Normal-fault development in two-phase experimental models of shortening followed by extension and comparison to natural examples

Field- and seismic-reflection-based studies have documented the influence of pre-existing thrust faults on normal-fault development during subsequent extension. Published experimental (analog) models of shortening followed by extension with dry sand as the modeling medium show limited extensional reactivation of moderate-angle thrust faults (dipping > 40º). These dry sand models provide insight into the influence of pre-existing thrusts on normal-fault development, but these models have not reactivated low-angle (< 35º) thrust faults as seen in nature. New experimental (analog) models, using wet clay over silicone polymer to simulate brittle upper crust over ductile lower crust, suggest that low-angle thrust faults from an older shortening phase can reactivate as normal faults.

In two-phase models of shortening followed by extension, normal faults nucleate above pre-existing thrust faults and likely link with thrusts at depth to create listric faults, movement on which produces rollover folds. Faults grow and link more rapidly in two-phase than in single-phase (extension-only) models. Fewer faults with higher displacements form in two-phase models, likely because, for a given displacement magnitude, a low-angle normal fault accommodates more horizontal extension than a high-angle normal fault. The resulting rift basins are wider and shallower than those forming along high-angle normal faults.

Features in these models are similar to natural examples. Seismic-reflection profiles from the outer Hebrides, offshore Scotland, show listric faults partially reactivating pre-existing thrust faults with a rollover fold in the hanging wall; in crystalline basement, the thrust is reactivated, and in overlying sedimentary strata, a new, high-angle normal fault forms. Profiles from the Chignecto subbasin of the Fundy basin, offshore Canada, show full reactivation of thrust faults as low-angle normal faults where crystalline basement rocks make up the footwall.