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Normal faults are commonly listric, that is, the dip flattens with depth. Movement along this type of fault is instrumental in formation of several types of structural traps (e.g., rollover anticlines and upthrown-fault-block closures). Some listric faults are restricted to sedimentary rocks, whereas others offset basement rocks. Theoretical data, rock-mechanical and simulated model experiments, and foundation-engineering tests and failures suggest that this type of fault may occur where brittle rocks overlie ductile rocks in an extensional regime. In some places the ductile section may be thin and bounded sharply at its top. Also, the extensional regime may be locally derived within a broader stress regime of another type, as evidenced by transtension associated with str ke-slip movement and arched strata in a compressive setting.
The flattening of the fault reflects an increase in ductility of the rocks with depth and, in some cases, deformation of the fault due to compaction or tilting of the upthrown block. The dip angle may vary along the strike of the fault in response to changes in throw. In cross section, a listric fault may consist of relatively short, en echelon fault segments. This geometry may be particularly characteristic of growth faults. Sedimentary faults may sole in ductile strata, or they may represent the brittle part of a fault-flow system. Fault patterns commonly are characterized by bifurcation, some of which may occur near the ends of individual faults comprising a zone.
Although unequivocal recognition of listric normal faults requires unusually extensive outcrop data, close subsurface control, or high-quality seismic data, their presence is suggested indirectly by such features as increasing dip with depth toward the controlling fault ("reverse drag"), thick progradational sandstone overlying ductile strata, and in some cases arcuate fault patterns, basins, or uplifts.
Listric normal faults form during rifting, drifting, and evolution of passive continental margins with concomitant basinal development. Listric faults confined to the sedimentary prism are common features on passive margins, especially in progradational, post-evaporite sequences. The basement is offset by listric faults as a fundamental element in the development of other types of basins, including those which formed during postorogenic extension. They also occur as secondary extensional features in an overall compressive stress regime due to plate convergence and during transform or strike-slip faulting.
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