Formation of systematic joints in metamorphic rocks due to release of residual elastic strain energy, Otago Schist, New Zealand

We present the first regional study of joints in the Jurassic-Cretaceous Otago Schist, New Zealand. The purpose of this study was to explore the origin and mechanism of joint formation in metamorphic rocks, especially any possible association between brittle and previous ductile deformation. The Otago Schist is cut by numerous systematic joints, up to tens of metres long, at any one exposure. We measured the orientation of joints, schist foliation planes, and quartz rods/mineral lineations at 46 sites across the Otago Schist, and calculated the spherical angles between their means. In relatively high metamorphic grade schists (greenschist facies) typically one systematic joint set has developed sub-perpendicular to penetrative foliation and lineation, irrespective of foliation and lineation orientations. This relationship also holds in lower grade schists (pumpellyite-actinolite facies), but more than one joint set is occasionally present. The flanking unfoliated schist protoliths (prehnite-pumpellyite facies) contain no systematic joint sets. A Late Cretaceous age for schist joint formation is indicated on the basis of lack of joint continuation into Late Cretaceous conglomerates that unconformably overlie jointed schists, cooling history, consistent orthogonality of joints with foliation and lineation, and lack of relationship of systematic joints to late Cenozoic plate-boundary features. We propose a model for joint formation during Late Cretaceous exhumation of the schist, and suggest that the systematic joints formed due to release of residual elastic strain energy preserved in the schists from Early Cretaceous ductile deformation.