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We show that the key to understanding the tectonic complexity of large strike-slip fault systems is fault stepovers. Secondary structures such as folds, thrust or reverse faults, cracks, dikes, normal faults, or smaller strike-slip faults, known to occur in strike-slip environments, localize in stepovers between en echelon faults. Two types of en echelon geometry are recognized: (1) strike-slip faults that are en echelon in map view with discontinuities along the strike of faults; (2) strike-slip faults that are en echelon in cross-sectional view with discontinuities along the dip direction of faults.
Depending on the sense of stepover, discontinuities along the strike of faults result in pull-apart basins and push-up ranges, several examples of which are presented to illustrate the associated structures and their complexities. Discontinuities along the dip direction of strike-slip faults are poorly known because of the lack of field observations. Data from seismicity, however, can be used to fill this gap. One example of such en echelon fault geometry is found along the Calaveras fault, California. It is inferred that stepovers along the dip direction of strike-slip faults may produce secondary strike-slip faulting on inclined planes connecting the en echelon segments of the major fault. As the amount of overlap increases, features similar to pull-apart basins or push-up ranges ar expected to occur.
Causes for the formation of discontinuities and control of the sense of stepover are not well known. Some possible factors are: spatial variability of the coefficient of friction, spatially variable elastic moduli, high pore pressure, and interaction between neighboring faults in an array of faults. The first two would give rise to both senses of stepover, whereas the last two lead only to one sense of stepover, which induces pull-apart basins.
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