Abstract
A theoretical, composite flow law is presented for mylonite containing interconnected layers of a weak mineral undergoing power law creep and porphyroclasts of a stronger mineral undergoing fracture and frictional sliding. Such mylonite, termed clastomylonite, is said to undergo frictional–viscous (FV) mylonitic flow. Its bulk strength is expressed as a function of bimineralic composition, temperature, effective pressure, and shear strain rate, as well as the material parameters for the constituent minerals. The FV flow law predicts that the rheology of clastomylonite is predominantly non-linear viscous and only slightly pressure-sensitive for most bimineralic compositions. FV mylonitic flow is shown to occupy a depth interval between cataclastic flow involving adhesive wear in the upper crust and fully viscous mylonitic flow at greater depths. The transition from adhesive wear to FV mylonitic flow is related to the onset of dislocation creep (glide-plus-climb) in the weakest phase and is inferred to coincide with a crustal strength maximum. A peak strength of about 80 MPa is calculated with a combination of Byerlee's constants for frictional sliding of granite and the FV flow law for granitic clastomylonite (30 vol.% quartz) at elevated fluid pressures. This value falls within the range of flow stresses independently obtained from quartz palaeopiezometry in many greenschist facies, granitic mylonite zones.
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