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Faulting
Beneath a Ductile Layer: Experimental
Modeling of Deformation Patterns in the Cover Sequence
WITHJACK, MARTHA OLIVER, J. SYBIL CALLAWAY
We studied deformation patterns produced by normal faulting beneath a ductile
layer (salt and shale) using scaled physical models. In the models, a 1-cm-thick
layer of silicone putty represents the ductile layer, and a 3-cm- thick layer of
dry sand represents the cover sequence. Movement on a precut, 45 degree-dipping
surface below the silicone putty simulates normal faulting. We examined the
effect of viscosity on structural development by varying displacement rates, and
used surface photographs and reconstructions of model cross sections to define
structural evolution.
Initially, a monocline develops in the cover sequence above the sub- ductile
normal fault. Soon after, a detached graben forms near the footwall hinge of the
monocline, and later a detached reverse fault forms near the hanging-wall hinge.
As displacement on the sub-ductile fault increases, the detached normal fault
that dips in the same direction as the sub-ductile normal fault becomes the main
detached fault. The other detached faults (normal and reverse) move down and
toward the hanging-wall side of the sub-ductile fault. Eventually, they become
inactive, and are replaced by parallel faults closer to the main detached fault.
This process repeats itself throughout the experiment producing a systematic age
progression of faulting
in the cover sequence. Models with the higher
displacement rates (simulating a higher viscosity ductile layer) have the most
complex deformation patterns, the greatest strike variations, and the greatest
disruption of the fault-age progression.