Joint formation in granitic rock of the Sierra Nevada

A single steeply dipping joint set in the Mount Givens Granodiorite, central Sierra Nevada, was studied to clarify the mechanics of fracture and joint formation in granitic rocks. The joints were filled with fluid during, or immediately following, formation; these fluids deposited epidote and chlorite within the joints. Examination of lithologic markers in outcrop and thin section demonstrates that relative displacements are normal to the joint surfaces. These observations rule out a shear origin for these joints. The measured extensional strain acommodated by joint dilation is on the order of 1 × 10 −4 to 5 × 10 −4 . A few joints in the area exhibit small strike-slip offsets. In these joints, the mineral fillings are sheared, indicating the strike-slip motion postdated the jointing. Individual joints consist of numerous subparallel, planar segments. The lengths of joints range from metres to nearly 100 metres. Shorter joints are more abundant than longer joints. The observed distribution of joint lengths is thought to result from the elastic interaction of adjacent joints. Shorter joints are prevented from further propagation by their long neighbors. Between mapped joints, small cracks that have lengths of several centimetres are found parallel to the longer joints. These cracks represent a growth stage between grain-scale microcracks and macroscopic joints. A method is developed for estimating the tensile stress responsible for initiating joint growth. The method requires knowledge of the extensional strain accommodated by joint dilation and the spatial density of joints, both of which can be determined by field observations. Calculations based on observations of joints in the Florence Lake area indicate relative tensile stresses (average remote stress plus internal fluid pressure) of approximately 1 MPa to 40 MPa. These values of stress and estimates of initial crack length are used to estimate the quasi-static fracture toughness of the granodiorite. The calculated fracture toughnesses range from 0.04 Mpa·m 1/2 to 4 Mpa·m 1/2 . The stress and fracture toughness estimates are compatible with existing data from laboratory fracture experiments.