Deformation pattern, paleostress, and paleofluid evolution in the Pag anticline, External Dinarides of Croatia

The NW-SE striking Pag anticline, in the External Dinarides fold and thrust belt, provides an appropriate field site for studying fold- and fault-related deformation patterns at different scales in a transpressional setting due to outstanding exposures. We performed a multiscale structural analysis together with petrographic and isotopic characterization of syntectonic calcite cements. Results indicate that the Pag anticline, is a box fold developed mainly by detachment folding in response to NE-SW oriented compression. Depth-to-detachment calculation indicates that the basal thrust of the anticline is located in the Upper Jurassic evaporitic complex, at a depth of about 2.5 km. The geometry of the fold is strongly controlled by the interaction and overstepping of a major thrust-backthrust fault pair. In the northern sector, backthrust activity produced a northeastern facing, steeply dipping to near vertical backlimb. Moving southward, the forelimb gradually becomes vertical to overturned and fold asymmetry switches to a southwestward facing. Late- to post-folding tightening resulted in non-cylindrical and compartmentalized deformation by near vertical N-S right-lateral, and E-W, left-lateral, strike-slip faults trending oblique to the fold axis. These fault sets make a wider angle than expected for transversal conjugate strike-slip faults commonly associated with folding, possibly due to lateral propagation from inherited, folded soft-sediment extensional faults. Paleostress analysis indicates that the evolution of the Pag anticline occurred in a stress field that switched from contractional to transpressional configuration, maintaining a N40-50° oriented major stress axis. Petrographic and isotopic data support infiltration of meteoric fluids into exposed carbonates in the pre-folding stage, followed by mixing with marine fluids during folding at shallow burial conditions and, eventually, meteoric fluid circulation along strike-slip faults in the late to post-folding stage. Stable isotope ratios suggest that fluid flow evolved from a bedset confined system to an open one in the late to post-folding stage. As such, a major role to control paleofluid flow is played by the transversal sets of low-displacement near orthogonal strike-slip faults. This suggests that, in reservoir structural characterization, particular attention should be paid to the presence of low-displacement strike-slip faults because of their role to enhance fluid flow mixing and channeling. The presence and, particularly, the abundance of such deformation features are difficult to constrain in buried fractured reservoirs by seismic reflection imaging because of low displacement values. We, consequently, stress the importance of studying field analogues by multidisciplinary approaches for better understanding the relationships between folding, faulting, the associated incremental deformation patterns, and the impact on fluid flow.