Flow folding

Abstract

Principles of fluid mechanics are used to interpret the structures in high-grade metamorphic rocks which behaved like viscous fluids and thus were deformed by nonuniform laminar flow. The structures investigated are in a typical Grenville assemblage of gneisses, marbles, and quartzites in the Westport area of southeastern Ontario. These rocks are metamorphosed to granulite and upper amphibolite facies, and the contrasting rock units are ideal for structural study. Like most high-grade metamorphic rocks, they possess an irregularly spaced stratiform foliation that traces out the folds and is parallel to the original bedding. The structural pattern is one of dominantly upright, roughly similar folds in which a single horizon may be repeated at different structural levels by refolding about the same axis. The distribution of map units is thus complex, but the major folds persist for long distances and have parallel axial planes that represent flow laminae. Synchronous refolding appears to be an inevitable part of the flow process and is the result of unsteady flow. It occurs primarily when advancing parts of the flow pattern (anticlines) contribute material to adjacent retarded zones (synclines), a phenomenon also characteristic of the movement of oil scum on water. Stratiform foliation develops along bedding planes which are interfaces between layers of different composition having different rates of flow. Hence it is a byproduct of flow and does not develop in originally homogeneous rocks. Three "end member" types of fold are discussed. These are: 1) flexural slip folds (concentric); 2) shear folds (similar folds with axial plane cleavage); and 3) flow folds (similar folds without visible axial plane cleavage or foliation). These varieties can all occur in a single environment but they are, respectively, most common in: 1) unmetamorphosed rocks, 2) low-grade metamorphic rocks, and 3) metamorphic rocks in upper amphibolite facies or higher. They result from the changing nature of strain from solid shear to viscous flow with increasing metamorphic grade, and not from different types of stress.