ACTIVE FOLDING, GROWTH STRATA, AND TERRACES AT THE NORTHERN APENNINE MOUNTAIN FRONT, BOLOGNA, ITALY

The Apennines of northern Italy, long recognized as a classic Cenozoic fold and thrust belt, share diverse interpretations of contemporary deformation, including active extension, active shortening, or tectonic quiescence. Collaborative research associated with the RETREAT project near Bologna has identified clear evidence for high angle normal faults both oblique and parallel to the northern Apennine mountain front. These structures are likely embedded in larger contractional structures including an emergent, growing frontal anticline. The proposed anticline lies foreland of a Pliocene piggy-back, epi-Ligurian synclinal basin whose growth is documented by fluvial deltaic and neritic strata. A middle Pleistocene beach deposit, preserved in the proximal Po foreland basin and correlative into the range front along fluvial strath terraces marks progressive unconformities and continued growth of the anticline into the Quaternary. We model anticlinal growth using a simple tri-shear approach constrained by several uplifted and folded strath terraces along the Reno River and their correlative downdip synorogenic deposits beneath the Po Plain. The model does not provide a unique solution; however, the field observations can be explained by a simple fault-propagation fold geometry where the propagation to slip ratio is 1.5, the anticlinal limb is pinned, and the fault has either ruptured the surface or flattened near the surface within the past 250 ka. Preliminary results from an ongoing GPS experiment indicate that the growing anticline is likely an intrawedge structure serving to steepen the prowedge that currently has a slowly deforming toe, but is actively shortening across a 100 km-wide zone stretching from the range crest to the middle Po foreland. Such a large, active thrust fault along the northern Apennine mountain front, should it be locked, presents a potentially serious seismic hazard to Bologna and other large cities in the Po foreland.