Although the number of observations of deep fault structure is small, the available exposed faults provide some information on the deep structure of a fault.
A fault “zone” consists of several smaller regions defined by the style and amount of deformation within them.
The center of the fault is the most deformed and is where most of the offset or slip between the surrounding rock occurs.
The region can be quite small, about as wide as a pencil is long, and it is identified by the finely ground rocks called cataclasite ( the ground up material found closer to the surface, gouge).
From all the slipping and grinding, the gouge is composed of very fine-grained material that resembles clay.
Surrounding the central zone is a region several meters across that contains abundant fractures.
Outside that region is another that contains distinguishable fractures, but much less dense than the preceding region. Last is the competent “host” rock that marks the end of the fault zone.
