Out-of-sequence recent thrusting revealed by surface and subsurface data under the Po Plain, Italy

Recognizing recent activity at buried out-of-sequence thrusts is a complex task but has important implications for seismic hazard and risk assessment. Here we propose an example of methodology that includes subtle geomorphological and hydrological observations at the surface correlated with upper crustal 3D reconstruction by seismic sections and stratigraphic logs. The densely inhabited Po Plain (Italy) hides the front of the Northern Apennines fold-and-thrust belt, buried under a thick succession of Plio-Pleistocene deposits. The Emilia Arc, located in the center of this front, is composed of a series of folds and south-dipping thrusts. Hindward of the thrust front, at the surface, high resolution Digital Elevation Models and GPS measurements allowed to discover topographic anomalies that denounce recent uplift and broad tilting. These have been accompanied by ample late Pleistocene-Holocene migration of the river network, and river over-excavation. In correspondence of these features, subsurface data show north-verging folds and reverse faults involving Pleistocene deposits. From this integration, a scenario where two out-of-sequence thrust zones contributed to modulate shortening during the late Pleistocene and, possibly, Holocene, emerges. Hinterland migration of deformation has been favored by the higher sedimentation load above the outer thrusts and their close confrontation with the thickened Southern Alps buried fronts.