The roles of complex mechanical stratigraphy and syn-kinematic sedimentation in fold development: insights from discrete-element modelling and application to the Pico del Águila anticline (External Sierras, Southern Pyrenees)

Abstract

Abstract A 2D discrete-element modelling technique is used to explore the effects of complex mechanical stratigraphy and syn-kinematic sedimentation in the development of the Pico del Águila anticline (External Sierras, Southern Pyrenees). The stratigraphy (Middle Triassic–Oligocene in age) involved in this structure is characterized by a gross interlayering of competent and incompetent units, which leads to a striking variation in outcrop-scale deformation of the units observed in the field. The numerical model attempts to reproduce the stratigraphic variation seen in the field by using a mechanical stratigraphy that contains a complex interlayering of competent/incompetent units. Two experiments are presented. Model 1 tests the response of this complex mechanical stratigraphy to shortening under conditions that lead to the formation of a detachment fold. This experiment shows that folding mechanisms vary abruptly depending on the mechanical properties of the materials involved: the incompetent units are strongly internally deformed, accommodating much layer-parallel shearing; the competent units deform by rigid-body translation/rotation, localized faulting and minor internal shearing. Model 2 tests the effect of syn-kinematic sedimentation under identical boundary conditions: these sediments stabilize the fold against gravitational instabilities and cause a concentration of deformation in the core of the structure, leading to a tighter, narrower fold.