Crossing conjugate normal faults

Normal faults commonly develop in two oppositely dipping sets having dihedr al angles of around 60 degrees, collectively referred to as conjugate norma l Faults. Conjugate normal faults form at a range of scales from cm to km. Where conjugate normal faults cross each other, the faults are commonly int erpreted to accommodate extension by simultaneous slip on the crossing faul ts. Using two-dimensional geometric modeling we show that simultaneous slip on crossing conjugate normal faults requires loss, gain, or localized redi stribution of cross-sectional area. In contrast, alternating sequential sli p on the crossing faults can produce crossing fault patterns without area m odification in cross section. Natural examples of crossing conjugate normal faults from the Volcanic Tableland (Owens Valley, California), Bare Mounta in (Nevada), and the Balcones fault zone (Texas) all indicate formation by sequential rather than simultaneous slip. We conclude that truly simultaneo us activity of crossing normal faults is likely to be limited to extremely small displacements due to rate-limiting area change processes. If their as sociated movement is truly simultaneous, crossing normal faults are virtual ly unrestorable and should show evidence of significant cross-sectional are a change (e.g., area increase may be indicated by salt intrusion along faul t, area decrease by localized dissolution or mechanical compaction may be i ndicated by extreme displacement gradients at fault tips). In the absence o f such evidence, even the most complicated crossing fault pattern should be restorable by sequentially working backward through the faulting sequence. In common with other structures that affect permeability and that cross at high angles, conjugate normal fault systems are likely to produce bulk per meability anisotropy in reservoir rocks that can be approximated by a prola te (elongate) permeability ellipsoid, with greatest permeability parallel w ith the line of intersection. Characterization of the fault pattern in a fa ulted reservoir provides the basis for interpreting the bulk permeability a nisotropy in the reservoir, an important step in optimizing well placement.