Fission-track analyses in the area of the southern Upper Rhine Graben

Abstract

Fission-Track (FT) analysis of the Palaeozoic crystalline bedrock in the area of the southern Upper Rhine Graben (URG) showed complex and rapid changing upper crustal thermal conditions during the Mesozoic and the Cenozoic (Timar-Geng et al. 2004, 2006a). While an Eo-Oligocene thermal pulse accompanying the rifting of the URG is well documented by apatite FT modelling results (Timar-Geng et al. 2006a,b), the Jurassic hydrothermal period leading to a broad scatter of zircon FT data (Timar-Geng et al. 2004) are only weakly constrained. Additionally, due to a large sedimentary hiatus between the Upper Jurassic and the Late Eocene the pre-rift evolution remains still unclear. Knowledge about the thermal evolution of the area is essential for development of crustal scale models, which evaluate the rift-evolution. This study aims to clarify the timing of the Jurassic thermal pulse(s) and its potential to heat the Mesozoic sediments, which covers the Palaeozoic basement. Especially, the temperatures and geothermal-gradients that were reached are of mayor interest. During a hydrothermal period is convective heat transport the important mechanism, which influences the reached temperatures in the upper crust. Therefore, addresses a localised detail study at the URG main border fault the potential of fault-bounded thermal anomalies caused by ascending hot fluids to heat the surrounding rocks. The FT method, which is the base method used here, is a low-temperature thermochronological method widely used to quantify the thermal history of igneous, metamorphic and clastic sedimentary rocks. In particular, the FT analyses of detrital samples provide the advantage to discriminate between a pre-depositional thermal history of the provenance regions and a post-depositional basin related thermal history (e.g. Brandon 1998, Carter 1999, Bernet & Garver 2005, Armstrong 2005). The FT analyses oft the Permian and Mesozoic sediment column in the URG area, this study, compared with previous FT studies from the Black Forest and the Vosges (Michalski 1988, Wyss 2000, Timar-Geng et al. 2004, 2006a, b) led to a complete overview of the FT age signatures of URG pre-rift units. This is a requirement for the interpretation of Cenozoic detrital syn-rift deposits of the region by the FT method. New FT analyses on Cenozoic samples from the southern URG area led to basal insights in the syn-rift river drainage patterns.