Plate tectonics idealizes oceanic transform faults to be conservative two-dimensional strike-slip boundaries where lithosphere is neither created nor destroyed. Ingo Grevemeyer and co-authors present a compilation of high-resolution multibeam bathymetric data from oceanic transforms and their adjacent fracture zones and find that transform valleys are systematically and unexpectedly deeper than their adjacent fracture zones. Accretion at ridge–transform intersections appears strongly asymmetric, with ‘outside corners’ showing shallower relief and more extensive magmatism while ‘inside corners’ have deep nodal basins and appear magmatically starved. The authors use 3D numerical models to show that this can be explained by plastic shear failure within the transform deformation zone resulting in the plate boundary becoming an increasingly oblique shear zone at depth. This makes accretion at transform systems a two-stage process, fundamentally different from elsewhere along mid-ocean ridges.
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