Stratigraphic evidence of a Middle Pleistocene climate-driven flexural uplift in the Alps

This interdisciplinary study on the subsurface stratigraphy of the Po Plain (northern Italy) brings new evidence in support of a climate-driven erosional unloading of the Alps since the Middle Pleistocene. A newly acquired, high-resolution seismic profile and a critical review of industrial seismic lines were integrated with sedimentologic observations on four magnetostratigraphically dated continental cores to reconstruct a three-sequence evolution of the Pleistocene clastic infill in the northern Po basin. During the first sequence (PS1; 1.4-0.87 Ma), characterized by sedimentation rates of 34 cm/kyr outpacing regional subsidence, meandering river systems prograded over the basin passing downstream to a cyclothemic shelfal succession. The second sequence (PS2; 0.87-0.45 Ma), heralded by a regional unconformity (R surface) correlated to the onset of the major Pleistocene glaciations, was characterized by widespread continental sedimentation of generally distal braidplain. The third sequence (PS3; 0.45 Ma to present), marked at the base by another regional unconformity (Y surface), is characterized by proximal braided fluvial deposition under combined conditions of confinement, erosion, and bypass. We interpret the PS3 sequence as deposited under the effects of a flexural uplift of the northern Po Plain during the Middle Pleistocene starting at 0.45 Ma, in response to the long-term erosional unloading of the Alps triggered by the waxing and waning of Alpine glaciers since the late Early Pleistocene global cooling (0.9 Ma). According to our modeling, erosion on a relatively limited area of the Alpine mountain chain, ranging from 1.3 to 1.7 mm/yr in the axial sector to 0.1-0.3 mm/yr at the margins, has been able to trigger rock uplift over a wider area including the proximal peripheral basins.