11 Multiple Fault Sets‌

First, a fracture of any type rarely occurs alone. Structures such as fracture sets and multiple fault sets are part of an ontology that provides information for understanding complex systems and facilitates building the historical progressive evolution of systems. Second, fault patterns that look similar to a casual observer may be formed by quite different mechanisms. As an example, the map in Figure 31 shows a complex fault system primarily in the Aztec Sandstone in the Valley of Fire State Park, Nevada, USA. This pattern is similar to the conjugate shear bands pattern in Figure 30, but it developed through a completely different mechanism, which is sequential shearing of initial joints and the resulting splays (Myers and Aydin, 2004; Flodin and Aydin, 2004; de Joussineau and Aydin, 2007a).

Figure 31 – Map showing a well-organized fault system primarily in the Aztec Sandstone that covers much of Valley of Fire State Park, Nevada, USA. Both the left- and right-lateral faults are distributed in a systematic pattern. This pattern is referred to as “apparent conjugate” to differentiate it from the conjugate shear band pattern shown earlier. The formation mechanism is referred to as segment and strand interactions in the next series of maps. From Flodin and Aydin (2004).

Complex fault patterns such as those in the map of Figure 31 may be analyzed by breaking them down into their components. The first step in this process is to decipher fault segments and their sequential interaction products. The maps in Figure 32 and Figure 33 are examples of the application of these concepts, the basis of which is the process of fault growth through interaction between adjacent segments and neighboring fault strands. Some parameters related to this process are defined in the diagram in Figure 34.

Figure 32 – Identification of fault segments along a primarily left-lateral strike-slip fault with over 80 m of slip. Blue lines are for left lateral (LL) fault traces and red lines for their splays, which later are often subjected to right-lateral (RL) shear. The fault traces are from Flodin and Aydin (2004) and the segment identification and interaction from de Joussineau and Aydin (2009).

Figure 33 – Fault segments and their influence on the localization of the next generation of faults with specific kinematics. The map identifies fault segments (LL: left-lateral, RL: right-lateral) for deciphering and breaking down complex fault systems. Segment identification from de Joussineau and Aydin (2009).

Figure 34 – Sketch defining parameters related to splay fractures. The fault and splay length are correlated: the longer the fault, the longer the splays. Modified from de Joussineau and others (2007).

The application of these concepts to field settings is not straightforward because fault growth is a continuous process. However, a field visit provides a snapshot of one instant in time that is the result of the full history. An analogy for reconstructing the fracturing history can be made to making a movie from still pictures of the system at discrete points in time.

Figure 32 is a map of a left-lateral fault with a little greater than 80 m offset (from Flodin and Aydin, 2004). The segments and the next generation of faults (i.e., those color-coded red) are due to shearing of the splays of the main segments with right lateral kinematics. The next generation of fractures, color-coded green in Figure 33, are the splays of the second-generation faults, which are better developed along faults labeled #1 and #2.

The impact of the fault networks presented in Figure 31, Figure 32 and Figure 33 on fluid flow is complex. The main fault segments and their largest splays typically accommodate meters to hundreds of meters of slip and have well-developed and continuous cores with low (cross-fault) permeability (Flodin et al., 2005). Consequently, they compartmentalize this reservoir. Their minor splays, however, are highly permeable open fractures enhancing the fluid flow. The largest of them could increase the connectivity of the fault networks at a scale of 100 m.

This process is also critical for normal fault patterns. For example, the Moab Fault (Figure 35) in Utah, USA, is a well-studied example in this category (Foxford et al., 1998; Davatzes et al., 2005). Figure 36, as compiled by Davatzes and others (2005), shows lateral relays (a and b, top) and a large intersection (b, bottom) along the Moab Fault and how these complexities control the nature and distribution of associated structures.

Figure 35 – Map showing the traces of the Moab Fault and its strands in southeast Utah. The Moab Fault is dominantly a normal fault with 900 m of maximum slip. Modified from Davatzes and others (2005).

Figure 36 – Localization of deformation bands (DBs) and sheared joint type faults at lateral relays along the Moab Fault. The variation of the secondary structures appears to be a function of small components of strike-slip on the fault segments. a) Fault relay zone. b) Fault relay (top) and fault intersection (bottom) zones. Modified from Davatzes and others (2005).