Morphometric analysis of a reactivated Variscan fault in the southern Bohemian Massif (Budějovice basin, Czech Republic)

Abstract

Quantitative geomorphic parameters are used to assess active vertical displacements at the NW–SE striking Hluboká fault and the NNE–SSW striking Rudolfov fault in the southern Bohemian Massif. The faults are part of a late Variscan fault system that was repeatedly reactivated in Mesozoic, Miocene, and Pliocene times forming the margins of the Budějovice basin. This basin is filled with up to 340-m-thick Cretaceous to Quaternary sediments and forms morphological lowland surrounded by hill country.We compared the basin-facing hillslopes along the faults with other slopes that are not fault-controlled. All creeks and drainage basins share similar geological and hydrological settings and the common base level of the Vltava River. Morphological differences in valley shapes, stream profiles, and drainage basin geometry therefore are likely to reflect different uplift of the crystalline basement with respect to the Budějovice basin.All calculated geomorphic parameters characterise the hillslope along the Hluboká fault as a very straight mountain–piedmont junction with a morphology that is influenced by uplift along the fault. Differential uplift is indicated by extremely low values of mountain front sinuosity (Smf 1.01 to 1.06), high stream length gradients (SL up to 200), and very low valley floor width to height ratios (Vf 0.05 to 0.26). The values are clearly distinct from the values observed at the other hillslopes. Streams showing convex-up thalweg sections with marked single knickpoints close to the Hluboká fault and previously published geodetic data (Vyskočil, 1973) support the interpretation of active vertical fault displacement. Values observed at the Hillslope crossing the Rudolfov fault (Smf 1.17 to 2.20, SL up to 130, Vf 0.29 to 2.5) and convex-up stream profiles of creeks crossing the fault may classify the slope along the Rudolfov fault as moderately active. The interpretation is again corroborated by published geodetic data.The analysed part of the margin of the Budějovice basin that is not fault controlled revealed geomorphic parameters that are not indicative for differential vertical displacements (Smf 1.30 to 4,16, SL up to 57, Vf 2.10 to 4.80). Concave-up stream profiles of creeks draining into the basin are characteristic for equilibrated streams.