Late-Variscan fluid migration at the Variscan thrust front of Eastern Belgium: numerical modelling of the palaeothermal and fluid flow field

Abstract

Modelling of the palaeothermal field at the Variscan thrust front in eastern Belgium indicates significant temperature modifications by late-Variscan palaeofluids migrating from internal to peripheral parts of the orogen. A detailed set of calibration data (chlorite geothermometry, microthermometry, organic rank) gives evidence of temporary palaeotemperature variations at the Variscan thrust front obviously connected to the migration of hot, low saline palaeofluids. These thermal events likely enhanced organic maturation (vitrinite reflectance, conodont alteration) of Devonian and Carboniferous sediments, which accumulated long before the Variscan orogeny occurred. Numerical simulation (2D Finite Element method) of the palaeothermal field includes coupled heat transport by thermal conduction and fluid flow. Palaeothermal scenarios yield successive palaeotemperatures (200-300°C), which are indicated by the control data, due to relatively short-term fluid ascent along the detachment and the imbricate thrust front. The simulated flow velocities are up to tens of metre per year lasting several thousand years (non-steady-state solution). In the scenarios modelled, these thermal events occur in a realm of enhanced bulk temperatures due to elevated basal heat flow densities (90 mW m -2 ) and an additional burial depth of some kilometres. The simulated temperature enhancement due to fluids ascending at the Variscan thrust front is several tens degrees. The scenarios demonstrate long-distance fluid migration during or after deformation of the Palaeozoic basin and its effect on the palaeothermal field.

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

“…Within the upper Carboniferous of the Wurm syncline vitrinite reflectance increases towards the south and the Aachen thrust (Babinecz 1962;Steingrobe 1990;Teichmüller and Teichmüller 1979;von Winterfeld 1994). Increased organic maturity in this area is explained by enhanced heat flow which was caused by fluid discharge in the footwall of the Aachen thrust (Knapp 1980;Lünenschloss et al 1997Lünenschloss et al , 2008Lünenschloss 1998;Teichmüller and Teichmüller 1979). A similar thermal overprint in the temperature range of 210-310°C can also be assumed for the rock units of the RWTH-1 well.…”

“…Abundant indications of fluid flow are given by high coalification grades in the hanging wall and in the footwall close to the Aachen thrust (Babinecz 1962;Teichmüller and Teichmüller 1979;von Winterfeld 1994) that are interpreted to be the result of Carboniferous and thus Variscan (Knapp 1980) hydrothermal fluid flow. Similarly, Lünenschloss et al (1997Lünenschloss et al ( , 2008 concluded from the regional distribution of vitrinite reflectance that coalification anomalies resulted from significant fluid flow at the Variscan thrust front. Other valuable monitors of fluid flow are mineralised veins that are ubiquitous in many metamorphic settings.…”

Novel crushing technique for measuring δ18O and δ2H values of fluid inclusions (H2O) in quartz mineral veins using Cavity Ring-Down Spectroscopy

“…The outcrop along the river consist of the alternating layers of siltstones and low grade metamorphosed sandstones called psammite [51]. During the late Carboniferous, the progressive deformation of Variscan fold-and-thrust belt in Rhenish massif led to an initial burial metamorphism of siltstones and sandstones layers that initiated the upward migration of warm fluids in the northern Variscan front zone driven by Variscan thrusting [52], [53], [54]. These migrated warm fluids are source of the quartz veins.…”