The Dynamics and Convective Evolution of the Upper Mantle

Abstract

This chapter summarizes current understanding of convective instability in the upper mantle based on observations and theoretical predictions, beginning with interpretations of the earliest observations on how cooling from above would be reflected in seafloor depth, heat flow, and geoid height as functions of age. The lithosphere is a thermal boundary layer of mantle convection and, in absence of other heat transport mechanisms, seafloor subsidence would be expected even to very old ages. Slower subsidence of old seafloor has been described in terms of plate models for upper-mantle thermal evolution which implies a heatflux to the base of thermal boundary layer either due to hot spots or thermal convective instability. Both heat transport mechanisms appear to be important but to differing extents in different ocean basins. A plate model assumes that convective instability develops only beneath old lithosphere (>70 My) and that cooling of the upper mantle is restricted to depths shallower than the plate thickness (<125 km). However, convective instability beneath the very young Nazca and Pacific Plates has been identified by gravity lineations aligned with plate motion. Inferences from seismic velocity structure indicate that shear-wave velocity in the upper mantle beneath the Pacific Plate continues to change with age at depths exceeding the plate thickness. Beneath other more slowly spreading plates, small scale convection at young ages might not be detectable or may be absent. The young onset of small scale convection beneath Nazca and Pacific Plates could be related to fast spreading rates or to physical properties of the upper mantle, such as water content or grain size, that reduce mantle viscosity. Convective instability in the upper mantle may also be driven by melting. Buoyant decompression melting is a candidate for explaining intraplate volcanism at spatial and temporal scales that do not fit the classical hot-spot paradigm. Buoyant decompression melting is considered as a possible explanation for intraplate volcanism at small spatial and short temporal scales. Upwelling and melting beneath spreading centers must play a major role in affecting the composition and physical properties of the upper mantle that ultimately govern convective instability. The compositional effects of melting under spreading centers may be significantly affected by mechanisms of melt migration and the development of buoyant instability beneath the spreading axis.