sgr ms thesis - University of Maine

friction along the closed crack surfaces caused by the compressive normal
stresses produce a local stress distribution slightly different from those cracks
that experience purely tensile fracture. Tensile cracks begin to form along the
direction of δ to allow accommodation of shear along the main body of the
fracture plane (Mode II and III). The orientation of the newly developed tensile
crack tends to migrate parallel to the σ 1 direction. Because of this, shear
fracturing in a compressed rock is actually dependent on the development and
growth of small tensile fractures (Grady and Kipp, 1987; Twiss and Moores,
2007).
5.1.2. The Self-Similarity of Fracture Patterns
Although Griffith fracture theory describes initiation of cracks at the
microscopic level, fracture behavior can be described this way at any scale.
Brecciated rocks display the scale independent, or self-similar, characteristics of
fracture propagation by the patterns produced along fracture surfaces and size
distribution of clasts. Physical brecciation is the result of fracture propagation
carried over many scales. Breccias form from the nucleation, propagation, and
intersection of these fracture paths (Laznicka, 1988). It is the self-similar pattern
of these fracture surfaces and frequency of intersections that define a breccia.
The form of the repeated pattern is dependent on the mechanism of
fragmentation, and study of these patterns can provide information on the
characteristics of the resulting breccia. The numerous different mechanisms that
produce breccias provide noticeable differences in their physical characteristics;
48