Deformation and metamorphism associated with crustal rifting : Permian to Liassic evolution of the Lake Lugano-Lake Como area (Southern Alps)

In connection with movements between Europe and Adria, the Southern Alps underwent extension leading to the formation of the Middle Jurassic South-Alpine passive continental margin. As a result of mainly S-vergent Alpine thrusting and folding, a substantially preserved Mesozoic crustal section, ranging from the surface to a paleo-depth of ca. 15 km is exposed in the Lugano-Lake Como area. Tectonic processes associated with Permo-Mesozoic extension can thus be investigated at different crustal levels. Rifting began with a thermal anomaly during which rocks at middle-crustal levels were sheared at ca. 650-750°C. Deformation was distributed on a several km wide zone and late-kinematic pegmatites were emplaced at this stage. Temperatures then decreased and beginning in the Norian, extension was accommodated by a major, E-dipping normal fault, the Lugano-Val Grande normal fault. The fault which can be followed down to a paleo-depth of ca. 12 km, was listric and flattened (to 20°) at ca. 7-9 km depth. In the upper 8-10 km, deformation was only brittle, whereas at deeper crustal levels greenschist mylonites were formed. With continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower-temperature ultramylonites and cataclasites. The thickness of the fault zone varies from some tens of meters in the upper 5-6 km to several hundred meters at deeper levels: deformation was thus discrete at a crustal scale.