Deformation history of a synorogenic sedimentary wedge, northern Cilento area, southern Apennines thrust and fold belt, Italy
In the northern Cilento area of the southern Italian Apennines, a deformed turbiditic succession of Langhian to lower Tortonian age (the Cilento unit) unconformably overlies tectonic units (Liguride and Sicilide) derived from an oceanic accretionary complex and, to the east, overlies Mesozoic–Cenozoic carbonates belonging to the Apulian continental paleomargin. The turbiditic succession, representing the sedimentary infill of a Miocene foreland basin developed over the flexured southwestern margin of the Apulian plate, shows a complete record of the deformation at the macroscopic and mesoscopic scales. Structures postdating synsedimentary (slump) features in the Cilento unit include Neptunian dikes and extensional veins perpendicular to bedding, all related to high pore-fluid pressures associated with burial. Early tectonic structures consist of minor thrust faults, often conjugate and at a low angle to bedding, produced by initial layer-parallel shortening. The main regional structure mapped in the area consists of a moderately inclined to recumbent (hinterland-vergent) backfold. This structure probably developed by shortening of the strata caught between shear surfaces that cut obliquely across bedding, within the framework of a noncoaxial progressive deformation. The main shear surface constituting the upper boundary to the shear zone is represented by a regional backthrust which brings Mesozoic-Cenozoic carbonates onto the terrains of the Cilento unit, forming a triangle zone where backfolding of the latter occurred. Minor open domes and basins deform both limbs of the main regional structure, probably as a result of progressive shear deformation and, possibly, of deformation above underlying thrust culminations. It appears that high pore-fluid pressures must have been important not only during the development of pretectonic (i.e., burial-related) structures, but also during the main tectonic deformation. Abnormally high fluid pressures, reducing the effective stress and favoring intergranular movements, suppressed cleavage formation and pervasive strain during the main folding event. Later in the deformation sequence, a decrease in pore-fluid pressure probably favored local cleavage development during refolding. The deformation sequence outlined above provides a case history of the structural evolution of synorogenic clastic deposits progressively deformed and incorporated within a thrust belt. This study, documenting the deformation of a sedimentary wedge dominated by backfolding associated with hinterland-vergent reimbrication, should contribute to a better understanding of the wide range of deformation sequences and processes characterizing the tectonic evolution of thrust-belt–foredeep systems.