Melt-bearing shear zones: analogue experiments and comparison with examples from southern Madagascar

Abstract

Analogue model experiments were conducted to investigate the influence of irregularly distributed weak sites in localising strain, as an aid to understanding shear zone development in partially molten rocks. The very weak inclusions consisted of Vaseline in a homogeneous matrix of paraffin wax, which has a power-law viscous rheology. Boundary conditions were those of pure shear at constant natural strain rate and confining stress σ 3. The inclusions were initially perfect cylinders with axes parallel to the intermediate bulk strain axis Y. Conjugate shear zones nucleate on the inclusions and link up to form an anastomosing pattern of high strain zones of concentrated shear surrounding much more weakly deformed pods of near coaxial strain. The zones initiate at angles near 45° to the bulk shortening axis Z but stretch and rotate towards the X axis with increasing bulk strain. All inclusions nucleate shear zones, so that with increasing development of the anastomosing pattern, weak material occurs only within the high strain zones. The restriction of migmatite leucosomes to shear zones in natural examples could also reflect a corresponding control of melt on the sites of shear zone nucleation, rather than implying accumulation from the surrounding wall-rock. The model geometry is very similar to that observed in small-scale shear zones in migmatites of southern Madagascar. Elongate zones rich in weak inclusions, originally either perpendicular or at 45° to the Z axis, were also modelled for direct comparison with the regional-scale geometry of the Pan-African high-grade ‘shear zones’ on Madagascar.