Normal faults

A normal fault is caused by in-situ stress conditions in which

$$\left\lbrace \begin{array}{l} S_1 = S_v \\ S_2 = S_{Hmax} \\ S_3 = S_{hmin} \\ \end{array}\right.$$ (5.3)

where $S_1 > S_2 > S_3$. These stress conditions are typical of tectonically passive and laterally extensional environments. For example, the Permian Basin in Texas is mostly under normal faultin stress regime. The fault plane is a shear rupture plane. Its orientation is ( $\pi/4 + \varphi/2$) in vertical direction from the horizontal plane (the plane perpendicular to $S_v$). The blocks move along the direction of $S_v$ and do work against $S_{hmin}$. At any point in the fault, the block above the fault is called the “hanging-wall” and the block below is the “footwall” (Fig. 5.11).

Figure 5.11: Single normal fault

Normal faults usually occur in pairs. Notice that the shear failure angle includes two possible solutions (for $S_1 \neq S_2 \neq S_3$). These are called conjugate solutions. The block that moves down in between two normal conjugate faults is termed “graben”, while the ones that move up relative to the footwall are called “horst”. These geological structures occur frequently in hydrocarbon systems with structural fault traps.

Figure 5.12: Conjugate normal faults forming graben and horst structures.