The role of passive margins on the evolution of Subduction-Transform Edge Propagators (STEPs)

A Subduction-Transform Edge Propagator (STEP) is the locus of continual lithospheric tearing which enables subduction of one part of a tectonic plate, while the juxtaposed part remains at the surface. A key question is the propagation direction of active STEPs, and we suspect passive margins to play a critical role in steering STEPs. We investigate the role of passive margins (width, orientation, and lateral strength contrast) on the STEP propagation direction through mechanical finite element models. For straight passive margins, we show that STEPs remain parallel to passive margins when within 15° from a trench-perpendicular geometry. In other cases, where passive margins change strike ahead of the active STEP, STEPs are captured by passive margins for abrupt strike changes (radius of curvature<lithosphere thickness) less than 25° from trench perpendicular. Outside this window, STEPs will propagate in the original direction. If a strike change (>25°) is made through a large radius of curvature (>lithosphere thickness), STEPs will also propagate along the passive margin. A STEP system evolves toward orthogonality, which may explain why STEP faults are approximately perpendicular to trenches in nature. STEP systems are relatively insensitive to small-scale details (due to large-scale stresses), propagating as straight features along rugged passive margins. Surprisingly, magnitudes of lithospheric strength variation across the passive margin and subduction history, which determines location and magnitude of density anomalies in the mantle, are less relevant for the STEP propagation direction.