Frictional-viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength

A theoretical, composite flow law is presented for mylonite containing inte rconnected layers of a weak mineral undergoing power law creep and porphyro clasts of a stronger mineral undergoing fracture and frictional sliding. Su ch mylonite, termed clastomylonite, is said to undergo frictional-viscous ( FV) mylonitic flow. Its bulk strength is expressed as a function of biminer alic composition, temperature, effective pressure, and shear strain rate, a s well as the material parameters for the constituent minerals. The FV flow law predicts that the rheology of clastomylonite is predominantly non-line ar viscous and only slightly pressure-sensitive for most bimineralic compos itions. FV mylonitic flow is shown to occupy a depth interval between catac lastic flow involving adhesive wear in the upper crust and fully viscous my lonitic flow at greater depths. The transition from adhesive wear to FV myl onitic 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 m aximum. 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 f rom quartz palaeopiezometry in many greenschist facies, granitic mylonite z ones. (C) 1999 Elsevier Science B.V. All rights reserved.