Pacific trench motions controlled by the asymmetric plate configuration

Abstract

[1] We present a novel explanation for absolute trench-normal motions of slabs surrounding the Pacific. Rapid subduction-zone retreat on the eastern side of the Pacific and slow advance in the west can result from the large-scale asymmetric plate configuration. We use simple fluid dynamics to explain the mechanical background of this hypothesis, and we use the results of a simple finite difference scheme to estimate the effect on trench motion velocities. The hypothesis is based on two key assumptions. First, we follow the concept of platescale horizontal counterflow in the asthenosphere driven by accretion of asthenosphere into lithosphere and by plate motion. Second, we assume that horizontally wide slabs without large slab windows drift passively in the mantle flow field and do not retreat as a result of flow around the slab. If the asthenosphere transfers flow-related horizontal shear stress into deeper levels of the mantle, an asymmetry in the plate configuration leads to different net pressure forces on the two slabs and thus affects the retreat behavior. In an ocean with an asymmetric ridge position, the slab of the smaller plate should retreat faster than the slab of the large plate, which may even advance. Also, the domain of a slower moving plate should collapse faster than the domain of the faster plate. Our model explains the counterintuitive negative correlation between slab age and retreat velocity observed in the Pacific. It also accords with the topographic asymmetry of the ridge flanks along the Pacific rise.

Superweak asthenosphere in light of upper mantle seismic anisotropy

“…Allowing for a further asthenospheric viscosity reduction for the slab‐only case (Figure b) increases the effect of the return flow close to the subducting slab in a way that is similar to what was discussed by Nagel et al . []. Comparing Figures c and b illustrates that the disruption of anisotropic patterns seen in Figure f and 3 is due to a combination of density anomalies driving vertical flow (such as around Hawaii), and slab‐induced, deep asthenosphere return flow as seen in Figure b.…”

Constraints on asthenospheric flow from the depths of oceanic spreading centers: The East Pacific Rise and the Australian‐Antarctic Discordance

“…Interestingly, the viscosity is low enough, given a layer thickness over a hundred kilometers, so that the viscous drag on a large, fast plate is smaller than the estimated ridge push force [e.g., Turcotte and Schubert , 2002]. Implications of buoyant asthenosphere flow for plate‐scale forces and the migration of subduction zones is discussed by Nagel et al [2008].…”

On the relation between trench migration, seafloor age, and the strength of the subducting lithosphere

“…The width of the slab appears as the main controlling factor as larger slabs stir larger volumes of mantle material (Funiciello et al, 2003;Bellahsen et al, 2005;Schellart et al, 2007). Another important parameter is represented by the length of the plate or the distance between the trench and the ridge, infl uencing the potential energy budget at the trench (Nagel et al, 2008). Among others, the upper plate motion shows signifi cant correlation with trench migration, plate motion, and back defor-mation (Otsuki, 1989;Uyeda and Kanamori, 1979;Heuret and Lallemand, 2005).…”

“…More recently, laboratory models (Bellahsen et al, 2005;Faccenna et al, 2007;Funiciello et al, 2008), numerical experiments along with conceptual models suggested that the strength of the slab, in particular its resistance to bend, could exert a primary control on the style of trench migration. Alternatively, from a global view, it has been proposed that the asymmetric plate confi guration across the Pacifi c and the tectonic forcing caused by upper plate motion could force the trench migration (Husson et al, 2008;Nagel et al, 2008).…”