Evolving mechanical stratigraphy during detachment folding

The combination of a complex lithostratigraphy and shortening can lead to a mechanical stratigraphy that changes character and mode of deformation during detachment folding. In the detachment-folded carbonates of the Carboniferous Lisburne Group of northern Alaska, a complex lithostragraphy led to a mechanical stratigraphy that evolved during folding, with mechanical unit thickness decreasing as deformation progressed and the fold tightened. Newly forming mechanical boundaries changed the dominant deformation behavior and subsequent geometry during folding of a given horizon. During early folding, flexural slip dominated in units comprised of thick bedded, competent lithologies while flexural flow dominated in units composed of thinner bedded, incompetent lithologies. After fold “lock-up” homogeneous flattening dominated in the less competent mechanical units. The final result was a fold with a complex geometry and a suite of fractures and other structures indicative of both flexural slip/flexural flow folding and homogeneous flattening.