The Formation of Orthogonal Joint Systems: New Observations from Limestone Beds in the Region of Havre-Saint-Pierre (Quebec, Canada)

Tessellated pavements, composed of vertical orthogonal joints, are a common feature in flat-lying sedimentary strata from foreland basins and platforms; however, their origin remains enigmatic. We report new results on orthogonal joints from autochthonous middle Ordovician limestone beds in the region of Havre-Saint-Pierre, Quebec, Canada (north shore of the Saint-Lawrence River). A set of cross joints (which connect longer systematic joints) have a spacing which is consistently larger than systematic joints by a factor of ~1.5. The mean joint-spacing-to-bed-thickness ratios (s/t = 4.3 for systematic joints, and 6.4 for cross joints) obtained in this study are much larger than values (s/t=~1.0) measured from the folded (higher strain) strata in the allochthonous zone along the south shore of the Saint-Lawrence River. This highlights the important role of tectonic strain in decreasing fracture spacing. The Weibull moduli obtained from the dependence of joint spacing on bed thickness are 1.82 and 1.52 for the limestone when systematic and cross joints formed, respectively. The formation of cross joints is interpreted to have been related to local swaps of the principal stress axes consequent to the development of systematic fractures rather than from a stress switch from compressive to tensile after a critical s/t ratio (1.0) of the systematic joints has been reached (as has been hypothesized in previous studies). The presence of ladder-shaped orthogonal joints is proposed to represent a state of effective stress where σ 1 *>>0>σ 2 *>σ 3 * and where (σ 2 *-σ 3 *) is smaller than the range of fracture strength scattering, thus providing a new mechanical model for the formation of orthogonal joint systems.