Low-temperature thermochronology of the flanks of the southern Upper Rhine Graben

Abstract

Low-temperature thermochronology of the flanks of the southern Upper Rhine GrabenAbstract The Upper Rhine Graben (URG) is the most perceptible part of the European Cenozoic Rift System. Uplifted Variscan basement of the Black Forest and the Vosges forms the flanks of the southern part of the graben. Apatite and zircon fission-track (FT) analyses indicate a complex low-temperature thermal history of the basement that was deciphered by inverse modelling of FT parameters. The models were tested against the observed data and independent geological constraints. The zircon FT ages of 28 outcrop samples taken along an E-W trending transect across the Black Forest and the Vosges range from 136 to 312 Ma, the apatite FT ages from 20 to 83 Ma. The frequency distributions of confined track lengths are broad and often bimodal in shape indicating a complex thermal history. Cooling below 120°C in the Early Cretaceous to Palaeogene was followed by a discrete heating episode during the late Eocene and subsequent cooling to surface temperature. The modelled time-temperature (t-T) paths point to a total denudation of the flanks of URG in the range of 1.0-1.7 km for a paleogeothermal gradient of 60°C/km, and 1.3-2.2 km for a paleogeothermal gradient of 45°C/ km since the late Eocene.

The south-western Black Forest and the Upper Rhine Graben Main Border Fault: thermal history and hydrothermal fluid flow

“…Our apatite FT ages, ranging between 55 ± 4 and 103 ± 9 Ma, do not differ significantly from the previously published ages for the crystalline basement of the central southern Black Forest (20 ± 2 and 83 ± 12 Ma; Timar-Geng et al 2006). TimarGeng et al (2006) reported a continuous cooling of the crystalline basement during the Cretaceous, Palaeogene, and the Early Eocene, based on apatite FT track length modelling, followed by short period of heating to temperatures of *120°C during Late Eocene to Oligocene.…”

“…2,5). This suggests that the formation of Cataclasite-II should have occurred before or contemporary to the Late Oligocene to Miocene rifting during which temperatures not higher than 120°C within its rift shoulders have been reported (Timar-Geng et al 2006). Otherwise, the temperatures estimated for the formation of Cataclasite-II has to cause apatite FT ages clearly younger than the observed of the fault related samples.…”

“…The zircon FT ages (Fig. 5) of the fault-related samples HD128 (109 ± 17 Ma) and HD130 (120 ± 20 Ma) are slightly -Geng et al 2006). Zircon samples of the Kandern Fault Zone yield younger FT central-ages than the reference sample sets, while the apatite FT central-ages do not differ significantly from the other sample sets younger than the zircon FT ages from the southern Black Forest (this study, Timar-Geng et al 2004, 2006.…”

“…Extensive hydrothermal activity with temperatures of *250°C during the Jurassic caused substantial overprint of the inherited apatite and zircon FT ages (Timar-Geng et al 2004, 2006. A moderate cooling below 120°C affected the uppermost crystalline basement of the Black Forest between Jurassic and Late Eocene times, followed by a short heating event with temperature of up to 120°C coeval with enhanced subsidence of the URG during the Late Eocene to Early Oligocene (Timar-Geng et al 2006). Although the rocks generally cooled since then the exact thermal conditions during the Miocene are still a matter of debate (Timar-Geng et al 2006, Ziegler andDèzes 2007).…”

Tracing sediment pathways by zircon fission track analysis: Oligocene marine connections in Central Europe

“…During the Jurassic, both basement and cover rocks experienced intense hydrothermal activity (Wernicke and Lippolt 1997;Brockamp et al 2003). ZFT ages of the Black Forest and the Vosges range between 164 and 247 Ma, but are predominantly Jurassic (Timar-Geng et al (2006) ; Fig. 4).…”