TWO-STAGE DEFORMATION, AND TWO STYLES OF LOCALIZATION, IN A DEEP CRUSTAL NORMAL SENSE SHEAR ZONE: MOUNT HAY BLOCK, CENTRAL AUSTRALIA

The >6 km thick, deep crustal, 1558.3 ± 8.2 Ma Capricorn ridge shear zone (CRSZ) is exposed in the Mount Hay block, central Australia, a massif exhumed and rotated during Devonian thrusting. The CRSZ restores to a ~40° NE-dipping extensional shear zone. It primarily contains noritic, gabbroic, and interlayered gabbroic-charnockitic gneiss units, with grt ± sil felsic gneiss in the latter two. Fine-grained mylonite zones weaving along the major lithologic contacts contain grt ± fsp ± sil as porphyroclasts in a bt + mag-rich matrix, unique in the Mount Hay block. Both gneisses and mylonites have subparallel foliation and lineation, the same shear sense, and evidence of flattening (conjugate shear bands, microfaults, and boudinage record elongation perpendicular to the stretching lineation). Contacts are sharp or are <1 m of anastomosing mylonite strands. Pseudotachylite and cataclasite bands occur in the gneisses bordering the mylonite zones. M-scale quartzite-filled tension gashes are common in the gneisses and dm-scale quartzite lenses occur in the mylonite. Samples across a gneiss strain gradient and adjacent mylonite indicate deformation started at >798 ± 33°C and >7.6 ± 0.7 kb and continued to <630°C and 2–3.5 kb (~7–13 km uplift).

Deformation occurred in two stages with two styles of localization. Fabric intensity mapping indicates strain localized adjacent to major lithologic contacts during transposition in the earlier stage, consistent with a combination of flattening and shear where the highest strain rates occur across competency contacts to maintain strain compatibility. These positions are where strain localized abruptly for the later stage. Sharp contacts and consistent gneiss grain size indicates the mylonite is not the product of a gradual grain size reduction driven by cooling during uplift. This is supported by thermobarometry results from structurally adjacent gneiss (739 ± 33°C, 7.0 ± 0.7 kb; hbl-pl, hbl-pl-qz) and mylonite (713 ± 25°C, 4.7 ± 1.2 kb; GASP, GBPQ). Rather, a (temporary?) jump in strain rate generated frictional structures with finer grain sizes along which subsequent ductile strain localized. Fe ± Si fluid filled tension gashes and fluxed through the mylonite zones producing the bt + mag-rich matrix, consistent with a regional metasomatic event in central Australia at this time.