The ‘orphan’ boundary type in plate tectonics: oceanic transform faults reconstructed

The plate tectonic revolution gave birth to three types of plate boundaries; two got most of the interest, i.e., the mid-ocean ridges where new ocean floor is formed and subduction zones where the lithosphere is recycled back into the Earth’s interior. In the oceans, the third type, the “simple” strike-slip conservative plate boundary or oceanic transform fault (OTF), was treated like an orphan in a Charles Dickens novel. However, recent observations challenge plate tectonics, revealing that OTFs show unexpected complex behaviour. The morphology of oceanic transform systems and numerical modelling suggests that OTFs are extensional below their strike-slip faults at the surface. Later in their evolution, before converting from an active fault into an inactive fracture zone at the ridge-transform intersection, OTFs may turn into accretionary features. Yet, how can a strike-slip plate boundary, generating magnitude >7 earthquakes, promote extension forming up to 18 km wide and 7 km deep valleys? Furthermore, a fault zone grading from a strike slip fault into an extensional feature at depth would be a unique geological feature and may control their major seismic slip deficit. TRANSFORMERS will reveal: (i) if OTFs are indeed wrongly classified in plate tectonics and are not conservative plate boundaries, but instead have to be re-classified as features where accretion occurs in two-stages, separated by a period of transform extension, revealing a process fundamentally different from predictions of plate tectonics, suggesting that fracture zones are structurally different from OTFs; (ii) how OTFs operate from top to bottom and why their seismic moment release is too low. The project will require major sea-going efforts, issuing seismological, geodetic and geological surveys on the ocean floor, mimicking a multiple year’s land campaign. The outcome will revolutionize our understanding of oceanic transform faults, adding a new chapter to plate tectonics.