MELT-BEARING SHEAR ZONES - ANALOG EXPERIMENTS AND COMPARISON WITH EXAMPLES FROM SOUTHERN MADAGASCAR

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. Th e very weak inclusions consisted of Vaseline in a homogeneous matrix o f paraffin wax, which has a power-law viscous rheology. Boundary condi tions were those of pure shear at constant natural strain rate and con fining stress sigma(3). The inclusions were initially perfect cylinder s with axe-parallel to the intermediate bulk strain axis Y. Conjugate shear zones nucleate on the inclusions and link up to form an anastomo sing pattern of high strain zones of concentrated shear surrounding mu ch more weakly deformed pods of near coaxial strain. The zones initiat e at angles near 45 degrees to the bulk shortening axis Z but stretch and rotate towards the,Y axis with increasing bulk strain. All inclusi ons nucleate shear zones, so that with increasing development of the a nastomosing pattern. weak material occurs only within the high strain zones. The restriction of migmatite leucosomes to shear zones in natur al 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 Madagas car. Elongate zones rich in weak inclusions. originally either perpend icular or at 45 degrees to the Z axis, were also modelled for direct c omparison with the regional-scale geometry of the Pan-African high-gra de 'shear zones' on Madagascar. (C) 1998 Elsevier Science Ltd. Ail rig hts reserved.