Can subduction initiation at a transform fault be spontaneous?

Abstract

Abstract. We present an extensive parametric exploration of the feasibility of “spontaneous” subduction initiation, i.e., lithospheric gravitational collapse without any external forcing, at a transform fault (TF). We first seek candidates from recent subduction initiation events at an oceanic TF that could fulfill the criteria of spontaneous subduction and retain three natural cases: Izu–Bonin–Mariana, Yap, and Matthew and Hunter. We next perform an extensive exploration of conditions allowing for the spontaneous gravitational sinking of the older oceanic plate at a TF using 2-D thermomechanical simulations. Our parametric study aims at better delimiting the ranges of mechanical properties necessary to achieve the old plate sinking (OPS). The explored parameter set includes the following: crust and TF densities, brittle and ductile rheologies, and the width of the weakened region around the TF. We focus on characterizing the OPS conditions in terms of (1) the reasonable vs. unrealistic values of the mechanical parameters and (2) a comparison to modern cases of subduction initiation in a TF setting. When modeled, OPS initiates following one of two distinct modes, depending mainly on the thickness of the overlying younger plate. The asthenosphere may rise up to the surface above the sinking old plate, provided that the younger plate remains motionless (verified for ages ≥5 Myr, mode 1). For lower younger plate ages (typically ≤2 Myr), the younger plate is dragged toward the older plate, resulting in a double-sided subduction (mode 2). When triggered, spontaneous OPS is extremely fast. The parameters that exert the strongest control over whether OPS can occur or not are the brittle properties of the shallow part of the lithosphere, which affect the plate resistance to bending, the distance away from the TF over which weakening is expected, and the crust density. We find that at least one mechanical parameter has to be assigned an unrealistic value and at least two other ones must be set to extreme ranges to achieve OPS, which we do not consider realistic. Furthermore, we point out inconsistencies between the processes and consequences of lithospheric instability, as modeled in our experiments and geological observations of subduction infancy, for the three natural candidates of subduction initiation by spontaneous OPS. We conclude that spontaneous instability of the thick older plate at a TF evolving into mature subduction is an unlikely process of subduction initiation in modern Earth conditions.

Anomalous Subduction Initiation: Young Under Old Oceanic Lithosphere

“…In nature, there are some incipient or fossilized subduction zones where the young plate likely underthrusts beneath the old plate. This is the case in the Hjort subduction zone and the Gagua Ridge (Arcay et al., 2020), and the Mussau trench (Hegarty et al., 1983), all shown in Figure 1. This configuration implies that subduction is not originated in a pure spontaneous way (by gravitational instability) but it also has some induced component within a compressional setting.…”

“…Therefore, they represent candidate locations for spontaneous or induced subduction initiation (Stern, 2004). There is some debate whether subduction can originate spontaneously along a transform fault in the current Earth (Arcay et al., 2020). Some authors argue that the large strength of the old lithosphere impedes the onset of spontaneous lithospheric collapse driven by its weight (Mueller & Phillips, 1991; Toth & Gurnis, 1998).…”

Plume‐Induced Sinking of Intracontinental Lithospheric Mantle: An Overlooked Mechanism of Subduction Initiation?

“…However, when subduction initiates along preexisting structures it can be challenging to discriminate between induced and spontaneous scenarios. Oceanic transform fault collapse, for example, was traditionally thought of as an “induced” mechanism that requires external forces (Hall et al., 2003) as simplified geodynamic models often have difficulties to reproduce this mechanism without using kinematic boundary conditions (Arcay et al., 2020). Yet, lateral buoyancy contrasts between different oceanic plates juxtaposed at oceanic transform faults have proven to be an adequate driving force to trigger subduction spontaneously.…”

“…Numerical models exploring the spontaneous mode of subduction initiation (e.g., Arcay et al, 2020;Dymkova & Gerya, 2013;Gerya et al, 2008;Nikolaeva et al, 2010Nikolaeva et al, , 2008 inevitably require a pre-defined sufficiently wide lithosphere-scale "weak" zone of hydrated rocks presumably inherited from previous tectonic activity. The presence of such pre-localized weak interfaces (usually characterized by significantly reduced brittle/plastic strength) is compulsory in these models to localize initial deformation and to decouple the plates, thus triggering slab foundering.…”

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, 2020;Gurnis et al, 2004;Leng & Gurnis, 2011;Zhou et al, 2018). 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, 2017;Turcotte & Schubert, 1982) as shown in Figure 8.…”