During the Laramide orogeny (approximately 70 million years ago), regional compression created deep-seated shear zones. Zachary Wells’ fissures align with a buried Precambrian suture zone. These deep cracks acted as weaknesses.
We propose that Z-Cracks arise from a ( G ) as a function of propagation angle ( \theta ). In standard materials, ( G(\theta) ) has a single sharp maximum. In Z-Crack materials, the anisotropy creates two local maxima within 15° of each other (Figure 2). The crack tip thus "dithers" between these modes.
: A technique using orthotropic photoelasticity to measure crack parameters in composite materials.