Oppositely-Concave Microfolds

Oppositely-concave folds can form when a rock volume experiences heterogeneous extension at a moderate or high angle to the anisotropy being folded (e.g. bedding or a tectonic foliation). These fold structures can form at a variety of scales, and where they have been observed the heterogeneous extension is caused by the presence of relatively stiff regions in a relatively homogeneous matrix. Below are two examples of OCMs.

Photograph from the Cooma Complex, southeastern Australia, showing oppositely-concave folds developed around a relatively rigid concretion. The well-developed, folded foliation was heterogeneously extended around the concretion during the folding event.

Photomicrograph showing oppositely-concave microfolds in and around a plagioclase porphyroblast from Queensland, Australia. The pre-existing, near-vertical foliation extended heterogeneously in the surrounding matrix, but not in the rigid porphyroblast.

Bell and Rubenach (1980, Tectonophysics, 68, T9-T15) first described oppositely-concave microfolds in and around plagioclase porphyroblasts, and Bell used these observations in the develpment of a model for how deformation partitions in deforming rocks (Bell, 1981, Tectonophysics, 75, 273-296). Owing to the supposed importance of this microstructure, I set out to examine it more closely. I cut 75 serial thin sections at 1.5 mm intervals through a sample from the classic Bell & Rubenach location, and my colleague Ross Moore (Mathematics Department, Macquarie University) and I used some of these serial sections to reconstruct OCMs in and around a plagioclase porphyroblast (Johnson & Moore, 1996; reconstruction details can be found here). Johnson & Bell (1996) then defined five types of OCMs, and showed that OCMs in general cannot be used as strain-path indicators, contrary to previous suggestions.

Cutaway view of reconstructed oppositely-concave microfolds in and around the plagioclase porphyroblast above, which is coloured green.

More recently, Johnson & Williams (1998) showed how OCMs can be used to make precise quantitative measurements of extension (also known as elongation). To do this, the initial spacing between two foliation surfaces is measured inside a porphyroblast and compared to the spacing between the same two surfaces in the matrix, which results in a measure of extension experienced by the rock during and/or after porphyroblast nucleation. Johnson & Williams (1998) made twenty-four measurements from 22 serial thin sections cut parallel to 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 best-fit line to a plot of initial length versus change in length gives an R-squared value of 0.998. Because the OCM method is particularly suited to metapelites, results may provide new insight into mechanisms of folding and crenulation cleavage development, pressure - temperature - time - deformation histories, mass transport during deformation and metamorphism, and kinematic studies of porphyroblast behavior (rotation vs. non-rotation) during ductile deformation.