A window into Alps and Apennines interactions and the development of the Northern Apennines fold-and-thrust belt

The tectonic framework of Northern Italy is characterized by the complex interaction between the south-verging Southern Alps, the north-verging Northern Apennines, and their shared foreland basin, the Po-Plain Basin. The Neogene evolution of the Northern Apennines gives rise to three buried structural arcs, each one with an increasing amount of shortening, from W-E, the Monferrato arc, the Emilian Arc, and the Ferrara arc. The eastern Emilian Arc is composed of three main thrust systems and related anticlines that, from south to north, are named Stradella-Belgioioso, San Colombano and Casalpusterlengo-Zorlesco structures, and the Caviaga-Soresina structures. The western Emilian Arc is defined by the prolongation of the Caviaga-Soresina, Cortemaggiore and Salsomaggiore structures. In the outcropping Northern Apennines, the Bobbio Tectonic Window preserves a record of the interactions between the buried front of the Emilian Arc with the buried front of the Southern Alps. 

Existing studies have focused on fault slip rate reconstructions based on the interpretation of seismic lines along the Emilian arc, but a comprehensive 3D model of the entire arc is still lacking. We developed a model that integrates the structural and exhumation history of the Emilian Arc and the Southern Alps. 

Our study sheds important information on the spatio-temporal evolution of the Bobbio Tectonic Window, with implications on our understanding of out-of-sequence deformation in the Northern Apennines. Specifically, more than 1300 TWT seismic reflection profiles and 200 wells with log information and 42 wells with time-depth curves (courtesy of ENI E&P), have been integrated to build a detailed 3D tectonic model of the Emilian Arc. A set of balanced cross-sections were also developed to calculate fault slip rates. Moreover, sandstones from the core of the Bobbio Tectonic Window (San Salvatore Sandstones) were analyzed for apatite (U-Th)/He low-T thermochronology to 1) constrain cooling and exhumation history, 2) assess relationships between deformation and exhumation of the Emilian Arc in response to Alps-Alpine tectonics.

Our preliminary thermochronological results from the Bobbio Tectonic Window show a Pliocene cooling signal between ca. 2 and 4 Ma. We interpret these results to represent out-of-sequence thrusting within the inner Apennine fold-and-thrust belt as a result of the collision between the frontal part of the Emilian Arc with the Southern Alps.  This study shows how far field geological structures can influence the kinematics of thrust systems and helps explain the generally decreasing Plio-Pleistocene tectonic activity of the Northern Apennine's buried thrust front.