Helium isotopic signature of modern and fossil fluids associated with the Corinth riftfault zone (Greece): Implication for fault connectivity in the lower crust

摘要

Helium isotopic ratios of modern groundwaters and of fossil fluids trapped in secondary calcites from faultplanes have been measured along the southern Corinth rift in order to investigate the nature of fluidscirculating in these fault zones and to set constraints on tectonic processes responsible for this extension. Theobserved isotopic ratios of modern fluids range from the air composition to moderately radiogeniccompositions (0.15-0.4 Ra, with Ra being the atmospheric ~3He/~4He ratio), whereas fossil fluids display moreradiogenic compositions (0.04 to 0.15 Ra), suggesting that they have been trapped deeper in the fault zone inthe absence of significant atmospheric contamination. These compositions result from mixing of a deepradiogenic and hot fluid with cold and shallow air-saturated aquifer waters. The absence of a mantle-He signal in the Corinth rift fluids is remarkable and evidences the lack of activevolcanic fluids in the basin. It further suggests that the fault system is not connected at depth with zoneswhere mantle-He might have been trapped. At the scale of the Corinth rift, this observation suggests thatfaults are rooted in the upper crust. If extension results from displacement of the crust above a north-dippingshear zone, then this shear zone should be relayed at depth through the lower crust by impermeable ductiledeformation. At a larger scale, it appears that basins in the western side of the Aegean host crustal helium whereas basinsin the Anatolian side present systematically mantle He. This He isotope distribution is difficult to explain bysymmetrical tectonic evolution of the Aegean domain, and could instead be related to the recent propagationof the North Anatolian Fault (NAF). Propagation of the NAF may have recently disturbed hydrothermalcirculations established after Miocene extension, by progressively reactivating the western basins with adifferent style of tectonic deformation, re-localising most of the fault controlled flow-pathway in shallowerbasins and as a consequence displacing the fluids mixture toward a crustal origin.