Experimental 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. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.