Metamorphic Remnants of the Variscan Orogeny across the Alps and Their Tectonic Significance

Abstract

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization.

Structural, lithostratigraphic and thermal features of a Permian lower crust from the Western Italian Alps (Valpelline Series, Valle d’Aosta)

“…2022; Roda et al . 2023) and subsequent orogenic collapse and lithospheric extension during late Carboniferous-Permian (Lardeaux & Spalla, 1991; Marotta & Spalla, 2007; Schuster & Stuwe, 2008; Roda et al 2018; Ewing et al 2023).…”

“…This could explain the close P-T conditions and similar mineral assemblage stability during the D 2 and D 3 events. However, the detection of kyanite in garnets by Gardien et al (1994) shifting the early path to higher pressure conditions followed by decompression heating may highlight a more complex evolution probably linked to Variscan orogenic crustal thickening during the Carboniferous (Carosi et al 2020;Schulmann et al 2022;Roda et al 2023) and subsequent orogenic collapse and lithospheric extension during late Carboniferous-Permian (Lardeaux & Spalla, 1991;Marotta & Spalla, 2007;Schuster & Stuwe, 2008;Roda et al 2018;Ewing et al 2023).…”

“…In the Western Alps, there are pre-alpine lower crustal slices such as the II Zona Diorito-Kinzigitica (IIDK; Carraro et al 1970), the Ivrea-Verbano Zone (IVZ; Quick et al 2003;Redler et al 2012Redler et al , 2013Ewing et al 2013Ewing et al , 2015 and the Valpelline Series (VP; Gardien et al 1994;Manzotti & Zucali, 2013;Caso, 2023), which have been affected by widespread hightemperature (hereafter HT) metamorphism and are now fragmented and found in different portions of the Western Alpine belt. The HT evolution is related to the post-Variscan collisional collapse and/or Permian lithospheric extension (Lardeaux & Spalla, 1991;Marotta & Spalla, 2007;Schuster & Stüwe, 2008;Roda et al 2018;Roda et al 2023). In particular, the VP in the Dent-Blanche Tectonic System (DBTS; Manzotti et al 2014;Manzotti & Ballèvre, 2017), located in the Valle d'Aosta (Western Italian Alps), represents a well-preserved exposure of a lower continental crustal section that has largely escaped the Alpine-age metamorphic overprint.…”