Abstract

In 1976-1993, AQUATER prepared general and detailed reconstructions of buried Neogene-Quaternary structures of the Po Plain from seismic and stratigraphic data. About 17,000 Km of reflection seismic profiles covering the entire Po Plain and Venetian flatlands were reinterpreted for neotectonic and seismotectonic purposes. Only a small part of these reconstructions was published. The Neogene-Quaternary, Apenninic deformation severely affected the central-southern side of the Po Plain subsurface and western side of the northern Adriatic Sea, and is known to have generated four asymmetric structural arcs, hundreds of Km long, running from W to E, namely: Turin Hill-Monferrato-Langhe-Maritime Alps (arc 1), Emilia-Romagna-Marche (arc 2), Ferrara (arc 3) and Adriatic (arc 4). The elongation of the arcs (subparallel to the adjacent northern-central Apennine chain) is approximately WNW-ESE in the Po Plain (arcs 1 to 3) and NW-SE in the northern Adriatic Sea (arc 4) and their external front is buried under Pliocene-Quaternary sediments. The arcs--to be seen as the buried extension of the Apennine chain--consist of high-angle, WNW-ESE to NW-SE striking, reverse faults with dextral, en-echelon arrangement in the northeastern side of each arc and of NNE-SSW to NE-SW striking, NW-vergent, thrusts in the western side. Subvertical cross faults affect each arc: dextral and sinistral striking WNW-ESE to NW-SE and N-S to NE-SW respectively, form a conjugate set developed in a NW-SE to NNW-SSE compressional field. The high-angle, en-echelon reverse faults of their NE side represent a dextral shear zone. The thrust is nearly orthogonal to the movement direction and is present at the front of each NW-shifting block, located south of the adjacent shear zone. The shear zone and the thrust form, in map view, an asymmetric arc with a longer NE side and a shorter NW side. The high-angle shear zones affect cover and basement. The genesis of the four arcs is thus attributed to progressive activation of four dextral shear zones, developing from W to E in Oligocene to late Pliocene time. Arc 1 began to develop in the Oligocene to Tortonian, arc 2 in the Messinian, arc 3 in the early Pliocene and arc 4 in the middle to late Pliocene. Arc 1 developed in an early ("Neo-Alpine") phase in a still unclear structural framework and could be connected with the Oligocene to Tortonian anticlockwise rotation of the Corsica and Sardinia and with a consequent NW to WNW compression NW of the pivot. The arcs 2, 3 and 4 developed subsequently ("Padan" phase) as result of progressive shortening caused by the opening and expansion of the central-northern Tyrrhenian Sea. During the second phase arc1 was deformed again. According to the proposed genetic and kinematic model, the arcs are the result of a westward movement of autochthonous crustal blocks pushed from ESE-SE in the first phase and from SE-SSE in the second phase, the main tectonic transport being to the NW.

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