Subduction initiation along the inherited weakness zone at the edge of a slab: Insights from numerical models

Abstract

S U M M A R YA preexisting weakness zone in the lithosphere is required to initiate subduction. Here, we focus on a new type of weakness zone, a Subduction-Transform Edge Propagator (STEP) fault, which is inherited from a tear along the edge of a slab. Using coupled thermal-mechanical models, we show that STEP fault-perpendicular convergence results in a dipping shear zone in any tectonic setting. At a continental margin, this shear zone dips towards the continent, which is an excellent starting condition for ocean-continent subduction. If (far field) convergence persists, STEP faults become new subduction boundaries. The trench moves landward during the earliest stages of convergence. When slab pull becomes a dominant driving force, after ∼80 km convergence, trench roll-back commences. The initial geometry and mechanical properties of the sub-crustal STEP fault zone affect the results; subduction initiation is facilitated by a wide (∼100 km) and low-viscosity weakness zone. Incipient subduction is easier for young oceanic lithosphere due to its lower flexural rigidity and is insensitive to the far field convergence rate. As STEP faults are commonly associated with young oceanic lithosphere, subduction initiation is thus relatively easy along them. Of particular interest are continent-ocean margins where STEP faulting has occurred.

Signature of slab fragmentation beneath Anatolia from full-waveform tomography

“…Özbakır et al (2013) propose that Africa-Aegean relative motion is partly accommodated by NW directed thrusting here. This high velocity anomaly may thus correspond to a gravitational instability that developed recently along the STEP fault as was predicted by Baes et al (2011) based on geodynamic model experiments.…”

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

“…This is shown by the pattern of strain localization that indicates that the STEP will propagate to the ESE. here (Billi et al, 2007;Baes et al, 2011). In our stylized model, this shows as a stress concentration in the Tyrrhenian Sea.…”

Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)

“…STEP faults can develop either in purely oceanic domains, as along both boundaries of the South Sandwich trench or along the northern boundary of the Tonga subduction zone (Govers and Wortel, 2005;Baes et al, 2011). However, they commonly develop along the ContinentOcean Boundaries (COB) of the same lithospheric plate, as in the South America -Caribbean plate boundary example (Clark et al, 2008a).…”