Structure and dynamics of the oceanic lithosphere-asthenosphere system

Abstract

The thermochemical evolution of oceanic lithosphere and its interaction with the underlying asthenosphere exerts a fundamental control on the dynamics of the Earth system. Since the 1960s, the range, accuracy and spatial coverage of geophysical and geochemical datasets have increased substantially. These additional constraints have helped to elucidate aspects of the lithosphere-asthenosphere system, but some apparently contradictory observations have presented additional interpretational challenges. Here, we summarise the merits, limitations and ambiguities of available observational constraints on the thermomechanical evolution of oceanic upper mantle. Newly developed cooling models are generally compatible with these constraints, although there is evidence for systematic differences in behaviour between different oceanic basins. Subsidence, magnetotelluric and seismological observations from the Pacific Ocean are consistent with plate rather than half-space cooling models, whereas results from the Atlantic and Indian Oceans are more equivocal. We provide an overview of proposed mechanisms for seafloor flattening and we show that regional deviations from globally averaged trends can be attributed to asthenospheric temperature variation and to changes in lithospheric thickness. Although the plate cooling model generally provides a good description of available observations, it is probably a crude approximation of the dynamic processes operating within the thermal boundary layer that underlies oceanic basins. By incorporating mantle density structure inferred from surface wave tomography into more sophisticated convection simulations, we show that the plate model can match the age-dependent behaviour of bathymetric and gravity fields. While the results presented here suggest a unified understanding of the lithosphere-asthenosphere system is within reach, unambiguous evidence for small-scale convection at the base of the lithosphere remains elusive. As a result, the precise mechanism responsible for sea-floor flattening has yet to be identified. It is also unclear why models that incorporate the effect of phase changes such as the garnet-spinel transition tend to fit observations less well than simpler counterparts. These outstanding controversies suggest further research is needed to develop a complete description of the structure and dynamics of the oceanic upper mantle but, encouragingly, the tools at our disposal have never been more powerful.