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Normal faults commonly develop in two oppositely dipping sets having dihedral angles of around 60°, collectively referred to as conjugate normalfaults. 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 virtuallyunrestorable 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.
David A. Ferrill received a B.S. degree from Georgia State University (1984), an M.S. degree from West Virginia University (1987), and a Ph.D. from the University of Alabama (1991). Before joining the Center for Nuclear Waste Regulatory Analyses (CNWRA) at Southwest Research Institute in 1993, he was an exploration geologist at Shell Offshore Incorporated. David is now a principal scientist and conducts analyses of faulting, fracturing, and seismicity related to radioactive waste disposal, and structural geological training and contract consulting for the oil and gas industry.Alan P. Morris received a B.S. degree from Imperial College (1973) and a Ph.D. from the University of Cambridge (1980). He has studied deformed rocks from Svalbard to the Basin and Range. Alan taught structural geology at Wayne State University (Detroit) and was a petroleum industry consultant for Geo-Logic Systems. He now teaches at the University of Texas at San Antonio, is a consultant to the CNWRA in San Antonio, and writes educational software for middle schools.
John A. Stamatakos received his B.A. degree from Franklin and Marshall College (geology, 1981) and his M.S. and Ph.D. from Lehigh University (paleomagnetism, structural geology, and tectonics; 1988 and 1990). Prior to joining the Southwest Research Institute in 1995, he spent two years as a postdoctoral research scientist at the Eidgenössische Technische Hochschule in Zurich (geophysics) and three years as a visiting professor and research scientist at the University of Michigan. John is currently a senior research scientist working on geological and geophysical issues related to safe disposal of nuclear waste.
Darrell W. Sims's early career as an archaeologist led him ultimately to structural geology. He earned a B.S. degree (applied science, 1986) and an M.S degree (applied geology, 1990) from the University of Texas at San Antonio (UTSA). After dividing his time between serving as a research and teaching associate at the UTSA and an independent consultant, Darrell joined the CNWRA at Southwest Research Institute in 1998. He is currently using physical analog and three-dimensional visualization techniques to investigate the evolution of faulted structures.