Sedimentological and stratigraphic signature of the Plio-Pleistocene tectonic events in the Southern Apennines, Italy: The Calvello-Anzi Basin case study

The Plio-Pleistocene Calvello-Anzi Basin (CAB) represents a synorogenic shallow marine to alluvial successiondeposited during the late phase of the Apennine Orogenic event. While the stratigraphy and sedimentary cycles arewell known, the precise role and timing of the tectonic phases affecting this area are unclear. In this paper wereport the major findings from a study aimed at unraveling the signatures of the Plio-Quaternary tectonic eventsaffecting the CAB and this portion of the Southern Apennines by using a field integrated approach characterized byfacies analysis, structural surveying, and basin architecture. The CAB is located in the axial portion of the SouthernApennine chain and has a roughly E-W orientation which is at a high angle with respect the main NW-SE strikingregional tectonic structures controlling the foreland-directed migration of the compression. In the study area, anapproximately 1,200-meter-thick package of Zanclean (early-middle Pliocene) to Santernian (middle Pleistocene)shelf, shallow marine, and alluvial deposits accumulated during the late stages of the Apennine orogenic evolution.Within the CAB deposits we recognize two main depositional motifs which roughly correspond to previouslyestablished regional cycles that are genetically linked to two corresponding tectonic phases of the SouthernApennine chain. Our results reveal how these two main tectonic phases played a primary role in controlling thedepositional architecture, facies, depocenter distribution, and the basin shape throughout the CAB's lifespan. In theproposed model, during the first depositional stage (Zanclean), the CAB formed as a wedge-top basin, controlled bythe subsurface propagation of NW-SE-trending thrusts related to the main NE-SW directed shortening of theApennines. This stage is recorded in the basal units of the CAB infill by progressive unconformities, the devel-opment of syn-tectonic shallow marine sedimentary wedges along a high gradient tectonically controlled de-positional surface, and a marked SE migration of the main depocenter. During the second depositional stage (upperPiacenzian to Santernian), the CAB was controlled by a series of roughly E-W-trending normal faults whichcrosscut the preexisting compressional structural fabric. Syndepositional activity along these faults played a directrole in creating an E-W oriented tectonic depression and controlling the accommodation space for the upperportion of the CAB infill. This second stage is marked by two points of evidence. The first point is revealed by asouthwestward migration of the CAB depocenter driven by ~90-degree rotation from a roughly NW-SE elongatedpattern to roughly elongated W-E transverse to the Pliocene regional structural fabric. This depocenter drift alsocoincides with an abrupt change in depositional regime from shallow marine to continental sedimentation. Thesecond and most important point of evidence is the presence of several basin-bounding extensional fault zoneswhich cut the lower Pliocene basin-fill units. We relate these features to the middle upper Pleistocene transition toalong-strike extension, widely reported in the Southern Apennines, which is related to the overall extensionalregime that is acting on the chain also at present. Data from this study provide additional insight into the evolutionof the Southern Apennine chain, the formation of the Plio-Pleistocene CAB, and the debated role of Quaternarytectonics on the basin's development during the post-collision orogenic phases. Given the proximity of the studyarea with several oil fields and considering that hydrocarbon traps in Southern Apennines are Plio-Quaternary inage, the constraints provided on the timing and the style of the regional deformation may improve the definitionsof hydrocarbon traps in the buried Apulian platform