Crossing conjugate normal faults

Abstract

Normal faults commonly develop in two oppositely dipping sets having dihedral angles of around 60o, collectively referred to as conjugate normal 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 interpreted to accommodate extension by simultaneous slip on the crossing faults. Using two-dimensional geometric modeling we show that simultaneous slip on crossing conjugate normal faults requires loss, gain, or localized redistribution of cross-sectional area. In contrast, alternating sequential slip on the crossing faults can produce crossing fault patterns without area modification in cross section. Natural examples of crossing conjugate normal faults from the Volcanic Tableland (Owens Valley, California), Bare Mountain (Nevada), and the Balcones fault zone (Texas) all indicate formation by sequential rather than simultaneous slip. We conclude that truly simultaneous activity of crossing normal faults is likely to be limited to extremely small displacements due to rate-limiting area change processes. If their associated movement is truly simultaneous, crossing normal faults are virtually unrestorable and should show evidence of significant cross-sectional area change (e.g., area increase may be indicated by salt intrusion along fault, area decrease by localized dissolution or mechanical compaction may be indicated by extreme displacement gradients at fault tips). In the absence of 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 permeability anisotropy in reservoir rocks that can be approximated by a prolate (elongate) permeability ellipsoid, with greatest permeability parallel with the line of intersection. Characterization of the fault pattern in a faulted reservoir provides the basis for interpreting the bulk permeability anisotropy in the reservoir, an important step in optimizing well placement. (Begin page 1544)