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

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 fou r Peaceful Nuclear Explosion (PNE) seismic sections of the 3500 km long pro file Kraton in Siberia. The scattered phases are consistent with energy ref lected from bodies embedded in a similar to 75 km thick low-velocity zone b elow the 8 degrees discontinuity at similar to 100 km depth. Inhomogeneitie s in the crust or uppermost mantle are not likely to cause the observed sca ttered phases. Reflectivity modelling of inhomogeneous media shows that P-w ave velocity fluctuations of about +/-1.5 per cent of the background veloci ty in the 100-175 km depth range can produce sufficiently strong arrivals t o explain the observations. From stochastic modelling we conclude that the thickness of the reflecting bodies is 4-7 km. Since the reflectivity modell ing approach assumes 1-D models. it cannot resolve the lateral extent of th e bodies. A high-density seismic record section acquired by airgun shooting during the BABEL experiment indicates that the lateral extent of reflectin g bodies below 100 km depth is less than 20 km in the Baltic Shield. At the average receiver spacing of similar to 15 km of PNE profile Kraton. tradit ional phase correlation is not possible for the strong arrivals from the re flecting bodies below 100 km depth, which suggests that the scattering bodi es have limited horizontal extent. In areas where the temperature distribution with depth is described by a ge neralized shield geotherm. the temperature is likely to be higher than or c lose to the solidus temperature For peridotite with small amounts of C-H-O below similar to 100 km depth. The P-wave velocity of mantle peridotite dec reases significantly as the temperature approaches that of the solidus. The refore. 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 similar to 100 km depth, because the melts are bound to solidif y during upward migration due to the higher melting point at shallower leve ls.