UPPER-MANTLE TEMPERATURES AND LITHOSPHERE-ASTHENOSPHERE SYSTEM BENEATH THE FRENCH MASSIF-CENTRAL CONSTRAINED BY SEISMIC, GRAVITY, PETROLOGIC AND THERMAL OBSERVATIONS

The new tomographic image of the mantle beneath the French Massif Cent ral reaching a depth of 270 km is interpreted in terms of mantle tempe rature, considering effects of anharmonicity and anelasticity on seism ic velocities as well as effects of mineral reactions, composition and partial melt. For every block of the tomographic model we calculate t he absolute temperature required to fit the observed velocity perturba tion, the average temperature of the tomographic layer being constrain ed by P-T estimates from mantle xenoliths and by surface heat flow. Fr om the 3-D temperature distribution we estimate the topography of the thermal lithosphere-asthenosphere boundary as well as 3-D distribution s of density, absolute P-and S-velocities and seismic attenuation. The observed velocity perturbations in the mantle beneath the Massif Cent ral can be explained nearly entirely by temperature variations. Temper atures approach the dry peridotite solidus in the depth range from 50 to 90 km just below Cenozoic volcanic areas, but no large-scale partia l melting is required to fit the seismic observations. Model temperatu res agree well with P-T estimates from mantle xenoliths and measured s urface heat flow. Model-predicted seismic velocities, seismic attenuat ion and density fit well the observations from seismic refractions, su rface waves and gravity. The model predicts a broad uplift of the ther mal lithosphere-asthenosphere boundary to a depth of 65-70 km with a 5 0-70 km wide band of stronger lithospheric thinning which crosses the main volcanic fields and strikes parallel to the direction of maximal compression in the crust. The Limagne Graben, which is the major rift structure of the Massif Central, has no clear expression in the topogr aphy of the lithosphere-asthenosphere boundary. Our interpretation sug gests a mantle plume below the central and southern part of the Massif Central with a potential temperature which is about 150-200 degrees C higher than the average potential temperature of the upper mantle. Th e structure of the lithosphere-asthenosphere boundary provides evidenc e for a possible thinning of the mantle part of the lithosphere beneat h the volcanic fields parallel to the direction of minima! horizontal compression in the crust.