The structural evolution of the Rotondo granite (Gotthard nappe, Central Alps): constraints on the strength and timing of weakening of the European upper crust during Alpine collision

Collisional dynamics, exhumation rates, and the large-scale geometry of orogenic belts are dependent on the relative strength contrast between colliding plates. In the Central Alps, the strong Adriatic lower crust indents into the thickened European upper crust, composed of stacked slices of weak, upper continental crust. The geological factors controlling this weak rheology and the timing of weakening are still debated.

To provide further constraints on what makes the European continental crust so weak, we have investigated the structural and tectonic evolution of the Rotondo granite through integrated field, microstructural, and in-situ (U-Pb on garnet, Rb-Sr on mica) petrochronological analyses. The Rotondo granite, an early Permian peraluminous granite (295 Ma, Rast et al., 2022), represents a strong inclusion in the polymetamorphic Gotthard nappe in the Swiss Central Alps. We have identified a sequence of four (D1-D4) main classes of deformation structures developed during the pre-Alpine, Alpine collisional, and exhumation history of the nappe.

D1 structures include brittle breccias, cataclasites and shear fractures, occurring pervasively throughout the pluton and pre-dating the Alpine peak metamorphic conditions. Garnet overgrew the brittle deformation fabric during Alpine peak metamorphic conditions at 580 ± 25 ºC and 0.9 ± 0.1 GPa at different times from 34 to 20 Ma (in-situ U-Pb dating on garnet). The following Alpine exhumation is recorded through the development of D2 reverse, ductile shear zones at 520 ± 40 ºC and 0.8 ± 0.1 GPa around 18-20 Ma (in-situ Rb-Sr on white mica). Exhumation perdured until 14 Ma, as inferred from in-situ Rb-Sr on synkinematic micas of D3 strike-slip brittle-ductile shear zones developed at 395 ± 25 ºC and 0.4 ± 0.1 GPa. The latest stages of upper crustal, brittle tectonics are shown by the development of D4 zeolite- and gouge-bearing fault zones at < 13 Ma (K-Ar illite dating).

This tectonic evolution is common to many other crystalline massifs of the External domains of the European Alps, and allow us to propose some large-scale implications on the rheological behavior of the continental (upper) crust during Alpine collision. The 34-20 Ma range of ages obtained from in-situ U-Pb dating on garnet suggests that the peak metamorphic conditions in the area likely persisted for more than 10 Myrs. After the peak, exhumation occurred at relatively fast rates (~3 mm/yr), and the internal deformation of the nappe was accommodated by weak ductile shear zones, localized on pre-existent (inherited) structural features. Indeed, meso- and microstructural considerations suggest that these shear zones were capable of sustaining differential stresses not larger than 10 MPa during collision and exhumation. The strength of the undeformed granite, limited by tensional veining, has been estimated to not exceed 60 MPa. This also demonstrates that the main weakening event of the crust occurred at retrograde conditions, during exhumation, after residing for a prolonged period of time at peak metamorphic conditions.

References:
Rast et al. (2022).  Swiss Journal of Geosciences, 115(1), 8. https://doi.org/10.1186/s00015-022-00409-w