Mechanism of exfoliation joint formation in granitic rocks, Yosemite National Park

Fractographic techniques reveal mechanical aspects of exfoliation in granit ic rocks at Yosemite National Park, and electronic surveying provides infor mation on their attitudes and dimensions. In the middle elevations of the c liff of El Capitan, exfoliation consists of early fractures a few meters to tens of meters in size which are fan-shaped. Fans at upper elevations poin t upwards, whereas fans at lower elevations point downwards parallel to the cliff. The fans interact with each other and merge into composite joints h undreds of meters in size, normal to the minimum compressive principal stre ss. This polarity indicates exfoliation by longitudinal splitting and buckl ing. The palaeostress causing the exfoliation on the southwestern side of H alf Dome is estimated to range between 0.01 MPa and 0.94 MPa. Differences i n fracture paleostresses of joints may be used in comparing their relative propagation velocities. It is suggested that the large exfoliation on the s outhwestern side of Half Dome had undergone a prolonged sub-critical growth before attaining rapid fracture velocities under post-critical conditions. (C) 1998 Elsevier Science Ltd. All rights reserved.