Episodic evolution of a carbonate thrust zone in the Southern Pyrenees

Tectonic Studies Group TSG: Virtual AGM 2021, Geological Society Events, January 5-8 2021. OnlineField structural data combined with petrographic and geochemical analyses (¿13C, ¿18O, 87Sr/86Sr, clumped isotopes and trace elements) applied to calcite veins, fault rocks and host rocks are used to reconstruct the evolution of a thrust zone in the southern Pyrenees, and to qualitatively evaluate the thrust zone permeability. The studied thrust offsets the southern limb of the Sant Corneli-Bóixols anticline placing a Cenomanian-Turonian carbonate sequence on top of a Coniacian carbonate unit. Successive fracture systems and related calcite cements are interpreted to record: (i) an episodic evolution of the thrust zone, resulting from a process zone development and subsequent thrust slip propagation, and (ii) compartmentalization of the thrust system, leading to the migration of different fluids and to the formation of different vein systems in the footwall and in the hanging wall. In the footwall, three systematically oriented fracture systems, sealed by three calcite cements, and a chaotic breccia, cemented by the two former cements, developed. The formation of the two earliest fracture sets and the chaotic breccia is consistent with dilatant fracturing around the fault tip (in the process zone) during initial fault growth, whilst the formation of the latest fracture set suggests hybrid dilational-shear failure during thrust propagation. The successive fracturing events and related calcite cementation phases indicate that the structural permeability was transient and that fractures created new pathways for fluids but were rapidily occluded with calcite. Clumped isotope thermometry reveals a progressive increase in precipitation temperatures from ~50ºC up to ~117ºC. This fact is interpreted as progressive burial associated with thrust emplacement. During this period, the fluid system changed from meteoric fluids to evolved meteoric fluids likely due to water-rock interaction at increasing depths and temperatures. Contrary to the footwall, in the hanging wall only a crackle breccia is present. This breccia is cemented by a distinct calcite cement, which is also found in the thrust plane and in the fault core. This later cement precipitated from formation fluids, at ~95 ºC, that circulated along the fault core and in the hanging wall, again supporting compartmentalization of the thrust structure