Migration pathways of crustal and mantle fluids during the formation of the Betic Cordillera (SE Spain).

Crustal-to-lithospheric-scale strike-slip faults can act as major pathways for crustal and mantle fluids, with implications for natural resources (e.g., geothermal, ore deposits) as well as natural gas storage and migration. The Betic Cordillera (SE Spain) records a complex geodynamic evolution, from slab retreat, tearing and delamination to the later inversion of a thinned continental margin. This extreme crustal thinning (~15 km) associated with metamorphic dome exhumation during the Miocene was accommodated by crustal to lithospheric-scale faults that are still active today (e.g. the Mw 5.2 Lorca earthquake in 2011).

To determine the geochemical origin of fluids, their migration pathways, and fault-controlled permeability through time, we analyzed noble gases (He, Ne, Ar) as inert, non-reactive geochemical tracers in both paleo- and present-day fluids. Noble gases in paleo-fluids were analyzed in quartz and calcite minerals associated with faults (INGV, Palermo). We also analyzed noble gases dissolved in water discharged from thermal springs at ~20 to 53°C (Eawag/ETH Zürich).

In calcite and thermal water, low ³He/⁴He ratios (R/Ra ≤ 0.5) indicate mixing between a dominantly crustal component and a mantle contribution or a mixed crustal-atmospheric origins. Quartz samples show stronger atmospheric contamination than in calcite, although ⁴⁰Ar/³⁶Ar ratios may suggest deep input (mantle vs crust; ⁴⁰Ar/³⁶Ar values between 490 and 1215). He-Ne isotopes in paleo-fluids reveal two areas that show a likely mantle-derived noble gas signature: Sierra Elvira, with ~3% mantle contribution suggested, and the Carboneras Fault Zone, with ~6%. In contrast, present-day fluids could reflect a ~4% mantle contribution in the northeastern Betics at Mula and Archena. We infer that mantle-derived signatures detected in paleo-fluids are not preserved in the same locations in present-day fluids. For instance, along a single fault system (e.g., the Carboneras Fault), paleo-fluids display up to four times higher mantle contributions (Rc/Ra ≈ 0.5) than present-day fluids (Rc/Ra ≈ 0.1). This contrast opens new questions regarding potential changes in mantle fluid sources or migration pathways during the evolution of the Betic Cordillera, the impact of tectonic inversion on deep fault permeability, the residence time of fluids in the crust, and the role of fault geometry in controlling fluid pathways.