Structural evolution, exhumation rates, and rheology of the European
crust during Alpine collision: constraints from the Rotondo granite -
Gotthard nappe
Abstract
The rheology of crystalline units controls the large-scale deformation
geometry and dynamics of collisional orogens. Defining a
time-constrained rheological evolution of such units may help unravel
the details of collisional dynamics. Here, we integrate field analysis,
pseudosection calculations and in-situ garnet U-Pb and mica Rb-Sr
geochronology to define the structural and rheological evolution of the
Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence
of four (D1-D4) deformation stages. Pre-collisional D1 brittle faults
developed before Alpine peak metamorphism, which occurred at 34-20 Ma
(U-Pb garnet ages) at 590 ± 25ºC and 0.95 ± 0.1 GPa. The reactivation of
D1 structures controlled the rheological evolution, from D2 reverse
mylonitic shearing at amphibolite facies (520 ± 40ºC and 0.85 ± 0.1 GPa)
at 18-20 Ma (white mica Rb-Sr ages), to strike-slip, brittle-ductile
shearing at greenschist-facies D3 (395 ± 25 ºC and 0.4 ± 0.1 GPa) at
14-15 Ma (white and dark mica Rb-Sr ages), and then to D4 strike-slip
faulting at shallow conditions. Although highly misoriented for the
Alpine collisional stress orientation, D1 brittle structures controlled
the localization of D2 ductile mylonites accommodating fast (1-3 mm/yr)
exhumation rates due to their weak shear strength (<10 MPa).
This structural and rheological evolution is common across External
Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the entire
European crust was extremely weak during Alpine collision, its strength
controlled by weak ductile shear zones localized on pre-collisional
deformation structures, that in turn controlled localized exhumation at
the scale of the orogen.