Kinematic and Metamorphic Evolution of the Lukmanier Pass Region (Switzerland): Extension Related History?

Schenker, Filippo and Reusser, Eric and Caddick, Marc J and Madonna, Claudio and Grassi, Daniele N. and Burg, Jean-Pierre (2012) Kinematic and Metamorphic Evolution of the Lukmanier Pass Region (Switzerland): Extension Related History? In: Alpine Workshop 2011, Corsica. (Unpublished)

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Detailed mapping, structural and petrological studies of the Lukmanier Pass area, in the Swiss Alps, document the kinematic and metamorphic evolution of a key segment of the northern margin of the Lepontine Dome. The mapped area covers three tectonic units, from north to south: (1) the Gotthard Massif, (2) its parautochthonous Mesozoic cover and (3) the Lucomagno Massif. The intermediate-pressure metamorphism increases from greenschist facies in the north to amphibolite facies in the south. The main foliation generally strikes E-W and dips steeply to the north. The contact between the Gotthard Massif and the parautochthonous Mesozoic series is characterized by top-to-the-S sense of shear, which indicates back-thrusting in the context of the Alps. The contact between the parautochthonous series and the Lucomagno Massif is characterized by top-to-the-N shear sense, thus defining ductile normal faulting. Structural and metamorphic relationships suggest that a single protracted deformation event formed these distinct and opposite shear senses (although geochronological data are required to confirm the ages of the shear). Elongated minerals of the peak temperature metamorphic assemblages (i.e. staurolite, kyanite and amphibole) locally define the kinematic lineation and attest to north-dipping ductile normal faulting. This juxtaposed the “cold” Gotthard massif in the hanging wall with the “hot” Lucomagno massif in the footwall, sandwiching the Mesozoic units during peak metamorphic conditions. The peak condition was calculated from a garnet-kyanite-staurolite-biotite metapelite near the contact with the Lucomagno massif. Detailed petrological work combined with thermodynamic modeling show a well constrained clockwise P-T path with an early temperature estimate of ca. 550 °C obtained from inclusions in prograde garnet and followed by a HT peak at ca. 650 °C and 8 kbar. The duration of these metamorphic phases will be discussed by means of 1D modeling of major element intra-crystalline diffusion in garnet. At a regional scale, the contact between the Gotthard massif in the north and the Penninic units in the south is characterized by top-to-the-NW thrusting with a dextral strike-slip component. This dextral component created a pull-apart extension responsible for top-to-the-N normal faulting in the Lucomagno region. The apparent back-thrusting of the Gotthard massif is probably related to a local mechanical instability during the top-to-the-N normal faulting.

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