Plate motions, slab dynamics and back-arc deformation

Abstract

Back-arc extension or compression is often related to a particular behavior of the slab (dip change and/or forward/backward migration) with respect to the upper plate. Slabs act, either passively when anchored in the deep mantle whereas back-arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench; or actively when slab pull – increasing with slab age – forces the trench to rollback. In addition to these two mechanisms, i.e., slab anchor and slab rollback, numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab, causing its forward or rearward migration with respect to the arc. Based on a compilation of upper plate absolute motion, trench absolute motion, back-arc deformation rate, upper plate strain regime and slab age for all oceanic subduction zones—excluding any kind of collision with continents, arcs or plateaus, we have examined how the combined effects of these parameters can account for the observed back-arc deformations. Our main results are: (1) a global correlation exists between upper plate absolute motion and back-arc deformation, i.e., back-arc extension when upper plate retreats and vice-versa; (2) there are as many advancing trenches as retreating ones, with trench motion globally limited to 50 mm y −1; furthermore, there is no positive correlation between trench retreat and slab age; and (3) upper plate absolute motion often fails to explain the back-arc deformation rates and the trench motions observed at several subduction zones, as Tonga, New Hebrides, Sandwich or Ryukyu; we propose that the trench migration of these subduction zones are on the influence of mantle flows. We conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion. Back-arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab (trench motion is limited to 50 mm y −1), even if locally, mantle flows force slab to move.