Abstract
The finite strain due to mylonitization of a quartzite has been determined, using measurements of deformed quartz grains and ‘ribbons’ developed from quartz grains, equant before the deformation. The mylonites are developed in long, narrow, steeply dipping mylonite zones within the Chewings Range Quartzite of Central Australia; this quartzite is very pure (≈98% quartz). The quartzite surrounding the mylonite zones appears unstrained. The samples examined exhibit severe flattening strains; k values of between 0.01 and 0.12, and r values of between 5 and 30 have been determined. Application of formulae to calculate volume change, assuming the model of an ideal ductile deformation band structure, indicate significant apparent volume losses, with Δ values between −0.69 and −0.95. Such values seem unreasonably high, as no geological evidence for large volume loss is seen. The band structure model is considered inapplicable for these mylonite zones. The observed structural features can best be explained by a deformation history involving severe bulk shortening normal to the zones.
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