Transitional-tensile fracture propagation: a status report

One model for the development of hybrid shear fractures is transitional-ten sile 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 t he hybrid of a joint and a shear fracture. Crack-seal veins with oblique fi bers are possible candidates. While these veins clearly show shear offset, this is not conclusive evidence that a shear traction was present at the ti me of initial crack propagation. Many recent structural geology textbooks u se a. parabolic Coulomb-Mohr failure envelope to explain the mechanics of t ransitional-tensile fracturing. However, the laboratory experiments cited a s 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 a cute angles, but these conjugate joints form neither simultaneously nor in the stress field required by the transitional-tensile model. Finally, linea r elastic fracture mechanics provides strong theoretical grounds for reject ing the notion that individual cracks propagate in their own plane when sub ject to a shear traction. These observations suggest that transitional-tens ile fracture propagation is unlikely to occur in homogeneous, isotropic roc k, and that it is not explained by a parabolic Coulomb-Mohr failure envelop e as several recent structural geology textbooks have suggested. (C) 1999 E lsevier Science Ltd. All rights reserved.