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 D 3, 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. D 4 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. © 2007 Birkhäuser Verlag, Basel.