Field relations in the footwall of the Badwater Turtleback fault provide evidence that macroscopically ductile deformation was directly responsible for early stages of brittle faulting during extensional denudation. Footwall rocks, which are typically mylonitie, consist of dominantly pegmatite, quartzofeldspathic gneiss, and dolomite and calcite marble. Because of the ductile deformation, these rocks are widely variable in thickness: bodies of rock that exceed 10 m in thickness may pinch out to zero thickness in distances of less than 100 m. The mylonitic rocks also display crosscutting relations that indicate that onset of bulk brittle behavior occurred at different stages of uplift for different rock types. Calcite marble was the last to deform by predominantly crystal-plastic mechanisms and therefore accommodated most of the shear strain during later stages of lower-plate, ductile deformation. Where the calcite marble pinched out into adjacent stronger rocks, the high strains caused brittle faulting in the adjacent rocks.

The process described here, of ductile deformation leading to extreme thickness variations and consequent brittle faulting, directly links mid-crustal, ductile deformation with upper-crustal, brittle deformation. It further demonstrates that brittle faulting can initiate at depths where ductile deformation accommodates the bulk strain.

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