Fault zones: A complex issue

摘要

As recognized by John Wesley Powell as early as 1870 (Fig. 1), faults are complex 4-D structures: volumes of complexly deformed rock that evolve in their structural and fluid flow properties through time. Fault zones are composed of rocks in variable degrees of deformation states, often with a lower strain fracture-dominated "damage zone" surrounding a more highly strained heterogeneous "core zone" containing one or several slip zones, gouge and breccias, and oblique Riedel shears. A zone of faults at the larger scale on the other hand, is often considered as a discrete slip plane in a volume of otherwise intact rock. However, this intact rock is rarely really intact, and studies over the last twenty or so years have shown that this depends critically upon the scale of observation: the closer you look, the smaller the faults that you can see, hence the more faults you can see. As deformation evolves, many of the smaller faults in a zone switch off leaving the larger ones to carry on getting larger (widening the core zone), often cannibalizing some of the smaller faults in the process.