DSDP drilling results showed that at site 439 the Sanriku Shelf of Northeastern Honshu has subsided nearly 3,000 m during the Neogene. Integrating these results with seismic-reflection data indicates widespread subsidence that is locally as great as 6,000 m. The authors propose that this subsidence resulted from the initiation of subduction of Mesozoic lithosphere at the Sanriku Margin beginning 25-30 m.y.b.p. Temporal cooling of the mantle below the forearc area caused contraction and subsidence; a simple model is presented to estimate the rate.