Pulsed melt flux controls the rheological evolution and widening of a melt-present high strain zone

High-strain deformation zones accommodate much of the strain during orogenic shortening through localised weakening. However, once a weakened zone develops, it should inhibit the formation of wide high-strain zones (hundreds of metres to kilometres across), making their origin contentious. We address this problem by studying the 100–300 m-wide, garnet–biotite-rich Cattle Water Pass shear zone in the intracontinental Alice Springs Orogen, central Australia. Microstructural features, including cuspate grain shapes, low dihedral angles, ‘string of beads’ textures and thin elongate grains of plagioclase and quartz, indicate deformation in the presence of melt. Quantitative crystallographic orientation data show three-dimensional plagioclase connectivity and limited quartz and ilmenite deformation in highly modified samples, consistent with syn-deformational melt crystallisation. Whole-rock geochemistry and major-, trace- and rare earth element variations demonstrate that deformation occurred in an open system, buffered by fluxing melt. Cathodoluminescence textures and local chemical variations in feldspar and biotite further support melt–rock interaction during deformation. Variably hydrated domains across the shear zone suggest spatially heterogeneous melt flux. The magnitude of rheological weakening caused by syn-deformational melt fluxing precludes formation of the entire high-strain zone in a single event; instead, the melt–rock reacted domains must reflect multiple episodes of melt ingress, reaction and egress. The absence of cross-cutting relationships indicates that each event preserved earlier domains, with new strain localising along weaker interfaces or in adjacent, less-strained regions. We propose a model in which successive melt pulses progressively widen the shear zone as the melt conduit network expands.