Shale diapirism in the Quaternary tectonic evolution of the Northern Apennine, Bologna, Italy

We use morphologic, stratigraphic and structural data to identify a set of diapirs just southward (uphill) of the northern margin of the Northern Apennine Range. The diapirs consist mainly of Ligurid chaotic shaley units, that reach a maximum thickness of over 3 km. We also reconstruct marine sedimentation/abrasion surfaces that record a 1 cm/a differential uplift induced by diapirism. Based on this data we use the lubrication approximation of the Navier-Stokes equations and analogue experiments to show that thrusting of the Epiligurid units can be generated by diapirism of the underlying Ligurid units in addition to regional compression. Due to a gravity component parallel to the strata, the structures formed by diapirism are similar to those generated by compressive tectonics, differing in that thrusting is lubricated by diapiric rise. Using the analogue experiments, we show that deformation begins with folding eventually evolving to thrusting, and we derive an empirical relation to infer the time remaining to the end of deformation. In our area diapiric deformation is still in the folding regime, but it should eventually evolve to the thrusting regime and terminate in about 105 a. We suggest that the whole north-eastern slope of the Northern Apennine is controlled by diapiric tectonics. Our interpretation contrasts with the more traditional one, which views regional tectonic compression as the only responsible for the structures observed in the field. Since our model is based on first principles, it could be applied to other areas with similar geology and topography.