Initiation of brittle faults in the upper crust: a review of field observations

A review of field observations reveals that faults in the upper crust initiate in one of three styles: linkage of pre-existing structures, linkage of precursory structures, or by localization of slip in a precursory shear zone. The first two styles can be characterized by three general stages. During stage 1, faults initiate by shear along pre-existing structures (formed during an earlier event), or by the initiation of precursor structures (formed earlier in the same deformation event). Stage 2 involves the pre-existing or precursor structures becoming linked by differently orientated structures, as stresses are perturbed within the developing fault zone. Linkage allows displacement to increase. A through-going fault develops during stage 3. The third style of fault initiation requires the development of a shear zone, which may change local mechanical properties and lead to faulting. These observations highlight the importance of mode I fracturing in the initial stages of faulting. Rock architecture and the orientation of layering with respect to the principal stresses exert a strong influence over the style of fault initiation. Most brittle faults in the upper crust initiate from precursory structures, and the mechanism for fault growth changes with scale. Thus we should not expect a simple, universal slip–length scaling relationship for faults.