ANALOG SIMULATION OF NATURAL ORTHOGONAL JOINT SET FORMATION IN BRITTLE VARNISH

Two orthogonal joint sets are commonly observed in ''posed sedimentary rocks with a wide variety of abutting and cross-cutting relationships. Brittle varnish analogue models are carried out in conjunction with field studies, in order to classify the different orthogonal fracture patterns and constrain the mechanical basis of orthogonal joint development. The results suggest that the stress which creates the second joint set can result from: (i) relaxation effects; (ii) slight tension due to warping of the bands defined by the first parallel fractures; and (iii) local and/or regional reversals between sigma2 and sigma3. A 'ladder' pattern, formed by the combination of an initial set of long parallel joints and associated non-cross-cutting joints of the second set, is obtained if the shear strength of the initial joints is low during the development of the second set. A 'grid' pattern, where both sets mutually cross-cut, occurs when the shear strength of the initial joint set is high, possibly the result of a high normal stress or healing and could result from two independent stress events. An intermediate pattern comprising cross-cutting and abutting orthogonal cross-joints can form if the shear strength of the initial joints was intermediate and/or variable during the development of the second set. Mutually abutting joint sets are observed within each pattern and could result from stress reversals or low differential stress during the final stages of joint development. The classical presentation of joint data using rose diagrams cannot distinguish between the wide variety of orthogonal joint patterns. Maps of fracture intersections should compliment the orientation data. We discuss methods to estimate the joint pattern where intersection data are absent.