Late- and post-Variscan cooling and exhumation history of the northern Rhenish massif and the southern Ruhr Basin: new constraints from fission-track analysis

Abstract

Apatite fission-track analyses were carried out on outcrop and core samples from the Rhenish massif and the Carboniferous Ruhr Basin/Germany in order to study the late-and post-Variscan thermal and exhumation history. Apatite fission-track ages range from 291±15 Ma (lower Permian) to 136±7 Ma (lower Cretaceous) and mean track lengths vary between 11.6 lm and 13.9 lm, mostly displaying unimodal distributions with narrow standard deviations. All apatite fission-track ages are younger than the corresponding sample stratigraphic age, indicating substantial postdepositional annealing of the apatite fission-tracks. This agrees with results from maturity modelling, which indicates 3500-7000 m eroded Devonian and Carboniferous sedimentary cover. Numerical modelling of apatite fission-track data predicts onset of exhumation and cooling not earlier than 320 Ma in the Rhenish massif and 300 Ma in the Ruhr Basin, generally followed by late Carboniferous-Triassic cooling to below 50-60°C. Rapid late Variscan cooling was followed by moderate Mesozoic cooling rates of 0.1-0.2°C/Ma, converting into denudation rates of <1 mm/a (assuming a stable geothermal gradient of 30°C/km). Modelling results also give evidence for some late Triassic and early Jurassic heating and/or burial, which is supported by sedimentary rocks of the same age preserved at the rim of the lower Rhine Basin and in the subsurface of the Central and Northern Ruhr Basin. Cenozoic exhumation and cooling of the Rhenish massif is interpreted as an isostatic response to former erosion and major base-level fall caused by the subsidence in the lower Rhine Basin.

Erosion rates on different timescales derived from cosmogenic 10Be and river loads: implications for landscape evolution in the Rhenish Massif, Germany

“…Such data indicate that the northern Rhenish Massif east of the Rhine experienced denudation rates of about 20-70 mm/ka in the Mesozoic and Cenozoic (Karg et al 2005). Accelerated cooling rates since the early Tertiary suggest that Cenozoic erosion rates were higher than those in the Mesozoic (Karg et al 2005); probably in the range of 40-70 mm/ka.…”

“…Such data indicate that the northern Rhenish Massif east of the Rhine experienced denudation rates of about 20-70 mm/ka in the Mesozoic and Cenozoic (Karg et al 2005). Accelerated cooling rates since the early Tertiary suggest that Cenozoic erosion rates were higher than those in the Mesozoic (Karg et al 2005); probably in the range of 40-70 mm/ka. A similar average denudation rate of *50 mm/ka since *120 Ma has been inferred from fission track data in the northern Rhenish Massif west of the Rhine (Glasmacher et al 1998) (Fig.…”

“…This observation suggests that also on a timescale of tens of million of years the landscape remained in or close to a steady state. Cenozoic exhumation and cooling have been interpreted as an isostatic response to former erosion and to a major baselevel fall caused by the subsidence of the lower Rhine Basin in the Tertiary (Karg et al 2005).…”

“…The incision rate of 65-90 mm/ka in the northeastern part of the massif is based on terraces described by Wortmann (1937) and Meiburg (1979). Denudation rates obtained from apatite fission track analysis for regions west and east of the Rhine are from Glasmacher et al (1998) and Karg et al (2005), respectively. See text for further explanation For the entire Rhine catchment, without the Alps, a Holocene erosion rate of 39 ± 11 mm/ka (Fig.…”

Separation of rifting and lithospheric folding signatures in the NW-Alpine foreland

“…Apatite (U + Th)/He data on samples taken above 500 m in the Vosges and AFT data in the Massif Central are consistent with cooling episodes between the middle Cretaceous and the Paleocene (Barbarand et al 2001;Bourgeois et al 2004;Peyaud et al 2005), whereas AFT and apatite (H + Th)/He data on samples taken at lower elevations in the Vosges and the Black Forest indicate successive cooling episodes in the Late Cretaceous-Paleocene, in the Oligocene and in the Miocene (Link et al 2003). In the Rhenish Massif, AFT data show a complex pattern of uplift since Late Paleozoic times (Karg et al 2005), whereas geomorphologic data support 250 m uplift since 0.8 Ma (Garcia-Castellanos et al 2000;Meyer and Stets 2002;Van Balen et al 2002).…”