Cooling-Induced Rheological Weakening Along the Nascent Plate Interface—A Mechanism for Catastrophic Subduction Initiation?

Abstract

Subduction initiation often begins with slow, forced convergence, switches “on” catastrophically as the slab collapses into the mantle, and then evolves to steady-state, self-sustained sinking that drives global plate movements. Numerical models suggest that the collapse phase implies sudden weakening of the plate interface. However, geological records of subduction infancy preserved as metamorphosed oceanic crust accreted beneath ophiolites (i.e., metamorphic soles) present a paradox. During the pre-collapse period that may last 2–15 million years, the nascent plate interface is hot, whereas during collapse, shear zone temperatures plummet, which typically strengthens rocks. So, how could cooling cause weakening? Here, we show microstructures of metamorphic sole rocks from Mont Albert (Québec, Canada) that demonstrate that upon cooling, metamorphic mineralogy became more heterogeneous, average grain size decreased, and deformation mechanisms shifted from dislocation-accommodated to fluid-assisted and grain size sensitive, which culminated in drastic rheological weakening. Quartz piezometry indicates that flow stress increased with cooling, but flow laws indicate that the colder rocks exhibited lower viscosity and therefore could localize strain. Interface viscosity initially rose with cooling, but upon reaching a threshold where major metamorphic minerals changed, dropped from >1018 to <1017 Pa-s. Cooling-induced mineral-mechanical changes thus drove rheological weakening, and provide a general mechanism explaining slab collapse and the transition to self-sustaining subduction. This implies that strain localization is inherent to modern metamorphosed oceanic lithosphere and does not require a “stress drop.” The next step to understanding subduction initiation is identifying causes of high temperatures and incipient cooling during the pre-collapse phase.