Structural and stratigraphic control on salient and recess development along a thrust belt front. The Northern Apennines (Po Plain, Italy)

The external part of the Northern Apennines accretionary wedge in northern Italy is buried beneath its fast subsiding and asymmetric foreland basin in the Po Plain. It is characterized by a diffused noncylindrical geometry resulting in salients and recesses in the study area, namely, the Cremona salient, the Parma recess, and the Ferrara salient. The interpretation of borehole and seismic reflection data suggests that the thrust belt is characterized by thin-skinned tectonic style. Two main décollement levels have been identified: a basal décollement located in the Upper Carnian units (San Giovanni Bianco Clay and Raibl Group) and a shallow décollement located in the late Eocene-Oligocene formations (Gallare Marls). The décollement surfaces dip SSW toward the hinterland of the accretionary prism, parallel to the steep (>10°) regional monocline. The geometry of the seismically active Northern Apennines system of salients and recesses is essentially controlled by the interplay of two factors: (i) the lateral facies variations of the stratigraphic units hosting and controlling the location and depth of the décollement levels and (ii) the slope of the basal décollement. Salients occur where, due to the inherited variable stratigraphy of the Mesozoic-Cenozoic Tethyan passive margin, the shaly formations hosting the two décollements are well developed allowing larger forward propagation of the thrust wedge. Recesses are instead associated to erosional-nondepositional areas. Moreover, salients are more pronounced where the flexural behavior of the Adriatic subducting slab has generated a steeper geometry of the foreland monocline and consequently of the basal décollement.