Platform subsidence behind an active subduction zone

NUMERICAL models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones^1-4. This 'platform subsidence' is predicted to cause an asymmetrical depression 500-1,000 km wide, with a maximum depth of ∼2 km if filled with water. Direct evidence linking platform subsidence to mantle flow has remained elusive, however. Here we offer evidence from western New Zealand for a complete link in both space and time between subduction initiation and platform subsidence. Because of the broad wavelength (>200 km) of the observed 1,500-m subsidence it is unlikely that the subsidence is due to a flexural foredeep. The observed subsidence does, however, match in both wavelength and amplitude that predicted by a model of subduction-induced mantle flow. The occurrence of platform subsidence has implications not only for understanding mantle convection, but also for explaining geoid anomalies over active margins⁵ and patterns of continental submergence⁶ and sedimentation⁷ that could otherwise be interpreted as a eustatic sea level change⁸.