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.