Physical modeling of normal faults and graben relays above salt

Le Calvez, Joel Herve
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In salt basins, thin-skinned extension is typically accommodated by graben development in sedimentary overburden and viscous flow in the evaporitic layer. This study presents a series of physical models in which grabens propagate along strike and interact in relay zones whose geometry depends on graben across-strike separation. I first describe the basis for physical modeling, including scaling analysis. I then review various experimental designs used by previous authors to model the interaction between laterally offset faults, finally I introduce a design in which faults nucleate at predetermined locations above small initial instabilities but that permits faults to propagate freely thereafter. I used series of systematic experiments to investigate the 3-D geometry and evolution of two laterally offset grabens interacting in a relay zone. Results x allow the determination of the influence of key geologic parameters on the evolution of relay zones. The main parameter, the across-strike offset between the grabens, controls the size (along both strike and dip) of the relay zone and its structural style. Where the offset is large, fault traces curve gently toward one another and fault throw and amount of diapir rise below the graben decrease progressively toward the relay zone. Some distance from that zone, old faults cease to slip as new faults form. In the relay zone old fault segments remain active and no new faults form. Some new faults hard-link along strike with active segments of older faults. Offset grabens and faults start interacting only when they have propagated along strike far enough to overlap. Syntectonic sedimentation can suppress new-fault formation during extension. Another experiment shows that inversion of graben relays by late shortening forms reverse faults rooted at depth on the crest of underlying diapirs. A second series of experiments, motivated by a field example in Canyonlands National Park, focuses on the genesis of graben arrays having regionally arcuate traces. Curvature is explained in terms of shear stresses between extending domains of the sedimentary cover, which is underlain by an evaporitic décollement, and lateral nonextending domains, a setting common along salt-bearing passive margins during thin-skinned extension.