Evidence of tectonic inversion in the Northern Apennines Hinterland basins, the example of the Valdera-Volterra basin (Central Tuscany)

Hinterland basins are low-lying and often heavily populated areas at the back of orogenic belts which have significant economic and infrastructural importance. The tectonic-stratigraphic and regional characterisation of hinterland basins is fundamental for evaluating their subsurface utilisation and potential geohazards.

This study focuses on the origin and development of the Tuscan hinterland basins of the Northern Apennines. These basins have been associated with a compressional regime lasting until the Late Pliocene-Pleistocene during which out-of-sequence thrusts and back-thrusts in the inner portion of the chain accommodated the compressive stress accumulated in the frontal zones. An alternative interpretation considers the evolution of these basins in an extensional regime as an effect of large-scale back-arc processes or gravitational collapse of thickened crust following the Apennine orogeny since the Early Miocene. In this tectonic regime, the basins have been interpreted as graben, half-graben or bowl-shaped basins evolving into graben.

Our work aims to determine the tectonic-sedimentary evolution of the Valdera-Volterra Basin through the analysis of  ~271.8 km of 2D seismic reflection profiles and wireline logs from two exploration wells. The Valdera-Volterra Basin basin is an NW–SE oriented depocenter ~60 km long ~30 km wide filled with a clastic succession of Miocene-Pleistocene fluvial-lacustrine to marine deposits up to ~2 km thick.

The analysis has revealed a polyphased tectonic history of the basin with a Messinian-Zanclean compressional phase deforming the basin-infill as indicated by seismic imaging of synformal geometries and strongly tilted unconformities. Such deformation is tentatively associated with E/NE vergent blind thrusts and SW vergent blind back-thrusts. During the Piacenzian, the activity of normal border faults and the presence in their hanging wall of associated sedimentary wedges thickening towards NE suggest an extensional phase following the earlier Messinian-Zanclean compression. Broad folding of the shallow Piacenzian units in the hangingwall of the normal faults suggests the occurrence of mild positive inversion at the end of the Piacenzian/Lowermost Pleistocene?.

This tectonic history has been associated with crustal shortening in the Northern Apennines hinterland, accommodated by thrusting, that occurred discontinuously until the end of the Pliocene/Lowermost Pleistocene?. The formation of the border faults during the Piacenzian has been related to a prolongated phase of tectonic quiescence that led to the collapse of the sedimentary pile and the Pre-Neogene substrate. In this setting, the positive inversion occurred at the end of the Piacenzian/Lowermost Pleistocene? represents the last compressive event related to crustal shortening.