RECRACKING OF JOINTED ROCK MASSES IN THE SYDNEY BASIN, NEW SOUTH WALES, AUSTRALIA

Fracture mapping of Late Permian-Early Triassic flat-lying sedimentary rocks in the Sydney Basin, New South Wales, Australia, shows that joints developed originally in extension were faulted in subsequent events. Joints with a regional distribution fall into two (early and late) formed groups. Group I joints propagated horizontally and never interfered with each other. These joints were subsequently reworked or recracked. Recracking commenced with jointing and continued with lateral slip. Faulted joints grew horizontally by the linking of recracked segments. En echelon arrays are the result of the vertical propagation of faulted joints into intact rock. Recracking of rock also resulted in the formation of sets of secondary joints (Group II). The sense of movement on conjugate faulted joints and the orientation of the sets of secondary joints are related to three compressional stress fields. The intensity of recracking and the amount of lateral slip is mostly related to the strength of infilling materials, the length and continuity of the parent joint, the angle between the existing fractures and the maximum compression direction, and the number of compressional events imposed on the fracture.