THE PROCESS ZONE - A MICROSTRUCTURAL VIEW OF FAULT GROWTH

The deformation associated with several small, brittle faults was inve stigated on both microscopic and macroscopic scales. While the dominan t macroscopic structures are solution cleavage planes and secondary sh ear fractures, the dominant microscopic deformation structures are hea led tensile microfractures. The fault-related microfractures display d ensities and orientations distinct from the background microfracture p opulation. These densities and orientations are consistent with format ion within the altered stress fields of propagating shear fracture tip s. This microfracture population is used to define the fault process z one associated with growth of the macroscopic fault plane. Process zon e microfractures show logarithmic density increases with proximity to the fault, a constant maximum density that is independent of fault len gth, and orientations which can be used to infer the direction of prop agation of the fault plane. The width of the process zone scales linea rly with fault length with a proportionality constant of the order of 10(-2).