Resolution of the master structures partitioning shortening and extension in the northern Apennines, Italy

We integrate existing and new geologic data (RETREAT project), focused on the origin, growth, and activity of the mountain front at Bologna, Italy into a new model that explains Apennine orogenesis in the context of a slab rollback - upper plate retreat process. Data are assembled from river terraces and associated Pleistocene growth strata, watershed-averaged erosion rate studies, geodesy including releveling surveys, reinterpretation of published reflection lines, a new high resolution reflection line, recently compiled seismicity, and mantle anisotropy assembled from shear-wave splits. The Bologna mountain front is revealed to be an actively growing structure with rock uplift of ~1 mm/yr, cored by a blind mid-crustal flat-ramp structure that accommodates ongoing shortening driven by Adria subduction at a rate of ~2.5 mm/yr. Geologic, geomorphic, and geodetic data indicate relatively steady rock uplift and exhumation for the Apennine foreland over the past 1 Ma despite the different time scales of the observational window. Apennine extension is recognized both in the foreland, as high angle normal faults accommodating modest stretching in the carapace of the growing mountain front, and in the hinterland, with larger normal faults that accomplish crustal thinning as the upper plate retreats. Nearly all of the active normal faults are clustered near the topographic crest of the range where underlying mantle anisotropy changes from orogen normal to orogen parallel and the crust abruptly thins 25 km. This co-evolution of extension and shortening shares some notable characteristics with other, basement-involved collisional orogens including the early Tertiary Laramide orogeny in the American west and the Oligocene to Miocene evolution of the Alps. These geologic, geomorphic, geodetic, and geophysical observations argue that Apennine deformation is partitioned among a fold and thrust belt where shortening has recently stepped-down to a single, deep structure presumably coupled to the subduction interface, and a tight zone of active extension coupled to an asthenospheric nose at the trailing edge of the retreating upper plate.