Lithosphere tearing at STEP faults: response to edges of subduction zones

Slab edges are a relatively common feature in plate tectonics. Two prominent examples are the northern end of the Tonga subduction zone and the southern end of the New Hebrides subduction zone. Near such horizontal terminations of subduction trenches, ongoing tearing of oceanic lithosphere is a geometric consequence. We refer to such kinks in the plate boundary as a Subduction-Transform Edge Propagator, or STEP. Other STEPs are the north and south ends of the Lesser Antilles trench, the north end of the South Sandwich trench, the south end of the Vrancea trench, and both ends of the Calabria trench. Volcanism near STEPs is distinct from typical arc volcanism. In some cases, slab edges appear to coincide with mantle plumes. Using 3D mechanical models, we establish that STEP faults are stable plate tectonic features in most circumstances. In the (probably rare) cases that the resistance to fault propagation is high, slab break-off will occur. Relative motion along the transform segment of the plate boundary often is non-uniform, and the STEP is not a transform plate boundary in the (rigid) plate tectonics sense of the phrase. STEP propagation may result in substantial deformation, rotation, topography and sedimentary basins, with a very specific time-space evolution. Surface velocities are substantially affected by nearby STEPs.