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_Journal of Structural Geology 32 (2010) 288–305_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Formation of systematic joints in metamorphic rocks due to release of residual elastic strain energy, Otago Schist, New Zealand_ _R. Weinberger a,*, Y. Eyal b, N. Mortimerc Geological Survey of Israel, 30 Malkhe Israel 95501, Jerusalem, Israel b Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel c GNS Science, Private Bag 1930, Dunedin, New Zealand_ _article info_ _Article history: Received 29 May 2009 Received in revised form 19 November 2009 Accepted 6 December 2009 Available online 21 December 2009_ _Keywords: Brittle deformation Ductile deformation Joint Lineation Schist Otago Schist Residual strain_ _abstract_ _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._ _? 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The most common geological structures created in the upper crust are joints. Joints are surfaces, fractures or partings in a rock across which there has been no displacement (Bates and Jackson, 1987). Systematic joints form regular, planar, sub-parallel sets. They profoundly control the shape of many spectacular landforms, and play an important role in subsurface transport of fluids such as water, magma, contaminants and hydrocarbons (e.g., Pollard and Aydin, 1988; Gross and Eyal, 2007). Establishment of reliable relationships between joints and their cause provides important tools for inferring the loading conditions and mechanical behaviour of rocks. It is widely agreed that joints form as mode I fractures perpendicular to the least compressive principal stress, and perpendicular to the maximum extension (elongation) for a coaxial strain field (e.g., Engelder and Geiser, 1980). The main jointing mechanisms (Engelder, 1985; Bahat et al., 2005) are responses of the host rock to a regional or local stress field, effect of pore pressure and hydro-fracturing, stress relaxation due to rock uplift, and or jointing due to material shrinking (e.g., columnar joints in basalts). These mechanisms are all different manifestations of the brittle (non-penetrative) deformation of cold rocks under low lithostatic pressure, whereas ductile (penetrative) deformation occurs in a different tectonic environment where rocks are hot and under high lithostatic pressure. Therefore, in most cases, ductilely deformed rocks should have cooled down and lithostatic pressure been relieved by exhumation before the beginning of jointing._ _? 2009 Elsevier Ltd. 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