Early to mid Tertiary crustal extension in the Central Alps: The Turba Mylonite Zone (Eastern Switzerland)

Abstract

In the Penninic‐Austroalpine boundary zone of Eastern Switzerland a clear distinction can be made between upper and lower stacks of nappes with contrasting tectonic evolution that are separated by the Turba mylonite zone (TMZ). The Austroalpine nappes, Platta ophiolites, and Malenco‐Forno‐Lizun ophiolites belong to the upper stack. These nappes were formed by predominantly west directed, late Cretaceous shortening and imbrication. The lower stack, comprising the Suretta nappe, Schams nappes, Avers schists, and Oberhalbstein flysch units, includes Tertiary sediments and was deformed and metamorphosed in early Tertiary time. Kinematic indicators in the mylonites of the TMZ, representing the boundary between the two stacks, show a top‐to‐the‐east directed movement, which is clearly distinct from the regionally prevalent top‐to‐the‐north direction of thrusting in the Tertiary. The main foliation in the shear zone is discordant with respect to the structures of the hanging wall, and concordant to the footwall. Vitrinite reflectance in shaly sediments above and below the TMZ in the northern part indicates that the TMZ coincides with a drop in peak metamorphic temperature from about 350°C in the top of the footwall to about 280°C at the base of the hanging wall. Therefore the TMZ is interpreted as an east dipping, low‐angle normal fault. The age of its activity is bracketed between about 45 and 30 Ma, based on overprinting relations with radiometrically dated features (45–30 Ma: main foliation in the footwall, older than TMZ activity; 30 Ma: Bergell granodiorite intrusion, younger than TMZ activity). Extension along the TMZ was followed by further north‐south shortening, as indicated by postmylonitic folding of the TMZ. Normal faulting along the TMZ accommodated relative uplift of the Lepontine structural dome, a large metamorphic complex in the Central Alps. Unroofing of this dome started before 30 Ma and continued in the Miocene with activity of further normal faults situated at deeper structural levels.