Abstract

The fractography and conditions of propagation of joints that cut Devonian siltstones in the Appalachian Plateau, New York, and Eocene chalks from the Beer Sheva Syncline, Israel, are investigated. The joints cutting the siltstones are marked by S-type and C-type plumes, and the joints cutting the Lower Eocene and Middle Eocene chalks are marked by coarse and delicate plumes, respectively. The four plume types propagated under sub-critical (slow propagation) conditions. On the semi-quantitative fracture velocity (v) versus the tensile stress intensity (KI) curves, the S and C plume types fall in the KI = 0.073–0.79 MPa m1/2 and v = 2 × 10−4–10−2 m/s and KI = 0.073–0.79 MPa m1/2 and v = 10−6–10−4 m/s ranges respectively. The coarse and delicate plumes fall in the KI = 0.03–0.17 MPa m1/2 and v = 10−6–4 × 10−5 m/s and KI = 0.03–0.17 MPa m1/2 and v = 10−4–5 × 10−3 m/s ranges, respectively. Generally, slow plumes are relatively short, show periodicity, and typically exhibit superposition of arrest marks. On the other hand, faster plumes are longer and continuous, occur particularly in thinner layers, and show no superposition of arrest marks. There is a clear distinction between two en échelon segmentation end-members in the joint fringe, the ‘discontinuous breakdown type’ and the ‘continuous breakdown type’. There are also ‘transitional’ variations between the end-members. Only curved ‘discontinuous breakdown type’ boundaries of en échelon fringes can be equated with mirror boundaries.

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