Observation of stagnant slab material in the lower transition zone beneath the Aleutian Trench using teleseismic underside reflections

Abstract

SUMMARY The goal of this project is to investigate the mantle transition zone (TZ) across the Aleutian subduction zone using SdS underside reflections. The Aleutian subduction zone is chosen as our study area because of its significant tectonic activity, coupled with a high-density data set of SdS midpoints, largely obtained from the U.S. Transportable Array. Seismic images were made using the wavefield iterative deconvolution stacking method. Our results are corroborated by comparing them with velocity anomalies observed in the 3-D GyPSuM Earth velocity model. The results of our investigation show that where the subducting Pacific Plate passes through the TZ, the 410 discontinuity is elevated by up to 20 km, and the 660 discontinuity is depressed by up to 40 km. We interpret the variations in the depth to the boundaries of the TZ in terms of Clapeyron slope of the olivine phase changes hypothesized to be responsible for these discontinuities. In this model, the 410 discontinuity is caused by a phase change of olivine to wadsleyite and has a positive Clapeyron slope, while the 660 discontinuity (phase change) represents a phase change of ringwoodite to perovskite and ferropericlase, and has a negative Clapeyron slope. Also, in the TZ, the 520 discontinuity (a phase change from wadsleyite to ringwoodite) occurs over a 30 km interval, resulting in a boundary that is too gradational to be observed globally in seismic imaging. However, in this study, the 520 is observed in regions close to the cold subducting slab in the Aleutian trench. We suggest this observation is a result of mantle chilling of the lower half of the TZ where the cold subducting Pacific slab does not penetrate the 660 km discontinuity, thereby cooling the mantle beneath the 520. This chilling of the mantle appears to sharpen the velocity contrast at the 520 depth. Finally, we infer that the Pacific slab pools atop the 660 discontinuity and undergoes dehydration that contributes to the observed deepening of the 660.