Rapid Weakening of Subducting Plates From Trench-Parallel Estimates of Flexural Rigidity

The percentage of slab pull force transmitted from the slab to the subducting plate depends on the slab strength (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, layered brittle-ductile layered), but applicability of these rheologic models cannot be distinguished based on trench-perpendicular plate bending models alone (Forsyth, 1980). Consequently, 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 within 100 km of the trench. The observed plate weakening provides further evidence for a plate rheology that leads to significant lithospheric-scale 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 energy dissipation related to plate bending compared to past calculations that assume constant strength, plate thickness and plate curvature. While a decrease in bending energy dissipation would provide more energy for slab pull, lithospheric plate weakening may decrease transmission to the subducting plate. Additionally, the high degree of lithospheric weakening suggests that plate age has a weaker influence on slab strength and energy dissipation then previously believed, as very old oceanic lithosphere weakens to effective elastic thickness predicted for relatively young plates. Billen, M. I., Gurnis, H.A., 2005. Constraints on subducting plate strength within the Kermadec trench. J. Geophys. Res. 110, B05407, doi:10.1029/2004JB003308. Conrad, C.P., Hager, B.H., 2001. Mantle convection with strong subduction zones. Geophys. J. Int. 144 (2), 271-288. Forsyth, D.W., 1980. Comparison of mechanical models of oceanic lithosphere. J. Geophys. Res. 85, 6364-6368.