Phyllonitization and development of kilometer-size extension gashes in a continental-scale strike-slip shear zone, north Goiلs, central Brazil

Several km-scale, vertical extension gashes occur in a low metamorphic grade, strike-slip shear zone in central Brazil. These mega-gashes show many of the characteristics commonly found in en échelon extension gashes of cm and outcrop scale, reflecting a wide range of scale-invariance for this phenomenon. The gashes are filled with quartz veins which commonly host gold mineralization. Microstructures show a progressive deformation of the original cavity-infilling vein structures towards the gash margins. Logarithmic plots of length vs thickness for gashes from thin-section, outcrop and air photograph scales define a power law L = 11.4 T0.96. Logarithmic plots of cumulative frequency define curves with power-law segments whose slopes indicate ‘fractal dimensions’ D around 1.4–1.5 for both length and thickness. The phyllonite zones adjacent to the mega-gashes are interpreted to exert a crucial role in their development. Calculations show that the amount of quartz depleted in the phyllonite zones correspond to the amount of quartz precipitated in the mega-gashes (~7 × 106 m3). Volumes of fluid in the order of 1010m3 must have been channelled through the opened fractures to precipitate such an amount of quartz. We conclude that these mega-gashes have developed from continuous propagation and opening of tension fractures in zones relatively preserved from phyllonitization (protomylonites). It is suggested that the development of kilometric extension gashes in the non-phyllonitic domains produces a volume gain in response to the volume loss produced in the phyllonite zones. The whole shear zone is envisaged, therefore, as an isovolumetric system with alternating lateral zones of volume loss (phyllonites) and volume gain (extension gashes).