Asynchronous extension of the late-Hercynian crust in Calabria

Quartz-monzodioritic dykes have intruded the Hercynian continental lower crust of Calabria at 323 ± 5 Ma. Pseudosection and average PT calculations have been performed using THERMOCALC. The obtained P (average PT) and T (pseudosection and average PT) values indicate that the quartz-monzodioritic dykes have intruded the 750 °C hot metapelitic–migmatitic country-rocks at a depth of about 32 km, with a temperature of ca. 850 °C. Microstructural analysis suggests that deformation of the dykes started at hypersolidus conditions during emplacement and continued after cooling at sub-solidus granulite facies conditions together with the country rocks. During this deformation event, which started not later than 323 ± 5 Ma ago, the main Sn + 1 schistosity evolved. Vorticity analysis of the quartz-metadioritic dykes and of the metapelitic country rocks shows that the Sn + 1 schistosity evolved during a general shear deformation characterized by 60 to 70% of pure shear. This high percentage of pure shear suggests that the main Sn + 1 schistosity evolved in the hot lower crust undergoing extension during vertical shortening. A horizontal attitude of this extensional schistosity is confirmed by retro-deformation of the today obliquely SE-ward dipping Sn + 1 schistosity. Extension continued with time and propagated upwards within the crust, affecting the middle crust in a time interval of 306 ± 1 to ca. 300 Ma when huge masses of granitoid rocks intruded it. The deepest batches of these granitoid bodies show a gneissic schistosity which developed under ductile conditions, while granitoids at higher levels became deformed in a brittle fashion. Our whole data set is consistent with a late Hercynian evolution in Calabria characterized by a progressively crust overlying a mantle that gradually rose during lithospheric extension. This extension was asynchronous, starting at 323 ± 5 Ma in the lower crust and affecting higher crustal levels at progressively later times.

► We present HT deformational evolution of unique Hercynian dykes in the Calabrian lower crust.
► The petrological and structural study of the mafic dykes and the metapelitic wall-rock was carried out.
► We found that decompression of the Hercynian crust started earlier and involved a significantly thicker crust than previously assumed.
► We conclude that extension was asynchronous and propagated with time from deeper to mid- and upper crustal levels.