Fluid flow in foreland basins: spatial and temporal scaling of their transport mechanisms

Foreland basins often host important ore deposits (like MVT deposits; Bradley and Leach, 2003) which are associated with deep and shallow fluid circulation. Those fluids, expelled from the orogen, can have different origins like meteoric water, diagenetic fluid, metamorphic fluid or even deeper crustal origins (Oliver J., 1986). However, whether these fluids are expelled to the foreland as continuous flow or as series of rapid pulses remains largely unexplored. Here, we combine numerical modelling with geochemical data and petrographic observation of a sandstone and its associated veins and reaction halos to identify spatial and temporal fluid flow and its transport mechanism(s) in foreland basins.

Thin sections from a Rotliegend red arkose-sandstone formation (German Permian Variscan foreland) were investigated using microprobe analysis and BSE-EDS-SEM imaging. The arkose-sandstone exhibited tapering lighter reaction halos around veinlets, most likely produced through redox reactions upon fluid infiltration into the sandstone. The model Elle from Koehn et al., (2022) was subsequently applied to link fluid transport mechanism to the patterns and geometry observed in the samples. Pore pressure applied from a crack toward the host rock and a concentration gradient were used to create fluid flow in the sandstone from which a range of values for advection, diffusion and a reaction rate were deduced. In this way, the model allowed to mimic the same pattern/geometry as the sample on several scales and enabled a systematic assessment whether fluid flow may have been constant or pulsating.

Combining petrographic, geochemical and modelling investigations revealed that the reaction halos in the sandstone were in fact formed upon a single rapid fluid flow event, that presumably was fast and channelised in the vein, and pervasive and comparatively slow in the surrounding host rock. These preliminary results imply that fluid flow and transport in foreland basins may be of a more pulsating nature rather than continuous steady state fluid flow and transport mechanisms may thus be similar to what has been previously reported for subduction zone settings (e.g., Kleine et al., 2016).

Bradley, D. C., & Leach, D. L. (2003). Tectonic controls of Mississippi Valley-type lead–zinc mineralization in orogenic forelands. Mineralium deposita, 38(6), 652-667.

Kleine, B. I., Zhao, Z., & Skelton, A. D. (2016). Rapid fluid flow along fractures at greenschist facies conditions on Syros, Greece. American Journal of Science, 316(2), 169-201.

Koehn, D., Kelka, U., Toussaint, R., Siegel, C., Mullen, G., Boyce, A., & Piazolo, S. (2022). Outcrop scale mixing enhanced by permeability variations: the role of stationary and travelling waves of high saturation indices. Geological Magazine, 159(11-12), 2279-2292.

Oliver, J. (1986). Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology, 14(2), 99-102.