Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decay within the first 10 million years as a result of pervasive hydrothermal circulation and alteration. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder heat advection. Here we show, however, that a systematic loss of ~20 % in the amplitude of the anomalies arises between the outer rise and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decay as reflecting the opening of normal faults and fissures caused by extension on the outer flexural rise, and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench.

Plain Language Summary

Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decrease within the first 10 million years as a result of the widespread circulation of hot seawater within the oceanic crust and the resulting alteration of its magnetic minerals. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder the free exchange of seawater between the crustal aquifer and overlying ocean. Here we show, however, that a systematic loss of ~20 % in the amplitude of the anomalies appear between the outer rise, an elevation caused by the flexure of the plate entering subduction, and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decrease as reflecting the opening of faults and cracks caused by extension at the top of the bent oceanic plate and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench.