Transverse faults in the thrust belt-backarc hinge zone (Eastern Tyrrhenian Sea Margin, Italy)

Abstract

Stratigraphy and architecture of the fault bounding the sedimentary basins developed along the Eastern Tyrrhenian margin provide information on the kinematic of faults transversal to the Apennine chain. To understand how changes in geodynamic processes control the structural evolution of transverse faults in the thrust belt-backarc hinge zone, we performed the interpretation of a 2D strictly spaced seismic data set, tied to stratigraphic data of exploration wells and onshore data constrains, and a 3D basin analysis.On the basis of geometry and age of the basin infill, we dated three events of fault activity and a complex kinematics of Pliocene-Quaternary transverse faults. The first tectonic phase produced the oldest normal faults developed along the Latium margin. These faults, active between 5.1 and 3.2 Ma (MPL2-MPL3 and MPL4 succession), bound sedimentary basins filled by a Transgressive/Regressive succession made up of sands, silts and clays. They gradually migrated from the NE- trending (transversal to the Apennine chain), Pliocene in age, toward the NW-trending (parallel to the Apennine chain) during the Lower Pleistocene (1.8 MA, MPL6 succession). The displacement along these normal faults was transferred, or relayed, from one to the next one along accommodation zones, corresponding to transfer faults. Accommodation zones along major bounding structures are sites of intra-basinal highs, characterized by thinner sedimentary covers. The transfer faults, orthogonal to the normal faults, offset the basin depocenters. Whereas a positive inversion structure located near a transfer fault deforms the central basin rift. During the Lower Pleistocene the transform faults are transversal to the Apennines chain. These latter developed from the Latium margin to the Campania Margin.The third phase of the development of the transverse faults corresponds to a second episode of rifting of the Eastern Tyrrhenian margin. This event is linked to the activity of NE-trending normal faults, during the Middle Pleistocene since the 0.7 Ma, producing half-grabens and a deepening of the basement in the northwestern part of the Campania Plain and in Naples Bay. The stratigraphic succession architecture records the tilting of the fault block. During the middle Pleistocene along the Campania margin the transverse faults were reactivated as normal faults.The great variability in the tectonic evolution of the Tyrrhenian margin has been interpreted as strictly related to the complex and rapid geodynamic evolution of the area during Pliocene-Quaternary times: Pliocene slab retreat of the Adria plate, followed by Pleistocene growth of a Subduction-Transform-Edge-Propagator (STEP) fault along the northern margin of the Ionian slab.