It is convenient to consider fault growth under two related categories: Longitudinal or vertical and transversal or lateral growth. The former results in increasing lengths and heights of faults, while the latter results in enlargement of fault zones sideways achieving greater widths or thicknesses. As pointed out earlier, these dimensional parameters are interrelated and scale primarily with the fault slip. The longitudinal fault growth occurs in three mechanisms as referred to under 'Fault Propagation:' (1) in-plane extension of faults as shear fractures, (2) out-of-plane extension of faults initially as opening mode (or closing mode) fracture at an acute angle to the fault, and (3) fault branching involving additional shear fractures at various configurations with respect to the main fault geometry. These are covered under the links, 'Fault Initiation,' 'Fault Propagation,' 'Fault Interaction,' 'Echelon Faults,' and 'Splay Faults.' We here cover the growth of fault zones by linkage and coalescence of nearby faults focusing on progressive evolution of faults and related fractures as systems.

Let's first recall that there are a number of mechanisms for fault initiation and growth controlled primarily by the failure modes which in turn depend on loading, strength, and rheology of rocks. Some faults form by localized strain into bands with large length-to-width ratios. We consider these under shear bands. Others form by shearing of initial weaknesses such as depositional features like bedding interfaces or deformation features which had formed by prior tectonic events in either opening mode or closing mode. Yet some others form by chemical transformation. At the expense of being repetitious, we recall that these prior structures are inherently discontinuous and have limited lengths, heights, and small thicknesses or apertures, and therefore, their growth onto mature faults requires elaborate linkage and coalescence. We select certain categories to focus on in this section to avoid variations due to lithology, rheology, and structural settings. The first part is based on the strike-slip fault development in Aztec sandstone exposed in Valley of Fire State Park, Nevada. Then, we briefly summarize growth of thrust faults and related folds and fractures in compressive tectonic environments. Finally, we consider extensional deformation of multilayered units made up of alternating stiff and soft units.

We discussed above the growth of primarily strike-slip faults in a massive and relatively stiff rock unit, an aeolian sandstone. Suffice it to say that the mechanism of other types of faults with various types of dip-slip in rheologically similar rocks under similar conditions should be the same with only certain variations in the orientations of the structural components, which have been described under 'Case Studies.'