Topographic inversion and changes in the sediment routing systems in the Variscan orogenic belt as revealed by detrital zircon and monazite U Pb geochronology in post-collisional continental basins

Abstract The basal alluvial and fluvial siliciclastic successions within the late Carboniferous intracontinental basins of the west-central Bohemian Massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse. We analyze this record using the U Pb laser ablation ICP-MS detrital zircon and monazite geochronology. Five distinct age groups have been identified across the examined stratigraphic succession: (I) Archean/Paleoproterozoic, (II) late Neoproterozoic, (III) Cambro–Ordovician, (IV) Late Devonian, and (V) mid- to late-Carboniferous. These age groups indicate mixing of both local and distal, extra-basinal sources and also point to a two-stage post-convergent geodynamic evolution of the Variscan belt. The first stage followed collapse of the central ‛Bohemian’ orogenic plateau at around 346–337 Ma and involved rapid cooling at ca. 340–320 Ma and removal of ca. 2–3 km of sedimentary overburden from the top of the plateau. No sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands. A paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca. 330–310 Ma. In turn, these uplifted domains, occupied by rapidly unroofed voluminous granite plutons, shed detritus into the ‛Bohemian’ sink. This was represented by the former plateau, collapsed to relatively low elevations, but still with significant internal paleorelief dominated by a system of tens of kilometers long ~NNE–SSW-trending paleovalleys. Some of the material was further transported in extensive river systems crossing the former plateau to the northerly Variscan foreland basin. Thus, the radial sediment routing system changed into axial (unidirectional). Finally, we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late-orogenic lithospheric-mantle delamination events. First-stage delamination occurred beneath the center of the Bohemian Massif to cause the plateau collapse, denudation, and radial sediment transport, whereas the second-stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport. End of deposition and magmatic shutdown at around 305 Ma may then define the very cessation of late-orogenic vertical movements and the transition to a post-orogenic, intra-plate setting.