Jf. Hippertt et Aj. Massucatto, PHYLLONITIZATION AND DEVELOPMENT OF KILOMETER-SIZE EXTENSION GASHES IN A CONTINENTAL-SCALE STRIKE-SLIP SHEAR ZONE, NORTH GOIAS, CENTRAL BRAZIL, Journal of structural geology, 20(4), 1998, pp. 433-445
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.