Triassic evaporites: a vast reservoir of brines mobilized successively during rifting and thrusting in the Pyrenees

Abstract

Triassic evaporites have a very particular location in the Pyrenees, close to detachment areas between the basement and the sedimentary cover, and constitute an enormous chlorine and potentially brine reservoir. During the two successive deformation cycles related successively to the Cretaceous rifting and the convergence during early Cenozoic, brines were expulsed and implied in fault activity, breccia formation and fluid–rock interactions. Fluid inclusions from fault infillings and alpine-style fissures sampled all along the Pyrenean chain have a maximal chlorinity close to that of halite–water equilibrium at temperatures between 250 and 350°C. Mixing of brines with low-chlorinity waters formed a series of fluids covering an extensive range of salinities. During syn-rift events, the hotter dilute end-member is likely derived from seawater infiltrated and heated near the exhumed mantle as no emerged areas were present at that time. During convergence and thrusting, brines again predominated and mixing occurred with a colder end-member, probably of meteoric origin, consistent with a significant period of relief formation. Brines played, therefore, an essential role in mass and heat transfer during the whole orogenic cycle in the Pyrenees.