Metamorphic and Deformation History in the Region of the Lukmanier Pass (Central Alps)

Schenker, Filippo (2008) Metamorphic and Deformation History in the Region of the Lukmanier Pass (Central Alps). Masters thesis, ETH Zurich.

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Detailed mapping, structural and metamorphic studies were carried out in the area of the Lukmanier Pass with the intent to determine the kinematic and metamorphic evolution in one crucial part of the northern margin of the Lepontine Dome (e.g. Ring [1992]). The studied area covers from north to south three tectonic units: the Gotthard Massif, the parautochthonous Mesozoic cover of the Gotthard Massif and the Lucomagno Massif. Generally, the structures strike NE-SW and dip steeply to the north. Structural relationships suggest a single deformation event overall the area, corresponding to a top-to-the-southmovement. This shear movement took place in the basement in two major backthrusts that developed folding and faulting in the Mesozoic cover. The Chiera and the Scopi synforms result from the movement of the southern and of the northern backthrust, respectively. EBSD analysis from a marble lens near the backthrust reveals a shear direction contrary to the top-to-thesouth movement; this opposite movement direction is attributed to flexural flow/slip. The metamorphic conditions were computed with PerpleX [Connolly, 2005], avPT [Powell and Holland, 1988] and GASP-barometry [Caddick and Thompson, 2008]. The rock bulk composition measured with XRF analysis computes the pseudosectionswith PerpleX, the mineral compositions used by avPT and GASP barometry were obtained with the electron microprobe. The metamorphic conditions computed are a temperature of 650◦C and a pressure of 8 kbar for a rock sample of Frodalera-Brönich. The high temperature compared to the regional background [Todd and Engi, 1997] suggests shear heat production generated during the backthrusting. Comparison with the Nufenen Pass area (Master Thesis of Claudio [2008]) allowed to distinguish two different shortening systems with a N-S compression during the Lepontine metamorphism. Shortening in the west occurred through a leading imbricate fan, where the structures have been ramped up in a sub-vertical position by the development of listric thrusts in the frontal part of the imbricate zone. In the east, the shortening took place through a synchronous system of conjugate thrusts, of which the backthrust at the Lucomagno Pass area represents the antithetic fault, responsible, together with the frontal thrust movement, for the vertical position of the units. Both backthrusts should branch on a basal detachment.

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