The process zone: A microstructural view of fault growth

The deformation associated with several small, brittle faults was investigated on both microscopic and macroscopic scales. While the dominant macroscopic structures are solution cleavage planes and secondary shear fractures, the dominant microscopic deformation structures are healed tensile microfractures. The fault-related microfractures display densities and orientations distinct from the background microfracture population. These densities and orientations are consistent with formation within the altered stress fields of propagating shear fracture tips. This microfracture population is used to define the fault process zone associated with growth of the macroscopic fault plane. Process zone microfractures show logarithmic density increases with proximity to the fault, a constant maximum density that is independent of fault length, and orientations which can be used to infer the direction of propagation of the fault plane. The width of the process zone scales linearly with fault length with a proportionality constant of the order of 10(-2).