Constraints on reflective bodies below the 8° discontinuity from reflectivity modelling

Abstract

Delayed first arrivals followed by a long wave train of scattered phases of up to 8 s duration are observed in the 800–1400 km offset range of the four Peaceful Nuclear Explosion (PNE) seismic sections of the 3500 km long profile Kraton in Siberia. The scattered phases are consistent with energy reflected from bodies embedded in a ∼75 km thick low velocity zone below the 8° discontinuity at ∼100 km depth. Inhomogeneities in the crust or uppermost mantle are not likely to cause the observed scattered phases. Reflectivity modelling of inhomogeneous media shows that P-wave velocity fluctuations of about ±1.5 per cent of the background velocity in the 100–175 km depth range can produce sufficiently strong arrivals to explain the observations. From stochastic modelling we conclude that the thickness of the reflecting bodies is 4–7 km. Since the reflectivity modelling approach assumes 1 D models, it cannot resolve the lateral extent of the bodies. A high density seismic record section acquired by airgun shooting during the BABEL experiment indicates that the lateral extent of reflecting bodies below 100 km depth is less than 20 km in the Baltic Shield. At the average receiver spacing of ∼15 km of PNE profile Kraton, traditional phase correlation is not possible for the strong arrivals from the reflecting bodies below 100 km depth, which suggests that the scattering bodies have limited horizontal extent. In areas where the temperature distribution with depth is described by a generalized shield geotherm, the temperature is likely to be higher than or close to the solidus temperature for peridotite with small amounts of C H O below ∼100 km depth. The P-wave velocity of mantle peridotite decreases significantly as the temperature approaches that of the solidus. Therefore, we interpret the reflecting bodies in the 100–175 km depth interval of the upper mantle as local accumulations of rocks, which are in a state close to or slightly above the melting point. The melt accumulations cannot rise above ∼100 km depth, because the melts are bound to solidify during upward migration due to the higher melting point at shallower levels.