Rapid weakening of subducting plates from trench-parallel estimates of flexural rigidity

The negative buoyancy force of sinking lithosphere (slabs) is the principle driving force for subducting plates, but transmission of this force to the subducting plate depends on the strength of the slab (e.g., Conrad and Hager, 2001). Slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies (i.e., perfectly elastic, perfectly viscous, perfectly plastic, brittle-ductile layered). Because the applicability of these rheologic models cannot be distinguished based on trench-perpendicular plate bending models (Forsyth, 1980), a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100 km of the Kermadec Trench (Billen and Gurnis, 2005). Using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the Tonga and Japan-Izu-Bonin subduction zones. The observed plate weakening provides further evidence for a plate rheology that leads to significant yielding (loss of elastic strength and reduction in effective viscosity) within the bending region of the subducting plate. This rapid weakening within the shallow, low curvature, region of the plate may significantly decrease estimates of energy dissipation related to plate bending, compared to recent calculations assuming high plate strength and constant plate curvature.

► The Tonga and Japan-Izu-Bonin subducting plates have significant weakening.
► Flexural rigidity decreases by 104–105 Nm within 100 km trenchward of the forebulge.
► Weakening indicates transition from elastic to visco-plastic dominated deformation.
► Plate weakening leads to decreased bending energy dissipation and less slab pull.