PHYLLONITIZATION AND DEVELOPMENT OF KILOMETER-SIZE EXTENSION GASHES IN A CONTINENTAL-SCALE STRIKE-SLIP SHEAR ZONE, NORTH GOIAS, CENTRAL BRAZIL

Several km-scale, vertical extension gashes occur in a low metamorphic grade, strike-slip shear zone in central Brazil. These mega-gashes sh ow many of the characteristics commonly found in en echelon extension gashes of cm and outcrop scale, reflecting a wide range of scale-invar iance for this phenomenon. The gashes are filled with quartz veins whi ch commonly host gold mineralization. Microstructures show a progressi ve deformation of the original cavity-infilling vein structures toward s the gash margins. Logarithmic plots of length vs thickness for gashe s From thin-section, outcrop and air photograph scales define a power law L = 11.4T(0.96). Logarithmic plots of cumulative Frequency define curves with power-law segments whose slopes indicate 'fractal dimensio ns' D around 1.4-1.5 for both length and thickness. The phyllonite zon es adjacent to the mega-gashes are interpreted to exert a crucial role in their development. Calculations show that the amount of quartz dep leted in the phyllonite zones correspond to the amount of quartz preci pitated in the mega-gashes (similar to 7 x 10(6) m). Volumes of fluid in the order of 10(10) m(3) must have been channelled through the open ed fractures to precipitate such an amount of quartz. We conclude that these mega-gashes have developed from continuous propagation and open ing of tension fractures in zones relatively preserved from phylloniti zation (protomylonites). It is suggested that the development of kilom etric extension gashes in the non-phyllonitic domains produces a volum e gain in response to the volume loss produced in the phyllonite zones . The whole shear zone is envisaged, therefore, as an isovolumetric sy stem with alternating lateral zones of volume loss (phyllonites) and v olume gain (extension gashes). (C) 1998 Elsevier Science Ltd. All righ ts reserved.