Syntectonic melt pathways during simple shearing of a partially molten rock analogue (Norcamphor-Benzamide)

Norcamphor-benzamide aggregates were used as analogues for partially molten quartzofeldspathic Drained, constant displacement rate, simple shearing of partially molten norcamphor-benzamide aggregates produces strain localization within melt-bearing, extensional shear fractures. These fractures interconnect to form shear surfaces whose orientation with respect to the shear zone boundary is like that of synthetic shear bands (C' surfaces) in naturally, deformed, mylonitic rocks. The shear bands channel overpressured melt from dilatant grain boundaries in the norcamphor-benzamide aggregate to undeformed, low-pressure areas adjacent to the deforming sample. The rapid expulsion of melt in the aggregate along shear bands hinders the attainment of a rheological critical melt percentage (20%) within the shear zone as a whole. However, this melt percentage is achieved within the melt-bearing shear bands. Deformation within the shear bands involves a strain-dependent switch from intergranular fracturing and dislocation creep to diffusion-accommodated, grain-boundary sliding, whereas the matrix adjacent to the shear bands continues to deform by dislocation creep. At a shear strain of approximately gamma = 0.7 the melt-bearing shear bands coalesce to form interconnected weak layers subparallel to the shear zone boundaries. The strength of the aggregate could not be measured but is inferred to decrease markedly once the melt-bearing layers interconnect. When the melt completely crystallizes and deformation ceases, the former presence of melt within the shear bands is only betrayed by the alignment of benzamide grain boundaries parallel to the shear bands.