Scales of structure in the lithosphere — images of processes

Abstract

Advances in Deep-Seismic Sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust-mantle transition. In this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near-vertical reflection and wide-angle refraction experiments. The abrupt termination of near-vertical reflections at the Moho and the coincident presence of a strong coda, accompanying the supercritical PMP reflection phase in record sections of continental seismic wide-angle refraction experiments, reveal a new property of the crust-mantle boundary. The Moho forms a sandwiched mix of crust-mantle material between lower crust and topmost mantle in combination with a step in mean velocity, a significant change in scale of the structural dimensions and of velocity variance. This view is strongly supported by the recognition of the subcrustal lithosphere as a high-frequency (5–10 Hz) scattering wave-guide for long-distance propagation of the Pn phase over more than 3000 km, detected during seismic experiments in Russia using Peaceful Nuclear Explosions. Evidence is brought forward that this high-frequency wave-guide is likely to be present also in the Phanerozoic upper mantle of western Europe.It is furthermore hypothesized that the global propagation of teleseismic Sn or corresponding Pn waves observed on continents and oceans combined with the recording of strong PMP-codas for wave paths on continents points to a world-wide change of scale of structure and velocities near the Moho. A change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the Moho. The global presence of the scattering Pn wave-guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle. Locations of present-day tectonic activity where horizontal decoupling between crust and mantle are likely to occur (e.g., subduction at continent-continent collision zones, change of tectonic regimes and continental transform faults) are discussed.