Development of tight anticlines in platform carbonates at the toe of transpressional thrust wedges: structural and paleofluid constraints from the Pag anticline, External Dinarides (Croatia)

Abstract

The long term evolution of orogenic wedges is characterized by complex interactions between tectonics, gravity, and surface processes. In the external sectors of fold-and-thrust belts, deformation involves interaction between folding and faulting in long-lasting deformation sequences. Describing and quantifying kinematic evolutions and fluid-rocks interaction processes is fundamental for understanding long-lived deformation processes involved in fold-and-thrust belts. At wedge toes, low amounts of deformation are expected and predicted by strain and kinematic models. However, multiple syn-orogenic contractional pulses and possible stress field variations, mechanical and stratigraphic properties of deformed successions, and fluid circulations at shallow crustal levels, may dramatically increase the complexity and intensity of deformation developed in the frontal parts of foreland thrust-and-fold belts. A better understanding of the relative role of these different factors to determine fold complexity at thrust wedge toes has important academic and economical implications. A world-class example of long-lasting contractional orogeny is provided by the Dinarides and, in particular, their frontal sector (i.e., External Dinarides), which offers outstanding exposures where studying the interplay between deformation, stratigraphy, and fluid-rock interactions in folded and faulted carbonate platform rocks. This Ph.D. thesis investigates on deformation structures developed during folding on a fault-related anticline involving a tight carbonate platform succession. Research was performed by a multidisciplinary approach to study the structural architecture and kinematic evolution of the anticline, the interaction between contractional folding and strike-slip faulting, and the role played by misoriented inherited faults on folding. Paleo-fluid evolution is also taken into account to better understand deformation structure timing and palaehydrological evolution, and the influence of deformation elements on fluid circulation through the anticline. The case study of the Pag anticline (External Dinarides, Croatia) allowed investigating on a shallow-depth detachment fold developed in proximal surface conditions at the toe of the late-Eocene Dinarides thrust wedge. Despite its structural position, the Pag anticline is characterized by high-structural complexity, including a non-cylindrical box fold geometry dissected by a pair of major overstepping conjugate forethrust and backthrust, and strongly compartmentalized by two sets of high-angle strike-slip faults, both striking oblique to the fold axis. The presence of a thin-skinned inherited fault system, oriented almost perpendicular to the fold axis, plays a major role during late-stage of transpressional folding. Fold evolution is characterized by the interaction between contractional and right-lateral strike-slip deformations since the early stages of layer-parallel shortening and folding. Palaehydrological evidence from structural diagenesis of folding- and faulting-related veins indicates that most of the long-lasting evolution of deformation progressed in “bedset confined” conditions, strongly influenced by stratigraphic-related eogenetic fluids, characterized by a meteoric signature and very slowly moving along pre-existing deformation structures. Only in the later stages, during fold tightening and compartmentalizaton, strike-slip fault zones and reactivated pre-tightening structures acted as fluid-conduits, triggering meteoric fluid circulation, showing the same isotopic signature through each stratigraphic unit. This suggests that the major hydrological connectivity for mineralizing fluid was reached during the latest stages of folding. Fault-controlled de-dolomitization processes occurred, recrystallizing fault-related dolomite bodies formed before folding. To conclude, this case study offered an outstanding opportunity to investigate and document folding-related deformation, fold evolution and architecture, and fluid circulation that occurred at the toe of the Dinarides foreland thrust-and-fold belt, involving platform carbonate rocks.