New Insights on Betic Cordillera Structure From Gas Geochemistry

The current lithospheric structure of the Betic Cordillera results from active geodynamic system related to slab retreat slowdown in western Mediterranean. A sharp change in lithospheric thickness has been imaged beneath the Betic Cordillera, potentially resulting from a near-vertical subduction-transform-edge-propagator fault toward the surface with possible mantle influx. In this study, we use helium isotopic composition of bubbling and dissolved gases in groundwater samples of the central part of the Betic Cordillera to evaluate the origin of gases and to set constraints on its lithospheric structure. We found that helium isotopic composition have a dominant radiogenic component with a mantle-derived He contribution reaching mainly 1% for the investigated area. Estimation of He diffusion within the ductile crust indicates that this process is potentially to slow to explain the low mantle-derived He contribution measured at the surface. A new analysis of the available data of the crustal metamorphic complexes allows us to suggest that the crust could be dissociated from the mantle with no evidence of asthenospheric influx. The weak mantle He signature could reflect a mantle material earlier incorporated in the crustal metamorphic complexes of the Betic Cordillera during their exhumation. In light of mass balance calculations, we propose that the slight 3 He excess observed in the present-day fluids might result from a fossil mantle signature diluted by local radiogenic production over time.