Abstract

A study of fracture surface markings (FSM) on three joint sets, striking at 190, 335 and 280°, cutting a 2.5 m thick slice of interbedded limestones and mudstones of lower Liassic age, yields evidence which enables determination of the orientation of the palaeostress field at the time of joint formation. A specific element of the FSM, namely the border planes, or the twist hackle, has been used to determine the orientation of σ₁, in that these twist-hackle structures are considered to be developed normal to the remote tensile stress σ₃. In the case of the joint sets striking at 335 and 190°, the sense of rotation of the twist plane would suggest that for both sets σ₁ was aligned N-S and almost parallel to bedding. Other FSM features suggest that the 190° set propagated rapidly with S-type plumose markings being the most common. The small angle of border plane rotation (10–15°) suggests that all of the joint sets propagated under relatively low confining pressures. The sense of border plane rotation on the 335 and 190° sets suggests that there is a possible conjugate relationship between them. However, the other main joint set which strikes at 280° cannot be accounted for by simultaneous development in a N-S aligned σ₁ which might have caused the development of the 335–190° joint sets.