Movie B-1. Extension experiment

Movie showing the cross-sectional evolution of structures. Two sections cross the domain with only a basal viscous layer overlain by brittle layers (upper two images). Both other sections cross the domain with a basal viscous layer and a second viscous layer interbedded in brittle layers (lower two images). Note the difference in fault evolution between the two domains. In the domain with an interbedded weak layer progressive extension results in decoupled conjugate normal fault systems in upper and lower brittle strata. In contrast, in the domain without a second interbedded viscous layer, the conjugate normal faults are fewer, deeper and wider, and faults extend all the way down to the top of the basal viscous layer. Initial graben width reflects the depth to the viscous layer. Although the normal faults at early stages of extension are straight in cross-section, they become slightly listric with progressive extension and fault-bounded blocks rotate about a horizontal axis. Faulting in the lower brittle compartment causes downbending of brittle strata overlying the interbedded viscous layer. The base of the model consists of a compressed assemblage of vertical foam bars (black) and plexiglass bars (dark grey). Initial width of model was 22.5 cm and initial height 3.5 cm. Extension increment between each movie frame is 5 mm. Viscous PDMS = dark grey, quartz sand = light to medium grey, corundum sand = light grey.