New fractographic aspects of natural and arti®cial fractures in chalks from the Upper Galilee, Israel, and experimental fracture in Perspex Dov Bahat a, *, Avinoam Rabinovitch b a Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel b Physics Department, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel Received 12 May 1999; accepted 5 May 2000 Abstract The fractographies of an arti®cial fracture that was formed by a blast and a natural fracture cutting chalks that display mirror planes and hackled fringes are similar. In both fractures the mirror and fringe are separated by clear boundaries, and the locations of the maximum hackle intensity and maximum f angle (produced between the mirror plane and the fringe) coincide with the positions where segments of the mirror boundaries that have dierent curvatures meet. On both the natural and arti®cial fractures, radial hackles associated with the dierent boundary segments focus on distinct origins. The f angle varies from 08 to 308 2 58 on the natural joint and from 308 2 88 to 508 2 88 on the arti®cial fracture. The coinciding positions of hackle intensity and f maxima observed in the two fractures match well with bifurcating fractures induced by experimental Hertzian fracture in Perspex. These are further supported by a geometric mismatch analysis. The present results demonstrate that the K I increases, which occur in positions where fractures of dierent ellipticities meet, enhance hackle intensity, crack- branching and the value of f. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction 1.1. General Fractures that develop both arti®cially and in nature often display asymmetric mirror planes that represent mergers of earlier smaller cracks (Bahat et al., 1999). The early cracks may propagate independently under dierent local stresses and will have dierent velocities. Therefore, their boundaries commonly dier in curva- ture, and when they coalesce their mirror boundaries may not lie on the same arc. Such geometrical dier- ences may also occur when the propagation of a single crack is disturbed by heterogeneous boundary con- ditions. Our results show that stress was concentrated at the junctions between mirror boundaries with dier- ent geometries. The present study relates to the eects of this stress concentration increase on fracture via the combined observations made on arti®cial and natural fractures, supplemented by fracture experimentation on Perspex and by a mathematical analysis. 1.2. Classi®cation Joints that have fringes (Hodgson, 1961) may be divided into distinct groups according to the following criteria: 1. Sharp boundaries between the parent joint and the fringe vs. non-sharp boundaries between the two (i.e. discontinuous vs. continuous transition from the planar joint to the en eÂchelon fringe, respect- ively, Bahat, 1986). 2. Fringes consisting of en eÂchelon segments vs. fringes populated by hackles (Bahat, 1986). 3. Natural vs. arti®cial joints (Bahat, 1991, p. 209). Journal of Structural Geology 22 (2000) 1427±1435 0191-8141/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(00)00055-9 www.elsevier.nl/locate/jstrugeo * Corresponding author. Fax +972-7-6472-997. E-mail address: bahat@bgumail.bgu.ac.il (D. Bahat).