State of stress and age offsets at oceanic fracture zones and implications for subduction initiation

The recycling of oceanic lithosphere back into the Earth’s interior through subduction is a central component of plate tectonics. The process by which new subduction zones initiate, however, remains poorly understood. A variety of mechanisms has been proposed by which a new subduction zone might initiate, including passive margin collapse (aided by sediment loading and/or rheological weakening due to the presence of volatiles) and forced convergence across a zone of preexisting lithospheric weakness. In this paper we focus on the latter model, which identifies three conditions necessary for subduction initiation: a zone of weakness such as a fracture zone, an age (and therefore density) offset along the fracture zone, and significant normal compressive stress which leads to shortening. We identify regions on the present-day Earth that meet these conditions and that may correspond to regions of relatively likely subduction initiation in the near future. Using a digital seafloor age model, we have created a database of oceanic fractures and quantified the associated age offsets. We have evaluated the state of stress on these lithospheric weak zones using two different global stress models. We find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present-day Earth, and that there is no indication of incipient subduction at regions identified as relatively likely for subduction initiation. Using the same technique, we have evaluated the state of stress and seafloor age offset at regions of inferred present-day incipient subduction, and find that most of these regions are not associated with both high far-field compressive stresses and large age (and thus density) offsets. Subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past, but our results indicate that the conditions needed for this type of subduction initiation are rare on the present-day Earth.