One model for the development of hybrid shear fractures is transitional-tensile fracture propagation, a process described as the in-plane propagation of a crack subject to a shear traction while held open by a tensile normal stress. Presumably, such propagation leads to a brittle structure that is the hybrid of a joint and a shear fracture. Crack-seal veins with oblique fibers are possible candidates. While these veins clearly show shear offset, this is not conclusive evidence that a shear traction was present of the time of initial crack propagation. Many recent structural geology textbooks use a parabolic Coulomb-Mohr failure envelope to explain the mechanics of transitional-tensile fracturing. However, the laboratory experiments cited as demonstrating transitional-tensile behavior fail to produce the fracture orientation predicted by a parabolic failure envelope. Additional attempts at verification include field examples of conjugate joint sets with small acute angles, but these conjugate joints form neither simultaneously nor in the stress field required by the transitional-tensile model. Finally, linear elastic fracture mechanics provides strong theoretical grounds for rejecting the notion that individual cracks propagate in their own plane when subject to a shear traction. These observations suggest that transitional-tensile fracture propagation is unlikely to occur in homogeneous, isotropic rock, and that is not explained by a parabolic Coulomb-Mohr failure envelope as several recent structural geology textbooks have suggested.
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