The role of compressive stresses in jointing on Vancouver Island, British Columbia

This study demonstrates that joint-parallel compressive stresses were integral to the development of joint sets on Vancouver Island, British Columbia. The mapped study area contains paleostress indicators in the form of calcareous concretions, which have multiple, internal fractures in precisely the same orientation as one of the surrounding joint sets in the sandstone matrix. Field and laboratory tests indicate that the stiffest concretions are the most likely to be fractured; however, results from numerical simulations using measured rock properties preclude an origin for the concretion fractures from either a far-field uniaxial tensile or compressive stress. Fracturing is only found to be possible if the concretions possessed a lower Poisson's ratio than the sandstone at the time of fracturing. In the latter case, a far-field uniaxial compressive stress may have generated tensile effective stresses in the vicinity of high modulus concretions, seeding the field site with an initial population of concretion fractures and joints. Given the close spacing of some joints, their extension cannot be satisfactorily explained without invoking grain-scale compression-driven tensile fracturing mechanisms. (C) 2002 Elsevier Science Ltd. All rights reserved.