Flow in upper-mantle rocks: Some geophysical and geodynamic consequences

Abstract

Flow mechanisms effective in the upper mantle and some of the parameters of the creep equation are determined from the study of peridotites from basalt and kimberlite xenoliths and alpine-type massifs. Creep controlled by dislocation climb, as inferred by Weertman, is the dominant mechanism. Evidence for superplastic flow is found in the deepest kimberlite xenoliths. Flow in the alpine-type massifs is ascribed either to intrusion in the crust when continental plates collide (lherzolite massifs) or to sea-floor spreading (harzburgite massifs included in ophiolites). The consideration of textures, crystal substructures and preferred orientations connected with P,T equilibrium conditions derived from pyroxenes, helps in deciphering the large-scale structure and flow of peridotites in the crust and in the mantle down to 200 km. For the first 150 km, the representative structures are those of the basalt xenoliths and the kimberlite xenoliths with a coarsegrained texture. They have many features in common and probably represent a static lithosphere with, in basalt xenoliths, possible evidence for the transition to the shear flowing asthenosphere. The porphyroclastic and mosaic-textured xenoliths, in kimberlites equilibrated at depth between 150 and 200 km and a few more superficial basalt xenoliths, reflect a much larger strain rate and applied stress and might be connected to vertical instabilities also responsible for magma genesis.