Surface joint systems, tectonic stresses and geomorphology: a reconciliation of conflicting observations

This note seeks to reconcile the widespread small-scale fractographic observations indicating that surface joints are extension fractures with the frequently observed occurrence of such joints as what appear to be nearly orthogonal conjugate sets, their strikes fitting the shear lines of the neotectonic stress field. It is suggested that the local appearance of joints as extension fractures may have nothing to do with their orientation in the large-scale neotectonic stress field, inasmuch as extrapolations from a local scale to plate-tectonic dimensions are quite speculative. Thus, the local extension-characteristics of the joint surfaces may be acquired at the latest stage of their genesis as a result of the corresponding rock faces becoming exposed, but the geometrically orientational attributes may be conditioned by the shear in the surrounding large-scale neotectonic stress field; a new possible mechanism for reconciling the conflicting local and large-scale observations is suggested.