A Partially Molten Zone beneath the Global 8° Discontinuity at ~100 Km Depth

Abstract

A distinct change in nature of the P-wave phases in long-range seismic and seismological sections reveals globally the 8° discontinuity, despite an up-to 6 s travel time difference between active and stable continental areas. The change from linear to scattered arrivals at ~8° offset indicates a laminated velocity structure to ~100 km depth above a low-velocity zone with seismic scatterers. Linear first arrivals at offsets beyond 12°±2° in “cold”, stable areas indicate the Lehmann Discontinuity in the 140–250 km depth range, whereas the scattering zone extends to near the 400 km discontinuity in “hot”, tectonically active areas. We interpret the zone beneath 100 km depth as partially molten. Solid state is again attained at the the Lehmann discontinuity. Hence, its depth may be an indicator of the thermal state of the continental mantle.