Reply to W. Franke on W. Dörr and G. Zulauf elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone

Abstract

In his comments on our paper about Variscan elevator tectonics, W. Franke (this issue) presents several reasons, which—according to his view—would not be compatible with overthickened crust in the Tepla-Barrandian unit during Upper Devonian/Lower Carboniferous continent/ continent collision. In the following paragraphs, we will show that (1) most points addressed by W. Franke are not relevant for our model of elevator tectonics, (2) correlations made by W. Franke between various crustal blocks of the Bohemian Massif are not consistent with data, and (3) the models presented by W. Franke are based on assumptions but ignore a complete set of robust data. W. Franke is pointing out that the Barrandian Paleozoic rocks underwent only minor shortening (less than 20 %). There would be no evidence for major thrust stacking under the Paleozoic cover, and the diagenetic grade of the Devonian rocks would preclude the once former existence of higher structural units. These observations would directly contradict the reconstruction of Fig. 15 of Dorr and Zulauf (2010), in which the Barrandian syncline is shown to occupy the central and upper part of a strongly thickened crust. All the facts listed by W. Franke are correct. His conclusions, however, are wrong. The metamorphic grade and the magnitude of upper crustal shortening cannot be used to constrain crustal thickness. W. Franke does not consider that the entire Tepla-Barrandian crustal block was thrust on top of Saxothuringian crust during Upper Devonian continent/continent collision. This scenario of doubling the crustal thickness has already been emphasized by Bues et al. (2002). It explains why mantle and lower crust slices occur inside the Bohemian shear zone. An almost complete oblique crustal section is exposed between Stod and the Hoher Bogen shear zone. Thrusting of the Tepla-Barrandian crust on top of Saxothuringian crust is also shown in the seismic section 9HR and is supported by gravimetric data published recently by Guy et al. (2011). Note that both the seismic and the gravimetric data are fully compatible with the elevator tectonics model of Dorr and Zulauf (2010). Northwest from the Moldanubian domain occurs an important gravity high corresponding to the Neoproterozoic basement of the Tepla-Barrandian Unit limited in the north by southeast-dipping reflectors of the Tepla suture, which is characterized by high-density eclogites and ultramafics. The footwall of the suture corresponds to lowdensity felsic crust of the Saxothuringian basement, and underneath the Tepla-Barrandian suprastructure, a lowdensity granulite channel was identified, the latter coinciding with vertical extrusion of felsic granulites in the Eger valley (Guy et al. 2011). The steep boundary between the Tepla-Barrandian and Moldanubian zone is interpreted by Guy et al. (2011) in terms of major vertical elevation of the orogenic lower crust supporting the Dorr and Zulauf (2010) elevator tectonics model. Concerning the magnitude of shortening in elevated domains, W. Franke should also consider recent analogs of the Tepla-Barrandian plateau. The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic cooling and denudation, yet shows no evidence for significant upper crustal shortening (Densmore et al. 2007). The same holds for the interior parts of the Tibetan Plateau. W. Franke believes that the amount of crustal thickening can be constrained from the level of erosion. He states that W. Dorr G. Zulauf (&) Institut fur Geowissenschaften, Universitat Frankfurt a.M, Altenhoferallee 1, 60438 Frankfurt, Germany e-mail: g.zulauf@em.uni-frankfurt.de