The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions

Abstract

Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

“…This requires an extra heat supply, such as asthenospheric upwelling that is compatible with slab and/or orogenic root-delamination after a continental collision, which promotes late orogenic extension Marotta et al, 2009;Spalla et al, 2014). Therefore, the two rock types have 23…”

“…The intrusion of the Sondalo gabbroic complex occurred initially at the mid-crustal level (0.5-0.6 GPa) and was progressively exhumed (<0.45 GPa), which generated partial melting of the host rock in the metamorphic aureole (Petri et al, 2016). The , 1989;Lardeaux and Spalla, 1991;Diella et al, 1992;Spalla et al, 1995;Handy et al, 1999;Schuster et al, 2001;Schuster and Stüwe, 2008;Marotta et al, 2009;Spalla et al, 2014;Marotta et al, 2018;Petri et al, 2017). Older Paleozoic ages (ca.…”

“…In the Alps, the end of the Variscan orogenic event is characterised by a high-temperature, low-pressure metamorphic evolution, which is associated with the emplacement of anatectic granitoids Spalla et al, 2014;Schuster et al, 2001, and refs therein).…”

“…The Permian-Triassic high thermal regime in the Alpine area is peculiar because a widespread HT metamorphism affected the Austroalpine and Southalpine continental crust. The resulting high-temperature metamorphism and the associated acidic and mafic igneous activity is long-lasting (from lower Permian to Norian, Spalla et al, 2014) and accompanied by extensional tectonics (Brodie et al, 1989;Lardeaux and Spalla, 1991;Diella et al, 1992;Handy et al, 1999;Schuster et al, 2001). Such a high T/P ratio has been interpreted as being the consequence of lithospheric thinning at the beginning of the Mesozoic continental rifting (Lardeaux and Spalla, 1991;Diella et al, 1992;3 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Dal Piaz, 1993;Schuster et al, 2001;Marotta and Spalla, 2007;Schuster and Stüwe, 2008;Spalla et al, 2014;Marotta et al, 2018), and the heterogeneous distribution of igneous and metamorphic rocks of this age suggests that the rifting was asymmetric.…”

“…The resulting high-temperature metamorphism and the associated acidic and mafic igneous activity is long-lasting (from lower Permian to Norian, Spalla et al, 2014) and accompanied by extensional tectonics (Brodie et al, 1989;Lardeaux and Spalla, 1991;Diella et al, 1992;Handy et al, 1999;Schuster et al, 2001). Such a high T/P ratio has been interpreted as being the consequence of lithospheric thinning at the beginning of the Mesozoic continental rifting (Lardeaux and Spalla, 1991;Diella et al, 1992;3 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Dal Piaz, 1993;Schuster et al, 2001;Marotta and Spalla, 2007;Schuster and Stüwe, 2008;Spalla et al, 2014;Marotta et al, 2018), and the heterogeneous distribution of igneous and metamorphic rocks of this age suggests that the rifting was asymmetric. This interpretation has been supported by the gradual transition from Permian-Triassic metamorphic and igneous ages in the continental crust, to the Mesozoic ages of the ophiolitic remnants (Marotta et al, 2009), and by the long-lasting extensional event that is characterised by regional scale normal faulting during the Permian-Jurassic Periods (e.g., Bertotti et al, 1993).…”