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

Abstract

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.

The Ionian and Alfeo–Etna fault zones: New segments of an evolving plate boundary in the central Mediterranean Sea?

“…Strain localization and therefore STEP propagation tracks the passive margin in this time slice (and in following). Nijholt and Govers (2015) conclude that this response is typical for passive margins with small (< 10°) variations in their orientation. An important element in the reconstruction at Time 1 is that the orientation of the passive margin at the southern end of the subduction zone changed orientation from roughly E-W north of Sicily, to the ESE along the Taormina line and parallel to the Sisifo fault (Billi et al 2007).…”

“…The fact that the AEF is not activated by STEP propagation means that its tectonics require an explanation in broader, regional context (see section 6.2). The results of the numerical model experiments show to be relatively insensitive to smaller scale details (Nijholt and Govers, 2015).…”

“…This may also have a steering effect on the propagation of the STEP. Both aspects were investigated using mechanical/numerical models (Nijholt and Govers, 2015). Here we present results of their work that are relevant for Calabria.…”

“…The active STEPs are located at the intersections of the trench and the STEP faults (indicated by solid red lines in Figure 14). Nijholt and Govers (2015) model the passive margins as parallel bands at the oceancontinent boundary to employ a transition in material properties from an old oceanic (Ionian) lithosphere to the continental (African/Nubian) lithosphere. Here, continental lithosphere is taken to be mechanically weak (see Table 3 for mechanical properties).…”

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

“…Slab edges are necessary ingredients of plate tectonics, and vertical tearing along the lithosphere at the termination of subduction trenches is a geometric consequence that enables the subduction to continue (e.g., Wilson, 1965;Millen and Hamburger, 1998;Govers and Wortel, 2005;Nijholt and Govers, 2015). A Subduction-Transform Edge Propagator (STEP) fault is the locus of continual lithospheric tearing at slab edges, which allows subduction of one part of a tectonic plate, while the juxtaposed part remains at the surface (Govers and Wortel, 2005;Nijholt and Govers, 2015). The evolution of the subduction zone along STEP faults results in sharp changes in the lithospheric and crustal thickness, which trigger lateral and/or near-vertical mantle flow associated with lithospheric tearing (e.g., Mancilla et al, 2015;Menant et al, 2016).…”