ACTIVE FOLDING IN A SUBSIDING SEDIMENTARY BASIN: A CASE STUDY IN THE PO BASIN (NORTHERN ITALY)

In subsiding basins the signals of active tectonics (e.g. folding) can be disguised by the sedimentary processes. Where the sedimentation rate is higher than the uplift a general flat topographic surface is observed without clear evidences of deformation, despite the subsurface can host active up to seismogenic faults. The Po Basin, with buried thrusts and related folds, has been identified as case study to design and test an approach based on 3D modeling, decompaction and sequential restoration aimed to distinguish the uplift rate of elusive folds from the sedimentary processes.

The Po Basin is the common foredeep of two opposite verging chains, the Southern Alps and the Northern Apennines, that influenced the evolution of the foreland basin from Paleogene onward. During the Pleistocene the complex interaction of tectonic processes, sea-level fluctuations, climate changes, and sediment supply drove a general regressive cycle that filled the foredeep. The most important tectonic phases can be recognized along the basin margins marked by the deformation and tilting of river terraces and of exposed syntectonic sediments; conversely their detection is difficult in the central-distal part of the basin, where the study of the buried folds relies only on the interpretation of industrial seismic reflection profiles. Our approach is based on a 3D geological model built after seismic interpretation. The 3D model allows the complete definition of the folds geometry, of the sedimentary units involved in the folding, and of the growth strata. In the analysis of these latter, 3D decompaction and regional tilting must be taken into account to assess the folding-related residual vertical separation.

We focused the study on the Piadena anticline (central Po Basin) that represents a local positive signal (uplift) in a general subsiding context, related both to sediment compaction and to more regional processes like the hinge retreat-related subsidence. In our analysis the foreland tilting has been subtracted from the topography resulting after unfolding and decompaction, for 6 time intervals; we obtained a residual signal related to the growing anticline, and the uplift rates of the structure during its Pleistocene evolution.