PLATFORM SUBSIDENCE BEHIND AN ACTIVE SUBDUCTION ZONE

NUMERICAL models of viscous mantle flow indicate that a broad subsiden ce 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 similar to 2 km if filled with water. Direct e vidence linking platform subsidence to mantle flow has remained elusiv e, however. Here we offer evidence from western New Zealand for a comp lete link in both space and time between subduction initiation and pla tform subsidence. Because of the broad wavelength (>200 km) of the obs erved 1,500-m subsidence it is unlikely that the subsidence is due to a flexural foredeep. The observed subsidence does, however, match in b oth wavelength and amplitude that predicted by a model of subduction-i nduced mantle flow. The occurrence of platform subsidence has implicat ions not only for understanding mantle convection, but also for explai ning geoid anomalies over active margins(5) and patterns of continenta l submergence(6) and sedimentation(7) that could otherwise be interpre ted as a eustatic sea level change(8).