Gravity influenced brittle‐ductile deformation and growth faulting in the lithosphere during collision: Results from laboratory experiments

Abstract

Because of the competition between brittle and ductile rheologies and their interplay with tectonic and buoyancy forces, lithospheric deformation results in very contrasting styles. In continental collision, especially with unconfined boundaries, deformation can be either homogeneously distributed or localized on complex fault patterns, and different deformation modes such as contraction, extension, and strike‐slip interfere. Using scaled lithospheric analog experiments made of dry sand, silicone putties, and dense honey, we investigate the mechanical parameters that control the deformation style in colliding systems, with a particular focus on the roles of buoyancy and brittle‐ductile coupling. The analysis of tens of experiments shows that the principal deformation features depend on two main parameters: a brittle‐to‐ductile strength ratio Γ, which controls deformation localization at the largest scale, and a buoyancy‐to‐strength ratio Ar, which fixes the relative amount of contractional, extensional, and strike‐slip structures. Strain localization occurs only for Γ larger than a critical value (∼0.5), and the range of Γ values, over which the transition from nonlocalized to localized deformation occurs, is small. The three main deformation regimes (contraction, strike‐slip, and extension), which coexist in most of the collision experiments, occur in relative proportions that depend mainly on Ar and on the nature of the boundary conditions.