Structural geology and petrography of the Naret region (northern Valle Maggia, N.Ticino, Switzerland)

Abstract

The Naret region has a complex geological history of Alpine polyphase folding and metamorphism that affected pre-Alpine rocks of the Maggia nappe, including the Matorello group (interpreted in this study as late-Variscan intrusives), the Lebendun nappe and Mesozoic rocks of the Bedretto zone. From field observations, four main ductile deformation phases (D₁ to D₄) can be distinguished and, in combination with thermodynamic modelling, the tectono-metamorphic evolution for the Naret region can be reconstructed. D₁ formed the initial nappe stack. During this phase, the Maggia nappe was thrust over the Lebendun nappe, at T ≤ 570°C and P ≤ 10 kbar. D₂ caused isoclinal refolding of the nappe pile, at around 610–640°C and 8.5–10 kbar, and produced both the main regional foliation (S₂) and a penetrative stretching lineation which is generally parallel to F₂ fold axes. D₂ is largely responsible for the current complicated geometry of the Lebendun nappe boundary. The main phase of porphyroblastesis occurred between D₂ and D₃, corresponding to a metamorphic temperature peak of ca. 640–650°C at pressures of ca. 8–9 kbar. D₃ produced open folds oblique to the general Alpine trend (“crossfolding”) and locally a crenulation cleavage with a well developed crenulation lineation, at estimated temperatures of 550–610°C. The last important phase, D₄, caused open backfolding of all pre-existing structures and is responsible for steepening of the main S₂ foliation, to produce the Northern Steep Zone, and for a regional rotation of S₃ and L₃. D₄ developed at T ≥ 550°C and P ≥ 3 kbar.

The Lebendun nappe is a complicated structure developed as the result of non-coaxial fold interference related to D₁ and D₂. From tectono-stratigraphic evidence, the rocks of the Lebendun nappe are interpreted as pre-Triassic in age.