Tectonostratigraphy of the southeastern part of the Bohemian Massif

Ophiolites and eclogite bearing units are important to reveal the tectonostratigraphy of orogenic belts as they define the position of suture zones. Especially an oceanic suture is a significant tectonic element as it separates former continental entities, which might have been far away from each other before oceanic closure and the subsequent continental collision. Conversely, the tectonic subdivision of an orogen should always reflect the former palaeogeographic relationships so that the 1^st order tectonic units can be defined in a comprehensible manner.

In the southeastern part of the Bohemian Massif, ophiolite slices and eclogite occurrences are known since several decades. They are likely remnants of the oceanic space (Proto-Rheic and/or Rheic oceanic domain) that separated peri-Baltic (e.g. Avalonia) and peri-African (e.g. Armorican) crustal elements before the Variscan collisional nappe tectonics. However, due to a post nappe-stacking metamorphic overprint reaching granulite facies and anatexis many shear zones responsible for nappe stacking are strongly recrystallized and now difficult to identify.

The tectonic nomenclature most commonly used to this day in geological maps dates from 1927 and divides the area according to a metamorphic discontinuity into the low-grade to amphibolite facies Moravicum and the granulite facies Moldanubicum. The Moravicum is the southwestern continuation of the Brunovistulicum, which also includes parts of the Variscan foreland. In addition, the late to post-deformational South Bohemian Batholith is distinguished and the strongly anatectic Bavaricum in the southwest is optionally separated.

In the course of detailed mapping in the Danube valley between Stein and Spitz, former peri-Baltic and peri-African rock units could be clearly separated based on lithological criteria as well as an intervening oceanic suture zone. This tectonic subdivision can be tentatively extrapolated to the entire southeastern part of the Bohemian Massif. In map view, the oceanic suture is running southwest-northeast. In general, the peri-Baltic derived units form the footwall in the East, whereas the peri-African derived units in the West are overlying the oceanic suture. However, the tectonic style is complex with flower structures and out of sequence thrusts formed in a transpressional environment.

The presently used terminology Moldanubicum vs. Moravicum only partially reflects these palaeogeographical aspects. The peri-Baltic units build up the Moravicum, but also the Drosendorf Nappe System of the Moldanubicum. The ophiolites and sediments of the Variscan oceanic suture zone are part of the Gföhl Nappe System and the eclogites occur in the easternmost part of the Ostrong Nappe System. Both these nappe systems belong to the Moldanubicum according to the classic nomenclature. The remaining Moldanubicum most probably originates from Armorica, i.e., the African sector of Gondwana. In order to create a more logical nomenclature we suggest the following improvements. The term Moldanubian Unit should exclusively be used for rock units derived from the African sector of the Gondwana margin. The oceanic suture zone could be included in a separate 1^st order unit (Raabs Unit). Nappes that consist of peri-Baltic rocks like the Drosendorf Nappe System should be affiliated with the Moravian and Brunovistulian units, whereas the Variscan foreland might be treated separately.