Trans-lithospheric diapirism explains the presence of ultra-high pressure rocks in the European Variscides

Abstract

The classical concept of collisional orogens suggests that mountain belts form as a crustal wedge between the downgoing and overriding plates. However, this orogenic style is not compatible with the presence of (ultra-)high pressure crustal and mantle rocks far from the plate interface in the Bohemian Massif of Central Europe. Here we use a comparison between geological observations and thermo-mechanical numerical models to explain their formation. We suggest that continental crust was first deeply subducted, then flowed laterally underneath the lithosphere and eventually rose in the form of large partially molten trans-lithospheric diapirs. We further show that trans-lithospheric diapirism produces a specific rock association of (ultra-)high pressure crustal and mantle rocks and ultra-potassic magmas that alternates with the less metamorphosed rocks of the upper plate. Similar rock associations have been described in other convergent zones, both modern and ancient. We speculate that trans-lithospheric diapirism could be a common process.

Discovery of Variscan orogenic peridotites in the Pelvoux Massif (Western Alps, France)

“…This relaminated lower crust would have then been locally extruded by up-doming (Janoušek & Holub, 2007;Lexa et al, 2011;Schulmann, Catalán, et al, 2014). This mechanism has been corroborated by recent thermomechanical modeling (Maierová et al, 2018(Maierová et al, , 2021, and could be a viable explanation to the presence of mantle enclaves in the lower crust of ECM, far from the subduction trench. Furthermore, it is rather consistent with (i) the extent of metasomatism observed in the enclaves, and (ii) the sampling of different mantle domains at different depths.…”

“…Therefore, these different units correspond to contrasted exhumation mechanisms, and mark different processes within the Variscan collision. While the plate-interface probably correspond to a former subduction mélange exhumed in the subduction channel, exhumation of Moldanubian peridotites and other UHP rocks might involve less conventional mechanisms of lower crustal "relamination" (Maierová et al, 2018(Maierová et al, , 2021Schulmann, Lexa, et al, 2014).…”

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

“…This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a 'Cascadia-type' active margin, which corresponds to a supra-subduction tectonic switching paradigm. The (ultra)high-pressure ((U)HP) granulite facies metamorphism characterized by clockwise P-T paths usually forms due to subduction of continental crust during continental collision (Dewey et al, 1993;Maierov a et al, 2021;O'Brien, 2000; J.X. Zhang et al, 2008) but can also form during homogeneous thickening of the upper plate continental root rocks (e.g., Le Pichon et al, 1997).…”

Compositional trends in Ba-, Ti-, and Cl-rich micas from metasomatized mantle rocks of the Gföhl Unit, Bohemian Massif, Austria

“…Subsequently, the crustal material was underplated beneath the orogenic root and later exhumed due to the indentation of the Brunia microcontinent under the Moldanubian Domain (Racek et al 2006;Guy et al 2011;Schulmann et al 2014;Kusbach et al 2015;Maierová et al 2021) The granulite massifs from the Saxothuringian Domain and the Gföhl Unit are predominantly formed by felsic kyanite and K-feldspar bearing granulites and gneisses, but they also contain a large number of garnet peridotite bodies associated with layers or lenses of garnet pyroxenites and eclogites. These latter rocks are interpreted as mantle fragments that were incorporated into the continental crust during or after the subduction of the Saxothuringian plate under the Teplá-Barrandian plate during the Variscan orogeny (Cooke 2000;Medaris et al 2005;Schmädicke et al 2010;Kusbach et al 2015).…”