Synkinematic porosity and ductile failure in mid-crustal ultramylonites from the Redbank Shear Zone, Central Australia

It is agreed that mylonitic shear zones are first-order fluid conduits in the crust, but we lack a systematic and comprehensive understanding of porosity and permeability generation in natural mylonitic shear zones. As a consequence, we are unable to predict their synkinematic fluid transport properties, which affects our assessments of fluid-mediated processes in shear zones. 

Here we present insights into the dynamic porosities in a quartzo-feldspathic layered ultramylonite from the Redbank Shear Zone (Australia) that formed during a stage of retrograde thrusting and hydration at lower amphibolite-facies conditions. In our analysis, we have combined non-invasive, high-quality x-ray microtomographic datasets from 5-mm-diameter core samples drilled orthogonally to the mylonitic foliation with high-resolution electron microscopy, electron backscatter diffraction and energy dispersive x-ray spectroscopy on the same samples. 

The sample is dominated by two fine-grained (<5 µm) microstructural domains, which differ by the relative proportions of Qz, Or and An, and the occurrence of Czo, respectively. Both deformed dominantly by grain-size-sensitive diffusion creep and grain boundary sliding. Newly grown Czo is thought to have resulted from the hydrothermal alteration of plagioclase at lower amphibolite-facies conditions during continued retrograde thrusting. Five types of synkinematic porosity were identified in the sample: pores at the boundaries, and dissolution pores inside of feldspar porphyroclasts, strain-shadow pores around Czo porphyroblasts, creep cavities, and pore sheets. These porosity types are the results of different mechanisms acting locally in the microstructure. On the sample scale, the porosity distribution is dependent largely on the distribution of porphyroclasts and porphyroblasts, and creep cavitation in the matrix. Porosity in the Qz-dominant layers, which lack Czo, is ‘localised’ around and inside shrinking feldspar porphyroclasts, whereas porosity in the fine-grained polyphase microfabric containing Czo porphyroblasts is more common and ‘distributed.’ The latter may allow more efficient but anisotropic fluid transfer. Creep cavities appear to have coalesced to form pore sheets along foliation boundaries or connecting strain-shadow pores. Our findings further corroborate the description of strain shadow porosity by Fusseis et al. (2023, Geology). We interpret that a feedback between clinozoisite growth and fluid ingress promoted further creep cavitation, and resulted in a greater potential for cavity coalescence to cause ductile failure in the fine-grained polyphase microfabric.