Heterogeneity of the uppermost mantle beneath Russian Eurasia from the ultra‐long‐range profile quartz

Abstract

The 3850‐km long Deep Seismic Sounding profile QUARTZ crosses six major geologic provinces in Eurasia and is sourced by 3 nuclear and 48 chemical explosions. We present the first interpretation of the entire data set, using two dimensional (2‐D) ray tracing and inversion, resolution analysis, and 1‐D amplitude modeling. Our interpretation shows a 42‐km‐thick, high‐velocity crust under the Baltic Shield, a 29‐km‐thick crust and high‐velocity upper mantle under the Mezenskaya depression, 52‐km‐thick crust with high‐velocity lower crust and uppermost mantle under the Urals, and 40‐km‐thick crust under the West Siberian basin deepening to 45 km under the Altay‐Sayan fold belt. High‐velocity (8.4 km/s) uppermost mantle is found under the Mezenskaya depression and under the east flank of the Urals. One almost continuous upper mantle boundary occurs at 65‐to 80‐km depth, and another with an approximately 40‐km‐thick LVZ occurs at 120‐to 140‐km depth. The shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity. Resolution analysis based on direct multivariate model perturbations, artificial neural network and principal component analysis, indicate the depth uncertainty of the 410‐km discontinuity within ±6 km, and also its trade‐off with dip and velocities above and below the discontinuity. Decreased near‐critical amplitudes of reflections from the 410‐km and 660‐km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15–20 km thick. Lithosphere thins, asthenospheric velocity decreases, and the 410‐km discontinuity dips to the SE approaching the Himalayan orogenic belt.