Reconstructing Devonian-Carboniferous subduction in the Northern Bohemian Massif

The Devonian to early Carboniferous geodynamic evolution of the Bohemian Massif is largely controlled by a “diffuse cryptic suture zone” (Schulmann et al., 2014), which crops out between the Saxothuringian Zone (forming the lower plate) in the north and the Teplá-Barrandian and Moldanubian Zones (collectively the upper plate) in the south. Oceanic passing to continental subduction within this suture zone has been linked to at least three discrete episodes of high-pressure to ultra-high-pressure metamorphism; the formation and obduction of Devonian age ophiolites; the up to 50 myr building of a continental magmatic arc; and potentially, the large-scale relamination of continental crust beneath the upper plate and its exhumation into the upper plate in the form of trans-lithsopheric diapirs.

Taken together, this would appear to require long-lasting subduction of a vast oceanic domain likely including old and dense oceanic lithosphere. Yet, significant separation between the lower and upper plates during the Early Paleozoic is not consistent with litho-stratigraphic, paleontological, or paleomagnetic data, which indicate a shared peri-Gondwanan shelf derivation of these units. Nonetheless, within the high-grade rocks of the suture zone itself, an exotic assemblage of Cambrian age volcanic-arc related rocks are identified. These rocks have been variably metamorphosed up to eclogite- and granulite-facies conditions during an early phase of the Variscan Orogen, but, also include lower-grade segments that experienced only lower amphibolite- or greenschist-facies conditions. A compilation of whole-rock geochemical, isotopic and zircon U-Pb and Lu-Hf data from this Cambrian arc assemblage is presented to argue for the exotic nature of this terrane including its possible derivation from the Baltica paleo-continent and for an association with old oceanic lithosphere (Stenian-Tonian age) likely captured from the circum-Rodinia Mirovoi Ocean.

Thus, it is proposed that the geodynamic evolution of the Bohemian Massif cannot be reconciled with a single-phase of oceanic passing to continental subduction. Instead, a three stage evolution is proposed involving: (1) initial subduction of an old oceanic crust and extinct Cambrian age arc terrane derived from the Baltica paleo-continent beneath the peri-Gondwanan margin; (2) transcurrent displacement of a strip of peri-Gondwanan crust behind the initial subduction zone; (3) a second phase of oceanic passing to continental subduction of this displaced peri-Gondwanan crust beneath the initial subduction zone.

Schulmann, K., Lexa, O., Janoušek, V., Lardeaux, J.M. and Edel, J.B., 2014. Anatomy of a diffuse cryptic suture zone: an example from the Bohemian Massif, European Variscides. Geology 42, 275–278.