The Maggia nappe: an extruding sheath fold basement nappe in the Lepontine gneiss dome of the Central Alps
The Lepontine gneiss dome represents a unique region of the Central Alps where Oligocene–Miocene amphibolite facies grade rocks and fold nappes of the deepest tectonic level of the Alpine orogenic belt are exposed in a tectonic window. The Cenozoic structures of the Maggia nappe reveals a giant tens of kilometre-scale tubular fold structure that cross-cut through the surrounding lower Penninic nappes from its root situated in the southern steep-belt of the Alps near Bellinzona. The Mesozoic sedimentary cover of the Maggia nappe is typical for the Helvetic stratigraphic domain. The age of formation of the lower Penninic fold-nappes by ductile detachment of the upper European crust during its underthrusting below the higher Penninic and Austroalpine orogenic lid and Adriatic indenter was estimated between 40 and 30 Ma. Maximal pressures of 8–9 kbars and temperatures of 600–700 °C were attended during and after the nappe emplacement some 30–22 Ma ago. The Maggia and surrounding nappes are crosscut by the isograds of the Barrovian regional metamorphism.