Structural regimes and metamorphic facies

Abstract

This paper is an attempt to bring together the physical and chemical aspects of metamorphism by relating rheology to metamorphic facies in the orogenic environment. Five regimes are considered and defined within an idealised orogenic crust. The first regime extends from below the level of diagenesis to the point at which metamorphic reactions commence. Experimental studies suggest that at this level deformation by creep has a logarithmic relationship with time, but it is known that other factors such as fluid-pressure will profoundly influence actual strains. The onset of metamorphic reactions defines the upper level of the second regime, which is characterised by the formation of micas and amphiboles at the expense of pre-existing sedimentary minerals. These reactions are all associated with the release of volatiles, which will bring about a marked increase in the creep-rate at this level. These reactions are accompanied by new structural styles, similar folds, widespread schistosities and recumbent isoclinal folds on all scales. The third regime occupies the levels approximately represented by the epidote-amphibolite and amphibolite facies, characterised by a restricted mineralogy and similar Theologies for all the common rock types. Deformation will be primarily by grain boundary or intra-lattice diffusion following the β-creep law ( ϵ = β t n ; n < 1) and will proceed at lower rates than in regime two. Regime four is equated with the higher pressure-temperature amphibolite facies and is distinguished from regime three by two groups of reactions, the first, the dehydration of the remaining common hydrous minerals, and the second, partial melting with the release of a granitic fluid. Both reactions effect an increase in the creep-rate, accompanied by the formation of gneissose structures, extensive flow folding and possible diapiric movements: rates of strain will vary widely with bulk chemistry of the system. Regime five corresponds to the granulite facies, extending into the upper mantle. Deformation is by prolonged laminar flow, causing simple structural styles, with all rock types possessing similar rheology.