Quantitative structural analyses and numerical modelling of ...

B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 16. The evolution of (a) shear plane-parallel melt preferred orientation at low melt fractions and(b) shear plane-perpendicular melt preferred orientation at high melt fractions. At low melt fractions,shear is accommodated by the grain boundary sliding parallel to the shear direction. Since grain boundarysliding means a loss of the cohesion along these boundaries, they represent an easy path for melt whichaccumulates at these boundaries (bottom left, Figure 16a). At higher melt fractions, a switch from shearplane parallel to shear plane-perpendicular melt preferred orientation can be observed. This is probablybecause of stress concentrations at grain boundaries oriented perpendicular to simple shear andaccompanying dilation/cavitation of these boundaries (bottom right, Figure 16b). See text for furtherexplanations.orientation of melt seams can be explained by a loss ofcohesion at boundaries oriented parallel to the foliation.Because of shearing, the melt concentrated at triple pointssuffers an increasing overpressure. We suggest that thecombination of loss of cohesion and build up of meltoverpressure leads to the injection of melt between activelymoving grain boundaries oriented at a high angle to theprincipal compressive stress, i.e., parallel to the macroscopicfoliation (Figure 16a). Oblique intragranular fractures canbe interpreted as Riedel shears channeling melt from overpressured,dilatant grain boundaries accompanying movementsalong foliation-parallel grain boundaries [Rosenbergand Handy, 2000]. The M1 and M2 samples are characterizedby a general constrictional deformation, lower aspectratio of grains and low melt fraction (