Fluid systems above basement shear zones during inversion of pre-orogenic sedimentary basins (External Crystalline Massifs, Western Alps)
In the inner part of the External Alps, inherited Liassic basins were buried and inverted during the Oligo-Miocene collisional phase of the Alpine orogeny. In northern Oisans, during crustal shortening, the basement was locally sheared while the cover was disharmonically folded above the main basement shear zones that did not propagate into the cover. In this contribution, we analyze the witnesses of paleo-fluid circulations associated with these crustal deformations, focusing particularly on Bourg d'Oisans and Mizoën basins (external Western Alps). On the basis of structural and microstructural observations coupled to geochemical analyses (cathodoluminescence, O and C stable isotopes, trace elements) of vein versus host-rock minerals, we show that in the cover, fluids mainly circulated over short distances (closed-system). However, trace element data also show that percolation of small amounts of basement-derived fluids occurred over several tens of meters in cover rocks right above basement shear zones. Indeed, the three successive vein sets recognized in the field display enrichments in basement-derived Ni, Co, and Cr, which indicate that fluid transfer from the basement was efficient since the beginning of basin inversion, therefore confirming the synchronous deformation of cover and basement. Fluid temperatures and pressures are estimated (microthermometry coupled to δ18O of vein minerals) to about 250-400 °C and 2-5 kbar for veins that most likely formed at or close to metamorphic peak conditions. These results coupled to literature data are finally integrated into a model of fluid circulation evolution through progressive deformation of the whole external Western Alps.