Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review

Abstract

Abstract During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data ( 207 Pb/ 206 Pb single zircon, SHRIMP, 40 Ar/ 39 Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by 40 Ar/ 39 Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older 40 Ar/ 39 Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at c. 300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and c. 275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC. Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.