Three-dimensional pattern of thrust and allochthonous salt emplacement in the Northern Calcareous Alps, Eastern Alps (Austria)

Abstract

Pre-existing salt structures and thickness variation in the supra-salt stratigraphy exert a strong control on the geometry and evolution of contractional systems. Here we explore a sector of the central Northern Calcareous Alps (Eastern Alps, Austria), and its evolution over a timespan of ∼40 Ma, during the Late Jurassic to Early Cretaceous. The nucleation points of structures and their obliquity were conditioned by the presence of pre-existing structures above a highly effective basal evaporitic décollement. The ratio of thickness between the evaporitic detachment and its overlying stratigraphy controlled the evolution of structures, with thicker supra-salt stratigraphy limiting the progression of deformation. Syn-tectonic sedimentation changed the map pattern of salt-to-cover thickness (overburden to salt ratio) during deformation. Increasing sedimentary load above the salt led to the transition from an initial stage of squeezing of salt structures, salt-fed thrusting and salt allochthony, to a stage of low-angle thrusting. Differential shortening across the area led to highly oblique transport directions, up to 90° to each other, and lateral motions within the emplaced units that were accommodated through syn-thrusting extensional faulting.

The structural evolution presented here further provides critical insights into the earliest phases of Alpine orogenesis, in the Late Jurassic to Early Cretaceous, and helps to clarify structural and temporal relationships that had not previously been fully understood.