Transitional±tensile fracture propagation: a status report T. Engelder Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA Received 17 August 1998; accepted 13 January 1999 Abstract 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 ®bers are possible candidates. While these veins clearly show shear oset, this is not conclusive evidence that a shear traction was present at 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 veri®cation include ®eld examples of conjugate joint sets with small acute angles, but these conjugate joints form neither simultaneously nor in the stress ®eld 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 it is not explained by a parabolic Coulomb±Mohr failure envelope as several recent structural geology textbooks have suggested. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Most structural geologists recognize two types of brittle fractures: joints and shear fractures. A common perception promulgated in many recent structural ge- ology textbooks is that joints and shear fractures are the end members of a spectrum of brittle fracture types, and that some fractures are a hybrid of both end members (cf. Hancock, 1985). Such fractures pre- sumably open as tensile cracks and simultaneously propagate in the same plane while subject to a shear traction. As an example, Price and Cosgrove (1990) show a vein for which ``it is clear that the planar frac- ture developed in hybrid extension and shear failure'' (Fig. 1). Similar fractures are known by a variety of names: oblique shear fractures (Dennis, 1972), hybrid shear fractures, conjugate hybrid joints, conjugate shear joints (Hancock, 1985), joints formed in the tran- sitional tensile ®eld (Suppe, 1985), hybrid shear/dila- tional fractures (Price and Cosgrove, 1990), hybrid members or hybrid joints (Bahat, 1991), mixed-mode fractures (Twiss and Moores, 1992), fractures that are a combination of extension and shear (Davis and Reynolds, 1996), and transitional±tensile fractures (van der Pluijm and Marshak, 1997). van der Pluijm and Marshak (1997) noted that ``Most fracture specialists claim that transitional±tensile fractures do not occur in nature, and point out that no experiments have yet clearly produced transitional±tensile fractures in the lab.'' The purpose of this short paper is to amplify on their comment about transitional±tensile fracture propagation and to move the structural geology com- munity toward a clearer understanding of the nature of hybrid shear fracturing. The hybrid shear fracture hypothesis has its origin more than a half century ago when engineers observed that the Coulomb±Mohr failure envelope was concave to the normal stress axis (Leon, 1934). Laboratory tests con®rm this behavior for rocks subject to com- pressive stresses (Fig. 2). Furthermore, a parabolic fail- Journal of Structural Geology 21 (1999) 1049±1055 0191-8141/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(99)00023-1 www.elsevier.nl/locate/jstrugeo E-mail address: engelder@geosc.psu.edu (T. Engelder)