Fluid systems above basement shear zones during inversion of pre-orogenic sedimentary basins (External Crystalline Massifs, Western Alps)
Résumé
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-␣uid 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, ␣uids main- ly circulated over short distances (closed-system). However, trace element data also show that percolation of small amounts of basement-derived ␣uids occurred over several tens of meters in cover rocks right above base- ment shear zones. Indeed, the three successive vein sets recognized in the ␣eld display enrichments in basement- derived Ni, Co, and Cr, which indicate that ␣uid transfer from the basement was ef␣cient since the beginning of basin inversion, therefore con␣rming the synchronous deformation of cover and basement. Fluid temperatures and pressures are estimated (microthermometry coupled to à18O 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 ␣nally integrated into a model of ␣uid circulation evolution through progressive deforma- tion of the whole external Western Alps.