Significance of multiple decollement in nature and models: Po Plain case (Italy)
D. Massoli 1, H.A. Koyi 2, M.R. Barchi 1 & S. Rogledi 3
1 Earth Sciences, University of Perugia, P.za dell’Università 1, 06100 Perugia, Italy.
2
Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden.3
ENI-Division AGIP, Via Emilia 1, 20097 San Donato Milanese (MI), Italy.The Po Plain is located in the Northern Italy and represents the foredeep of both Southern Alps and Northern Apennine Fold and Thrust Belts. Reflection seismic profiles have been used to characterize and describe the structural style of the Northern Apennine buried structures, consisting of a NE-NNE vergening imbricate fold-and-thrust system having arc shaped outer fronts.
Schematically, the stratigraphy of this region consists of Mesozoic-Paleogene passive margin successions (eavporites and carbonates) and Neogene-Quaternary turbidite sequences (marls, sandstones and marine clays and sands).
On the seismic reflection profiles two major decollement horizons can be detected; the deepest, represented by the Triassic evaporites (alternated anhydrites and dolomites) and the shallowest, corresponding to a marly formation (Marne a Fucoidi fm.), within the upper part of the carbonates. This arrangement generates two sets of compressional structures with different characteristics: 1. shallow folds and imbricated thrusts; 2. carbonate folds.
In order to study the effects of multiple superposed decollement on the structural evolution of a fold and thrust belt, a series of analogue models has been performed. The models contained two decollement layers respectively situated at two stratigraphic levels. All models were geometrically, kinematically and dynamically scaled to the Po Plain area. A series of measurements, taken at fixed intervals during the shortening of the models, and model results show that two types of structures with different "roots", wavelengths, shape, dispalcement, size etc., developed in all the models. The changes in the geometry and kinematics of the structures are attributed to the presence of multiple decollement levels.
Analysis of seismic reflection profiles and results of analogue models show that structural complexity and branching of deeper thrusts at shallow levels are due to the presence of weaker horizons acting as shallow detachments. Part of the complexity in the structural evolution is due to the decoupling between deeper structures by the shallow decollement levels. Although scaled to the Po Basin, our model results may be applicable to other fold-thrust belts with multiple decollements.