Strain, Metamorphism, and the Development of Differentiated Crenulation Cleavages at Cooma, Australia

Crenulation cleavages have formed during a noncoaxial strain history in anisotropic tectonites of the Slacks Creek Anticline near Cooma, Australia. A constant sense of rotation of strain increments has led to (1) the super-imposition of a differentiated crenulation cleavage S₂ on an earlier S₁ schistosity, (2) B₃ microfolds (undifferentiated) of S₂, and (3) a tightening and overturning of the major folds. Accompanying S₂ formation was syntectonic, retrogressive metamorphism involving the elimination of andalusite and cordierite between 500 and 560°C. Regrogression at low Pn Q in the lower part of this temperature range coincides with a field where SiO₂ solubility isobarically increases with decreasing temperature. At higher P_H₂O increased pore pressure along with decreasing temperature leads to increased SiO₂ solubility. Insofar as differentiation was a transient phenomenon phased out between B₂ and B₃, the mobilization of differentiate material was independent of stress or strain history. Long range solution transport leading to chemically defined cleavages appears dependent on metamorphic factors for enhanced solution, and on the microstructure of the tectonites for preferential fluid migration routes.