This study relates to jointing in granite that took place in the crust before uplift, thus offering an insight into paleo–fracture processes. Our investigation reveals a unique set of joints within granite of the South Bohemian Pluton exposed in the Borsov Quarry, southeast of Prague in the Czech Republic. These joints occur in close proximity and exhibit a great variety of fracture-surface morphologies. The joints in the Borsov granite may be divided into four groups based on fractographic features: (1) partial mirror planes without fringes, (2) mirror planes rimmed by en echelon fringes, (3) complex en echelon fringes, and (4) hackled fringes. On the basis of theoretical, experimental, and fractographic data obtained in outcrops, we have constructed a curve for the change in crack velocity V as a function of the stress-intensity factor KI, and we have plotted the various joints in the Borsov granite on this curve. The spread of the joints along the curve indicates that the joints terminated under widely different physical conditions, possibly responding to repeated extensional events and different fracture mechanisms. Hence, a new quantitative method of joint classification is introduced.
Such a great variety of fractographic features is unknown from joint sets that occur in a single quarry in sedimentary rock, because their formation always relates to a particular fracture mechanism. We argue that the joints in the Borsov granite formed before uplift in a fluid-enriched environment, while the granite responded to diverse, local effective-stress conditions.