Salt Tectonics Versus Shortening: Recognizing Pre-Orogenic Evaporite Deformation in Salt-Bearing Fold-And-Thrust Belts on the Example of the Silica Nappe (Inner Western Carpathians)

Recognizing salt-related structures and differentiating them from tectonic shortening-related anticlines and synclines can be a very challenging task in salt-bearing fold-and-thrust belts, especially in poor outcrop conditions. In this study, we explain several diagnostic structural and sedimentary features that may be used to distinguish pre-orogenic halokinetic structures from shortening-related structures on the example of the Silica Nappe (Inner Western Carpathians, Central Europe). Detailed structural mapping in this area resulted in the recognition of several pre-orogenic salt-related structures, including linear salt walls and minibasins. Initial evaporite movement started as early as the late Early Triassic, and widespread diapirism occurred during the Middle to Late Triassic, ultimately leading to facies differentiation, with carbonate platform growth in the subsiding minibasins and reduced basinal deposition on top of the diapirs. Later, the inherited salt structures localized the deformation during the Cretaceous Alpine orogeny, and exerted a strong control on the geometry and kinematics of the subsequent deformations. Our new interpretation explains previously unsolved structural problems in the Silica Nappe, like pre-orogenic thickness variations during the post-rift phase, frequent young-on-older type thrust contacts and multiple folding directions with variable vergencies. The results point out that the Silica Nappe is a fold-and-thrust belt, where pre-orogenic salt tectonics and its effects on the fold-and-thrust belt evolution can be studied in detail.