Can subduction initiation at a transform fault be spontaneous?

Abstract

Abstract. We propose a new exploration of the concept of spontaneous subduction, i.e., lithospheric gravitational collapse without any external forcing, at a transform fault (TF). We first seek candidates in recent subduction initiations at a TF that could fulfill the criteria of spontaneous subduction and retain 3 natural cases: Izu-Bonin-Mariana (IBM), Yap, and Matthew & Hunter. We next perform an extensive exploration of conditions allowing spontaneous gravitational sinking of the older plate in a oceanic TF using 2D thermo-mechanical simulations. Our parametric study aims at better deliminating the ranges of mechanical properties necessary to achieve the old plate sinking (OPS). The explored parameter set includes: crust and TF densities, brittle and ductile rheologies, and width of the weakened region around the TF. We focus on characterising OPS conditions in terms on (1) reasonable vs unrealistic values of mechanical parameters and (2) comparison to modern cases of subduction initiation in a TF setting. When modelled, OPS initiates following one of two distinct modes, depending mainly on the thickness of the overlying younger plate (YP). Asthenosphere may rise up to the surface above the sinking old plate provided that the YP remains motionless (verified for ages ≥ 5 Myr, mode 1). For lower YP ages (typically ≤ 2 Myr), the YP is dragged towards the OP resulting in a double-sided subduction (mode 2). When triggered, spontaneous OPS is extremely fast. The basic parameters to simulate OPS are the brittle properties of the shallow part of the lithosphere, controlling the plate resistance to bending, the distance away from the TF over which weakening is expected, and crust density. We find that all mechanical parameters have to be assigned extreme values to achieve OPS, that we consider as irrelevant. Furthermore, we point out inconsistencies between the processes and consequences of lithospheric instability as modelled in our experiments and geological observations of subduction infancy for the 3 natural candidates of subduction initiation by spontaneous OPS. We conclude that spontaneous instability of the thick OP at a TF evolving into mature subduction is an unlikely process of subduction initiation at modern Earth conditions.

Rapid subduction initiation and magmatism in the Western Pacific driven by internal vertical forces

“…The boundaries in this diagram would shift if other input parameters that have been observed to affect transform fault collapse were changed. These include: the initial width of the fracture zone; the ages of either plate; the rheology of mantle and crustal material; the weakening effects of fluids 20,26 . However, while the boundaries between each type of behaviour in the regime diagram may move, the overall trends are likely to be robust.…”

“…The only physical difference between compositions is the initial amount of "damage" (see below). The effect of changing many of these input parameters on the behaviour of this type of model has been explored by others 20,26,35 . We recognise that they may have an impact on the precise evolution of the model (and perhaps change the exact values of the applied forces needed in Vertically/internally driven with fore-arc basalts produced (Izu-Bonin-Marianas) Horizontally/externally forced with no fore-arc basalts produced ; e-h: the model displaying horizontally forced subduction initiation (taken at 1.1, 4.9, 5.1, 5.3 Myr from the start of the model run).…”

Subduction Initiation During Collision‐Induced Subduction Transference: Numerical Modeling and Implications for the Tethyan Evolution

“…On the other hand, the absence of SI in the Indian Ocean may also be a result of lack of proper weak zone according to our numerical models. The existence of weak zone is a common case on the Earth, which could be the faults, hydration zones, and highly deformed/fracture zones (Arcay et al, ; Gurnis et al, ; Leng & Gurnis, ; Zhou et al, ). These weak zones may not collapse into subduction zone under the typical tectonic force of ridge push of about 3.0 × 10 12 N/m (Mahatsente, ; Turcotte & Schubert, ) as shown in Figure .…”

A transdisciplinary and community-driven database to unravel subduction zone initiation

“…Irrespective of the somewhat misleading term spontaneous, SZI is, of course, a dynamic process and is always forced in one way or another (e.g., ref. 11 ).…”