[ 1] In a series of numerical experiments, arrays of parallel-dipping normal faults formed only if the model contained a thin viscous layer sandwiched between the brittle, faulted layer on top and a lower boundary that resisted vertical displacement. With this particular vertical stratification, parallel-dipping faults developed even if the viscous layer had a shear- stress-free lower boundary condition. This observation contradicts previous studies relating parallel-dipping normal faults to consistent horizontal shear stress in the brittle layer. We explain the formation of parallel-dipping normal faults in our experiments through the properties of flow in a thin viscous channel and how this flow accommodates and delocalizes faulting in the brittle layer on top. First, for closely spaced normal faults, an array of parallel-dipping faults minimizes the viscous work rate ( dissipation) in the viscous layer as it minimizes the distance between sources and sinks in the flow pattern. Second, a thin layer of moderate viscosity leads to the required formation of closely spaced single faults. A highly viscous substratum tends to reflect faults at the base of the brittle layer and hence promotes the formation of a local graben at each necking site in the brittle layer. With lower viscosities, a thick layer or a floating lower boundary condition usually leads to the formation of widely spaced core complexes in the brittle layer. Our scheme can explain varying dip polarity along strike, which is observed in many rift systems and difficult to explain through consistent shear stress.
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