Transient discontinuities revisited: pseudotachylyte, plastic instability and the influence of low pore fluid pressure on deformation processes in the mid-crust

Occurrences of pseudotachylyte cyclically introduced as melts into the Outer Hebrides thrust are demonstrably synkinematic with crystal-plastic mylonites. Classical interpretations of these rock types as, respectively, products of pressure-dependent frictional melting and thermally-activated intracrystalline deformation create a paradox in that these processes are, to a large extent, mutually exclusive. Detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy. The ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 ° C and stresses of 140–210 MPa, on which were imposed much higher transient stresses in response to heterogeneous, non-uniform flow. The latter conditions are sufficient for the development of plastic instabilities in feldspar-rich crust, if pore fluid pressures are sufficiently low. The latter is consistent with the absence of hydration during exhumation observed in the rocks under study. Low pore fluid pressure during thrust exhumation of deep crust enables activation of high-strength ductile processes dominated by dislocation glide which may be a prerequisite to instability. Whereas other studies have demonstrated the possible occurrence of such melt-generating instabilities, it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure-temperature conditions. Grain-size reduction to produce ultramylonites dominated by grainsize-sensitive flow is achieved by both deformation-induced dynamic recrystallization and crystallization of instability-generated melts.