How many subductions in the Variscan orogeny? Insights from numerical models

The Variscan belt is the result of the Pangea accretion, a prominent feature of the European continental lithosphere (von Raumer et al., 2003) . The debate on the number of oceans and the geodynamic evolution of the Variscan belt is still open (Faure et al., 2009; Franke et al., 2017). Two scenarios have been proposed:

1. Monocyclic scenario: assumes a single long-lasting south-dipping subduction of a large oceanic domain. Armorica remained more or less closed to Gondwana during its northward drift, in agreement with lack of biostratigraphic and paleomagnetic data that suggests a narrow oceanic domain (lesser than 1000 km; Matte, 2001; Lardeaux, 2014);

2. Polycyclic scenario: this geodynamic reconstruction envisages two main oceanic basins opened by the successive northward drifting of two Armorican microcontinent and closed by two opposite subductions (Lardeaux, 2014; Franke et al., 2017). The northern oceanic basin is identified as the Saxothuringian ocean, while the southern basin is identified as the Medio-European ocean (Lardeaux, 2014).

Models of single and double subduction have been developed to verify which scenario better fits with Variscan P-T evolutions from the Alps and the French Central Massif (FCM). From the comparison between model predictions and natural Variscan P-T-t estimates results that data from the Alps with high P/T ratios better fit with the double subduction model, supporting that a polycyclic scenario is more suitable for the Variscan belt evolution. Differently, data from the FCM with high P/T ratios that fit with both models have poorly constrained geological ages and, therefore, are not suitable to actually discriminate between mono- and polycyclic scenarios (Regorda et al., 2020). Moreover, the predictions of the models open to the possibility that rocks of the Upper Gneiss Unit of the FCM could derive from tectonic erosion of the upper plate and not only from the ocean-continent transition of the lower plate.

References

Faure M., Lardeaux J.-M. and Ledru P.; 2009: A review of the pre-Permian geology of the Variscan French Massif Central. Comptes Rendus Geoscience, 341, 202-213.

Franke W., Cocks L.R.M. and Torsvik T.H.; 2017: The Palaeozoic Variscan oceans revisited. Gondwana Research, 48, 257-284.

Lardeaux J.-M.; 2014: Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts. Part II: Variscan metamorphism in the French Massif Central – A review. Bull. Soc. géol. France, 185(5), 281-310.

Matte P.; 2001: The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of theArmorica microplate: A review. Terra Nova, 13(2), 122-128.

Regorda A., Lardeaux J-.M., Roda M., Marotta A.M. and Spalla M.I.; 2020: How many subductions in the Variscan orogeny? Insights from numerical models. Geoscience Frontiers, 10.1016/j.gsf.2019.10.005.

von Raumer J. F., Stampfli G.M. and Bussy, F.; 2003: Gondwana-derived microcontinents – the constituents of the Variscan and Alpine collisional orogens. Tectnophysics, 365, 7-22.