Late Carboniferous paleoelevation of the Variscan Belt of Western Europe

Abstract

We present the first stable isotope paleoaltimetry estimates for the hinterland of the eroded Variscan Belt of Western Europe based on the hydrogen isotope ratios of muscovite from syntectonic leucogranites that have been emplaced at ∼ 315 Ma. We focus on the Limousin region (Western Massif Central, France) where peraluminous granites are spatially associated with strike-slip and detachment shear zones that developed as a consequence of Late Carboniferous syn-to post-orogenic extension. In this region, we show that the north-east corner of the Millevaches massif (located at the junction between brittle and ductile fault systems) represented a pathway for Earth surface-derived fluids that penetrated the crust and reached the ductile segment of the low-angle Felletin detachment zone. Using microstructural, thermometry, hydrogen isotope and 40 Ar/ 39 Ar geochronological data, we show that these Variscan meteoric fluids interacted with hydrous silicates during high temperature deformation between ∼ 318 and 310 Ma. For paleoaltimetry purposes, we reference our hydrogen isotope record (δD) of ancient meteoric fluids from mylonitic rocks to ∼ 295 Myr-old records retrieved from freshwater shark remains preserved in the Bourbon l'Archambault basin that developed in the external zones of the orogen.A ∼ 76 difference in δD meteoric water values between the Millevaches massif (δD meteoric water value = − 96 ± 8 ) and the Bourbon l'Archambault foreland basin (δD water value = − 20 ± 6 ) is consistent with paleoaltimetry estimates of 3.4 ± 0.7 km based on a modern lapse rate of ∼ -22 /km for δD water values. The rather large difference in δD values between the foreland basin and the continental interior suggests that the hinterland of the Variscan belt of western Europe was high enough to act as a barrier to moisture transport from the south-south-east and induce an orographic rain shadow to the north.

Cenozoic mountain building and topographic evolution in Western Europe: impact of billions of years of lithosphere evolution and plate kinematics

“…Late Carboniferous elevation of ~3 km estimated for the Variscan belt (Massif Central) (Dusséaux et al, 2021) and evidence of the lack of marked rain shadow that suggests moderate elevation of 2000 m during late Carboniferous-Permian (Roscher and Schneider, 2006) supports this model. We have estimated that the mantle heat flow established during the Late Carboniferous-Permian (300-270 Ma) was maintained to high values of 32 mW/m 2 (Figure 9).…”

Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

“…These data indicate that meteoric water can infiltrate to 8-10 km from the surface, suggesting that they can flow downward and laterally along the STDS, and then mixed with deeply derived fluids. Previous studies have demonstrated that meteoric water can reach the brittle-ductile transition in the crust through faults or detachment shear zones, such as in the Cordilleran metamorphic core complexes (e.g., Fricke et al, 1992;Kerrich, 1988;Mulch, Teyssier, Cosca, & Vennemann, 2006;Wickham & Taylor, 1985), Variscan Shear Zone (Dusséaux et al, 2019(Dusséaux et al, , 2021Lemarchand et al, 2012), STDS (Gébelin et al, 2013(Gébelin et al, , 2017, Alpine Fault (Menzies et al, 2014), and Menderes metamorphic core complex (Hetzel et al, 2013). These previous studies have documented surface-derived fluids flowing down and mixing with deeply derived fluids (Gébelin, Teyssier, Heizler, & Mulch, 2015;Gottardi, Kao, Saar, & Teyssier, 2013;Person, Mulch, Teyssier, & Gao, 2007).…”

“…Fluids from different sources have distinct isotopic signatures (Yardley & Bottrell, 1992). A relatively low hydrogen (δ 2 H) and oxygen (δ 18 O) isotopic composition of fluid can be indicative of a meteoric component (e.g., Zang et al, 2019), and thus used as a palaeoaltimeter (e.g., Campani, Mulch, Kempf, Schlunegger, & Mancktelow, 2012;Dusséaux, Gébelin, Ruffet, & Mulch, 2021;Gébelin et al, 2013;Gébelin, Mulch, Teyssier, Page Chamberlain, & Heizler, 2012;Mulch & Chamberlain, 2007;Rowley & Garzione, 2007). Low δ 2 H and δ 18 O ratios recorded in extensional detachment systems indicate that meteoric fluids can circulate into different depths and even into ductile detachment zones (e.g., Gottardi et al, 2015;Morrison & Anderson, 1998).…”

Timing and duration of meteoric water infiltration in the Quiberon detachment zone (Armorican Massif, Variscan belt, France)

“…Assessing the source of fluids in such hydrothermal systems has a range of applications including constraining the conditions for mineralization and ore deposition (Ballouard et al, 2017(Ballouard et al, , 2018aBeaudoin et al, 1991;Boiron et al, 2003) or paleoaltimetry reconstructions if surface-derived (meteoric) fluids are present (e.g. Dusséaux et al, 2021;Gébelin et al, 2012Gébelin et al, , 2013Grambling et al, 2022;Mulch et al, 2004).…”

“…Gébelin et al, 2017Gébelin et al, , 2013, and in the Variscan belt of Western Europe (e.g. Ballouard et al, 2018aBallouard et al, , 2017Ballouard et al, , 2015Dusséaux et al, 2021Dusséaux et al, , 2019. Some of these studies (e.g.…”