Determining finite longitudinal strains from oppositely-concave microfoldsin and around porphyroblasts: a new quantitative method

This paper describes a precise new method for determining finite longitudin al strains in porphyroblastic metamorphic rocks, which makes use of opposit ely-concave microfolds (OCMs) formed by heterogeneous strain of the matrix around porphyroblasts. The initial spacing between two foliation surfaces i s measured inside a porphyroblast and compared to the spacing between the s ame two surfaces in the matrix, which results in a measure of extension (e) experienced by the rock during and/or after porphyroblast nucleation. A na tural example is provided by the well-known 'millipede' plagioclase porphyr oblasts from the Robertson River Metamorphics in Queensland, Australia. Twe nty-four measurements were made from 22 serial thin sections cut parallel t o both the X-Z and X-Y planes of finite strain, giving an average extension of 1.72 parallel to the X-direction of finite strain. The least-squares be st-fit line to a plot of initial length vs change in length gives an R-2 va lue of 0.998. A minimum estimate of maximum shortening (negative e) was als o made by measuring the total lengths of S-1 folia that had been crenulated during OCM formation, giving a value of -0.54, which falls short of the -0 .63 expected for constant-volume, plane-strain deformation. Because the OCM method is particularly suited to metapelites, results may provide new insi ght into mechanisms of folding and crenulation cleavage development, pressu re-temperature-time-deformation histories, mass transport during deformatio n and metamorphism, and kinematic studies of porphyroblast behavior (rotati on vs non-rotation) during ductile deformation. (C) 1998 Elsevier Science L td. All rights reserved.