Print this pageExperimental Investigation of Duplex and Triangle-Zone Development
COUZENS, BRENT A., DAVID V. WILTSCHKO, and BRUNO C. VENDEVILLE
We have used a set of dynamically scaled 
experiments
to model the effects of
variable mechanical stratigraphy on the development of duplexes. Dry sand
modeled strong, brittle rocks, whereas silicone modeled viscous decollements. We
used different silicones and varied the strain rates to simulate different
decollement viscosities in nature.
The duplexes formed in all the models are hinterland sloping and have bumpy roof thrusts. As decollement viscosity increases, duplex geometry changes from large displacement, overlapping ramp anticlines (antiformal stack) to small displacement, independent ramp anticlines. Also, with increasing viscosity, more horse blocks form, ramp spacing is closer, and each horse accommodates less shortening.
Duplex evolution is strongly affected by decollement viscosity. Low-viscosity
decollements allow for large penetrative strains in the brittle sand layers
before deformation localizes along faults. After thrusts form, shortening may
occur simultaneously on several structures. At higher decollement viscosities,
the amount of penetrative strain before localization is significantly less.
Also, structures develop forward sequentially from the model hinterland, with
much less simultaneous motion. At higher viscosities, horse-blocks are more
likely to locally underthrust the units above the roof, forming what is commonly
referred to as a triangle-zone. However, unlike kinematic models of
triangle-zones, all the experiments
have some shortening forelandward of the
duplex above the roof decollement.