Can subduction initiation at a transform fault be spontaneous

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.