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 develo pment. The results suggest that the stress which creates the second jo int set can result from: (i) relaxation effects; (ii) slight tension d ue to warping of the bands defined by the first parallel fractures; an d (iii) local and/or regional reversals between sigma2 and sigma3. A ' ladder' pattern, formed by the combination of an initial set of long p arallel joints and associated non-cross-cutting joints of the second s et, is obtained if the shear strength of the initial joints is low dur ing the development of the second set. A 'grid' pattern, where both se ts mutually cross-cut, occurs when the shear strength of the initial j oint set is high, possibly the result of a high normal stress or heali ng and could result from two independent stress events. An intermediat e pattern comprising cross-cutting and abutting orthogonal cross-joint s can form if the shear strength of the initial joints was intermediat e and/or variable during the development of the second set. Mutually a butting joint sets are observed within each pattern and could result f rom stress reversals or low differential stress during the final stage s of joint development. The classical presentation of joint data using rose diagrams cannot distinguish between the wide variety of orthogon al joint patterns. Maps of fracture intersections should compliment th e orientation data. We discuss methods to estimate the joint pattern w here intersection data are absent.