STRUCTURES AND WAY-UP CRITERIA IN MIGMATITES, WITH APPLICATION TO THE VELAY DOME (FRENCH MASSIF-CENTRAL)

A detailed structural analysis has been applied to the migmatite sequence of the southern limb of the Velay dome, France, in an attempt to study the structural implications of the occurrence of metatexites and diatexites. Metatexites have a plastic behaviour presumably dominated by the solid skeleton which largely takes up the deformation. In contrast, diatexites have a predominantly viscous behaviour because the solid framework has been disrupted and they are dominated by the granitic melt rather than the solid component. Therefore, the metatexite to diatexite transition has a major rheological and tectonic importance because it separates regions that record plastic (stable) deformation from regions that record viscous (unstable) deformation where gravitational instabilities may develop. Mesoscopic, syn-migmatitic way-up criteria, which assume that melt migration is partly driven by gravity, give evidence for a southward overturning of the Velay dome. This overturning is supported by sedimentary and plutonic way-up criteria seen in schists and granitic plutons, respectively. The N-S asymmetry of the Velay dome is related to late-Variscan, asymmetric extension on a regional scale. Consistency of data provides further evidence that syn-migmatization way-up criteria are regionally reliable structural tools applicable to other migmatitic terrains.