Andrea Tommasi, Geosciences Montpellier (France)
Veronique Le Roux, Geosciences Montpellier (France)
Vincent Soustelle, Geosciences Montpellier (France)
Jean-Louis Bodinier, Geosciences Montpellier (France)
Alain Vauchez, Geosciences Montpellier (France)
Carlos Garrido, Instituto Andaluz de Ciencias de la Tierra (Spain)
Recent results in seismology, petrology, geochemistry, tectonics, and geodynamical modelling converge on the idea that coupling between chemical and physical processes at the lithosphere-asthenosphere boundary play an essential role in the interaction between tectonic plates and the convecting mantle. Orogenic peridotite massifs are ideal natural laboratories to investigate this coupling. Recent detailed structural mapping, petrophysical and geochemical analyses on the Lherz and Ronda orogenic massifs and mantle xenoliths show that magma-rock reactions play a major role on the rejuvenation (or asthenospherization) of the lithospheric mantle (Le Roux et al., 2007 EPSL; Lenoir et al., 2001 EPSL). In these massifs lithospheric rejuvenation is marked by km-scale modal/chemical variations resulting from partial melt redistribution, local annealing of deformation microstructures, grain growth, and deformation in presence of small melt fractions.
From top to base, processes vary from refertilization of aged, refractory (harzburgite) lithosphere to partial melting of previously refertilized material (lherzolite and websterite). Both processes are associated with narrow fronts where major microstructural, modal and chemical variations occur within a few meters to tens of meters. Melting and refertilization fronts display striking resemblances (local annealing of preexisting deformation textures and small-scale chemical heterogeneities) as well as significant differences. The melting front observed in Ronda is a clear-cut structure that can be followed over >10 km, suggesting that its formation was thermally-controlled at a regional scale. In contrast, the refertilization front of Lherz is extremely convoluted, suggesting a formation by coalescence of relatively narrow ( 10 m) melt infiltration channels. In both massifs, analysis of structural relationships in harzburgites, refertilized lherzolites and websterite layers, and crystal-preferred orientations (CPO) of olivine and pyroxenes highlights a strong feedback between reactive percolation of basaltic melts and deformation under near-solidus conditions at the lithosphere-asthenosphere boundary. Refertilization and melting fronts observed in these massifs are thus considered as the upper and lower boundaries of transient, moving asthenosphere-lithosphere transition zones during thermo-mechanical erosion of the lithospheric mantle by upwelling asthenosphere.