Clarke, A.P., Vannucchi, P 2020
Structural anisotropy: Using image analysis to quantify block-in-matrix fabrics
Abstract
Block-in-matrix rock assemblages — such as mélanges — are structurally complex units whose fabric is primarily defined, not by planar or linear features as most rock units are, but by the orientation and aspect ratio of irregularly shaped blocks. Despite this, previous attempts to quantitatively characterize mélange fabrics have predominantly focused on measuring the foliation using traditional techniques. Here we introduce a method of characterizing block-in-matrix fabrics using image analysis of field photos to discern the aspect ratios and orientations of blocks and define the structural anisotropy of the rock unit. We also include the software to calculate structural anisotropy and the trend of the fabric from image analysis data.
Summary
The discipline of structural geology largely concerns itself with the shape and orientation of either planar features (such as bedding, foliation, and fractures) or linear features (such as fault striations and elongated minerals). Describing the planar and linear features of a rock mass is the appropriate way of characterising most types of rock units. However, mélanges are a type of rock unit which are instead characterised by their “block in matrix” texture, meaning that the dominant structural features are blocks of one rock type within a matrix of another. While they often do have planar and linear features, these often deflect around the blocks rather than forming a consistent orientation. While planar and linear features can be easily measured by traditional (e.g., compass-clinometre) and modern (e.g., picking from a photogrammetric model) techniques, block-in-matrix fabrics are more difficult to quantify.
This paper presents a means to quantify the orientation and degree of alignment of mélange blocks using image analysis. On 2D outcrop surfaces and on photographs, mélange blocks can be represented as ellipses. Using ImageJ, we extract the aspect ratio (meaning the degree of elongation) and the orientation of the long axis of each ellipse. We then plot this data on a radial graph which produces a new ellipse representing the average aspect ratio and orientation; the shape and orientation of which allow use to quantify the block-in-matrix fabric.
To demonstrate this technique, we performed image analysis on photos from three mélanges around the world: the Gwna Mélange in Wales, the Osa Mélange in Costa Rica, and the Shimanto Belt in Japan. These images consisted of drone-acquired aerial imagery, field photos, and microscopy imagery.