Fault breccia refers to an incohesive fault core rock with randomly oriented medium- to coarse-grained angular rock fragments that make up more than 30% of the rock mass (Figure 1), formed by brittle processes.

Figure 1. Outcrop photo of fault breccia.

Fault breccia is known to play an important role in hydrocarbon generation, migration, and presence in some area of the world because the dilation associated with breccia provide fast flow pathways for fluid and gas. Figure 1 provides an evidence of this behavoir, showing an oil-filled breccia from the Monterey Formation, California.