Three-dimensional geometry and tectonostratigraphy of the Pennine zone, Central Alps, Switzerland and Northern Italy

Continental collision during Alpine orogenesis entailed a polyphase deformation history (D1–D5) in the Pennine zone of the Central Alps. The regional tectonostratigraphy was basically developed during D1 and D2, characterised by isoclinal, typically north-closing recumbent anticlines, separated by pinched-in synclines, on the scale of tens of kilometres. Later deformation phases (D3 and D4) warped the stack into wavy to open folds. Exhumation of this zone resulted locally in later vertical shortening and folding of already steep fabrics (D5). dimensional models of the nappe pile were constructed, based on geostatistical assessment of the regional foliation field and considering the abundant structural field data. These models indicate the existence of five principal tectonostratigraphic levels developed during D1 and thus equivalent to nappe units s. str.: the Gotthard, the Leventina–Antigorio, the Maggia–Simano (and probably the Monte Leone as well as the Composite Lepontine Series), Lebendun–Soja and Adula–Cima Lunga levels. All these tectonic units formed part of the passive continental margin of Europe prior to the onset of the Alpine orogenesis. dual isoclinal post-nappe folds reflect relative displacements on the order of 40 km or more. The most prominent D2 post-nappe structure is the Wandfluhhorn Fold, structurally equivalent to the northern closure of the Leventina–Lucomagno Antiform. The Lebendun and Monte Leone folds are of similar magnitudes and also affect the whole nappe pile, whereas the smaller Mogno and Molare synforms only refold the Maggia–Simano nappe internally. Principal D3 and D4 structures are the tight Mergoscia Synform directly north of the Insubric Fault between Bellinzona and Locarno (Southern Steep Belt), the Maggia Steep Zone, forming the steep western limb of the Campo Tencia Synform and subdividing the Lepontine dome into the Simplon and Ticino subdomes, the Chiéra Synform steepening the dominant foliation in the north (Northern Steep Belt) and the Vanzone and Claro Antiforms steepening the dominant foliation in the south (Southern Steep Belt). The current geometry of the Northern and Southern Steep Belts reflects an interplay between D4 and D3, involving both fold interference and reactivation/tightening.