Duplexes are a common feature in thrust belts at many scales. Their geometries vary significantly from antiformal stacks with significant forethrusting in the cover (e.g. southern Pyrennes, Spain) to triangle zones (e.g. foreland Canadian Rockies) to low-displacement individually spaced ramp-anticlines (e.g. Sub-Andean thrust belt, Bolivia). We present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style. The models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers. The strength of the decollements in the models is a function of the shortening rate applied to the model. The relative strength of the decollements and surrounding rocks affects the development of active- or passive-roof duplexes (triangle zones). It also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps, which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp-anticlines. Model results indicate that specific associations of structural features form systematically under similar theological and boundary conditions. The presence of relatively strong decollements promotes local underthrusting of the cover, individual ramp-anticlines, internal deformation of thrust sheets, low early layer-parallel shortening, and sequential towards-the-foreland propagation of structures. Weak decollements promote forethrusting of the cover, antiformal stacks, coeval growth of structures, and low internal strain, with the exception of significant early layer-parallel shortening. No underthrusting at a regional scale occur-red in any model. (C) 2003 Elsevier Science Ltd. All rights reserved.
