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The subduction of the Farallon Plate has altered the temperature and composition of the mantle transition zone (MTZ) beneath the United States. We investigate MTZ structure by mapping P-to-S conversions at mineralogical phase changes using USArray waveform data and theoretical seismic profiles based on experimental constraints of phase transition properties as a function of temperature and composition. The width of the MTZ varies by about 35km over the study region, corresponding to a temperature variation of more than 300 K. The MTZ is coldest and thickest beneath the eastern United States where high shear velocity anomalies are tomographically resolved. We detect intermittent P-to-S conversions at depths of 520km and 730km. The conversions at 730-km depth are coherent beneath the southeastern United States and are consistent with basalt enrichment of about 50%, possibly due to the emplacement of a fragment of an oceanic plateau (i.e., the Hess conjugate).

Plain Language Summary The Earth's mantle transition zone between depths of about 400km and 800km is characterized by mineralogical phase transitions that depend on pressure, temperature, and composition. Seismic waves that propagate through mineral phase transitions can change polarization: a compressional wave (P wave) can change into a transverse wave (S wave). In this study, we analyze P-to-S converted waves using recordings of earthquakes from a network of seismographs in the United States (i.e., the USArray). From estimates of the arrival times and amplitudes of P-to-S conversions, we map the depths and strength of mineral phase transitions and the temperature and composition of the transition zone. Our seismic observations confirm models of the geologic history of Farallon Plate subduction along the western margin of the North American continent. The mantle transition zone is coolest beneath the eastern United States, and fragments of former oceanic plateaus are still present at the base of the transition zone beneath the Gulf of Mexico region.