Strain studies of cleavage development in the Chelmsford Formation, Sudbury Basin, Ontario

Abstract

Cleavage formation in the Precambrian Chelmsford Formation of the Sudbury Basin, Ontario, is directly related to the finite strain and deformation path. Finite strain was measured at the outcrop scale from the shapes and orientations of deformed concretions within sandstones, at the microscopic scale from the relative positions of quartz grain centers, and via quantitative correlations between the anisotropy of magnetic susceptibility and strain. Two-dimensional strains measured in different sections at each site were combined into three-dimensional strain ellipsoids at 16 sites distributed throughout the Chelmsford Formation outcrop. Using the magnetic susceptibility correlation, a further 527 individual strain ellipsoids were obtained throughout the sites. A prominent strain gradient is mirrored by the progressive development of cleavage in the Chelmsford Formation, with strain and fabric development increasing in intensity from NW to SE. At the least deformed sites, cleavage is weak or absent. The finite strain reflects a combination of vertical compaction followed by coaxial, tectonic layer-parallel shortening acting in a subhorizontal, NW-SE direction. Pressure solution is the dominant deformation mechanism responsible for the formation of cleavage. Material removed by pressure solution is redeposited as pressure fringes on quartz grains, an observation which is compatible with the form of the deformation path which indicates little effect due to volume loss in cleavage formation in these sandstones. Folds within the Chelmsford Formation indicate formation by buckling but have very low limb dips. Decomposition of the finite strain tensor into a compactional part and a layer-parallel shortening part suggests that fold initiation and amplification was preceded by about 38% layer-parallel shortening. The large number of strain ellipsoids obtained from the Chelmsford Formation provide a significant data base from which a number of important tectonic problems can be examined including the predeformational shape of the Sudbury structure.