of an older age, challenging a major concept of plate tectonics. Thus, instead of showing the predicted age-dependent subsidence, the seafloor shallows at ridge-transform intersections (RTI). It therefore might be reasonable to question if they are indeed conservative plate boundaries.

We will provide observational evidence suggesting that OTFs are highly dynamic features, showing both features of accretion and the occurrence of tensional tectonics, indicating that we have to revise our understanding of how OTFs operate. We will provide constraints from a global compilation of bathymetric data, show predictions from numerical simulations and show observational evidence from micro-seismicity at slowly slipping OTFs in the Atlantic Ocean. Micro-earthquakes outline a diffuse activity over a broad area, cutting across the inside corner domain between the spreading centre and the transform fault before focusing along the trace of the fault. In the vicinity of the ridge-transform intersection, focal mechanisms reveal transform-normal extensional tectonics instead of supporting transcurrent motion, while strike-slip tectonics occurs only away from adjacent spreading segments. These observations support a scenario based on numerical simulations showing that at RTIs the right-angular plate boundary at the seafloor develops into an oblique shear zone at depth, causing crustal thinning and consequently forming transform valleys. However, before turning into a tectonically inactive fracture zone magmatic activity at RTIs buries transform valleys, suggesting that OTFs and fracture zones differ structurally from each other. Therefore, tectonic processes shaping transforms are divers, arguing for a revision of the concept of conservative plate boundaries to account for their morphology, strong lateral differences in seismic behaviour, and crustal structure.