A simple mechanism for mid-crustal shear zones to record surface-derived fluid signatures

Ion microprobe analyses of garnet porphyroblasts from three separate splays of the mid-crustal Walter-Outalpa shear zone, Curnamona Province, South Australia, indicate homogeneous δ¹⁸O values of <3‰. Integrated Lu-Hf geochronology and electron microprobe compositional mapping demonstrate that closed-system growth of these isotopically light garnets initiated as early as 531 Ma, prior to peak metamorphism and deformation during the Delamerian Orogeny (514-490 Ma). We attribute this to the prograde burial and dehydration of altered fault panels under thick sedimentary sequences during pre-orogenic basin formation. Contrary to established fluid transport models, surficial fluid signatures were not imposed at depth by large fluxes of downward-penetrating fluids, but rather by the exposure and meteoric alteration of fault rocks that were subsequently buried and reactivated as ductile shear zones. The existence of low δ¹⁸O values in deeply exhumed shear zones may therefore be indicative of fault structures that have a prior history of surface exposure, weathering, burial and re-exposure.