Geometries of frontal fold and thrust belts: Insights from discrete element simulations

This study investigated the evolution of frontal structures of fold and thrust belts with 2D numerical simulations using the discrete element method (DEM). Of specific interest is the occurrence of triangle zones, and how mechanical stratigraphy affects the formation of the triangle zones. In our simulations, we varied the strength of a two unit stratigraphy, as well as the décollement horizons, to determine the controlling parameters of different types of frontal structures. For models with homogenous stratigraphy, deformation was concentrated along forethrusts, which involved both the upper and lower units. With the addition of a mechanically weak upper unit, deformation along forethrusts and popup structures largely occurred in the upper unit with relatively little deformation of the lower unit. With thicker décollement surfaces and strong upper and lower units, decoupling was enhanced and allowed for the formation of triangle zones in front of the deforming wedge. The triangle zones were uplifted along lower forethrusts, until a new frontal triangle zone formed. Results compared favorably with triangle zones found in the Canadian Cordillera in southern Alberta.