Low strength of the shallow mantle lithosphere: Data and geodynamical consequences

The shallow mantle is, except in extremely thin and hot plates, the stronger part of the lithosphere and hence a key layer for understanding plate tectonics. We will present mechanical and microstructural data (EBSD and TEM) from deformation experiments on olivine single crystals and polycrystals performed at low temperature (850-1100°C) and moderate pressure (300 MPa) under dry conditions. These results indicate that the shallow lithospheric mantle may deform by dislocation-based processes at much lower stresses than those predicted from the extrapolation of high-temperature experimental data for olivine. Although the experiments show heterogeneous mechanical behaviors, with hardening, the microstructural observations highlight that, even at the high strain rate of the experimental conditions, recovery processes are at play, suggesting that in nature, at lower strain rates and higher pressure, hardening by dislocation entanglement might be counteracted. Consistently, mantle rocks deformed in natural shear zones active at temperatures 850-900°C show microstructures characterized by coarse recrystallized grain sizes (≥100µm) and strong crystal preferred orientations, which are consistent with deformation dominantly by dislocation creep at rather low stresses (<100MPa). Based on our experimental results and on a compilation of previously published data on olivine deformation between 500 and 1000 °C, we propose a new semi-empirical exponential flow law, which might represent an upper limit to the uppermost mantle strength. Such a low strength shallow mantle lithosphere suggests a higher equilibrium recrystallized grain sizes, and less shear heating; further geodynamic implications will then be discussed.