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.
Highlights
The classical concept of collisional orogens suggests that mountain belts form as a crustal wedge between the downgoing and overriding plates
It was later shown that subduction of continental mantle beneath progressively accreted crust results in an asymmetric wedge-shaped architecture dominated by high-pressure (HP) metamorphism typical for Alpine type orogens[3]
The European Variscan belt is a large fossil and deeply eroded bivergent orogen characterized by high-temperature (HT) metamorphism and widespread crustal melting[5]
Summary
The classical concept of collisional orogens suggests that mountain belts form as a crustal wedge between the downgoing and overriding plates. Here the channel flow exhumation scenario fails to explain the occurrence of (ultra-)high pressure (U)HP continentderived rocks and mantle relics in the axial zone of the Variscan orogen several hundred kilometers away from the plate interface[8]. The scenario further assumes that subducted continental crust was relaminated beneath the Moho of the upper plate and emplaced into the crust of the orogenic root in the form of giant elongated diapiric bodies composed of UHP felsic rocks and mantle fragments.
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