On the evolution of Subduction-Transform Edge Propagators (STEPs)

Abstract

Subduction-Transform Edge Propagators (STEPs) are the locus of lithospheric tearing which arise to accompany the subduction of one part of a lithospheric plate, while the other part remains at the surface. The propagation direction of STEPs is an enigmatic question and it is suspected that passive margins may play a critical role in steering a STEP. In this thesis, the effect of passive margins on STEP propagation (direction) is investigated through mechanical, finite element models. Numerical model results show that propagation of a STEP along a passive margin-ocean interface is feasible for a range of models which show a passive margin-trench orientation within 20 degrees from a perpendicular setup. Passive margins are rarely straight features and with the inclusion of a change in strike of the passive margin ahead of the STEP, model results show that propagation will occur along the passive margin-ocean interface if this change is less than 15 degrees from a perpendicular setup. These results shed light on the evolution of the retreating Calabrian arc: slab rollback along the African margin is possible in an ESE direction through the formation of a STEP along the edge of the African-Ionian margin. The STEP reached the north-eastern edge of Sicily and continued to propagate along the edge of the African continent in a ESE direction, parallel to the current Sisifo fault. After it reached the passive margin bounding the Malta Escarpment, the Sicilian STEP propa-gated into the Ionian basin along-strike the Ionian fault at an ESE orientation. This Ionian fault is a major discontinuity in the Calabrian accretionary wedge, differentiating two distinct lobes. Comparison with data of the Calabrian accretionary wedge indicates that the western lobe has originated as an edge effect from the wider STEP-fault zone while the eastern lobe resulted from the differential motion between the subducting Ionian plate and overriding Tyrrhenian plate.