Effects of subduction and slab stagnation on the mantle transition zone: A perspective from ambient seismic noise

The mantle transition zone (MTZ) is a critical feature of the Earth's mantle, sensitive to both thermal and water-related influences due to surface geodynamics. This study investigates the MTZ using topside P-wave reflections derived from ambient seismic noise, an often underutilized resource in the investigation of upper mantle processes. Using a database of inter-station cross-correlations from the broadband Romanian National Seismic Network, we extract these topside reflections with a processing method inspired from common-conversion point stacking: we bin the data according to reflection points in a gridded approach, align them using an adjoint tomography model of Europe, and phase-weight stack them to estimate empirical Green's functions for each grid. We then calculate the travel-time difference between observed and expected arrival times and iteratively adjust the discontinuity depths in the velocity model to match the time delays. Comparisons with two regional tomography models reveal several key correlations, including a depressed 660 km discontinuity beneath a high-velocity MTZ region, often interpreted as an accumulation of cold, recycled oceanic slabs. The Vrancea slab is also observed to penetrate the MTZ, significantly uplifting the 410 km discontinuity. Using Clapeyron slopes, temperature anomalies from these depth deviations are estimated, providing insights into the extent of slab subduction and its influence on upper mantle deformation and thermal anomalies.