ACTIVE BASEMENT-CORED FORELAND SHORTENING IN THE NORTHERN APENNINES, ITALY

Published and widely available seismic, structural, and geologic data on the northern Apennines, Italy traditionally portray the orogen as an inactive, thin-skinned, foreland-propagating fold and thrust belt. Our work, as part of the multi-national and multi-disciplinary NSF-RETREAT project offers an alternative view that includes active shortening, hinterland-stepping deep crustal thrust faults, and shallow crustal normal faults atop a growing frontal anticline that defines the Bologna mountain front. Collectively, these features are similar in scale and rate of deformation to basement-cored foreland structures of the Laramide Rockies. We propose that this deformation style in the northern Apennines heralds the transition from subduction-driven shortening to continent-continent collision as Adria is consumed beneath Italy. The data and modeling we have collected and assembled to reach this conclusion include youthful growth strata such as deformed fluvial terraces, a high resolution reflection seismic line across the Bologna mountain front, trishear modeling of a crustal-scale blind thrust, active sesmicity, and a geodetic releveling line. We propose a seismogenic mid-crustal ramp tied to the subduction interface in the hinterland and projecting beneath the inactive frontal anticline in the Po foreland. The recognized middle crustal ramp can be considered as one of the most important seismogenic sources in northern Italy. The Quaternary evolution of this basement-cored foreland structure in the northern Apennines shares many similarities with the late Oligocene to Miocene evolution of the Alps and should the mid-crustal ramp continue to dominate, the Apennines will evolve into a orogen similar to the Alps where the core is characterized by an uplifted and crystalline basement massif. In this context, but differently from the Alps and the Laramide Rockies, extension due to the retreating upper plate acts on the upper crust of the Apennine hinterland and backarc and works to overprint the shortened foreland compressional regime. The diverse subduction history and dynamics of the Laramide Rockies and Italian Apennines point to multiple pathways to basement cored uplifts in the forelands of active orogens.