Crustal modelling of the Ivrea-Verbano zone in northern Italy re-examined: Coseismic cataclasis versus extensional shear zones and sideways rotation

New field and microstructural observations in the Ivrea-Verbano zone (IVZ), northern Italy, reveal extensive post-Hercynian cataclasis, indentation structures, microtectonic escape structures, microcataclastic injectites and occasional true pseudotachylytes. The deformation occurred in situ and was governed by pure shear. These observations are incompatible with prevailing crustal models of the IVZ. Most of these models assume the presence of lower-crustal, originally flat-lying extensional shear zones, sideways block rotation and exposure of the petrological Moho, although the original structural observations from the central IVZ suggested that the post-Hercynian deformation was cataclastic rather than shear-dominated. Other, already published information also contradicts the sideways rotation of a crustal block comprising the IVZ and the adjacent Serie dei Laghi (SdL) after the Early Permian, as assumed in the models. The IVZ and SdL have different detrital zircon populations and are separated by a steep shear zone and a transcurrent fault (not rotated listric faults as assumed in the models). Rotation is also prevented by a flat-lying zone of miarolitic cavities in the Early Permian Baveno granite and by contemporaneous, vertical basic dykes. The widespread in-situ cataclasis is interpreted as due to coseismic deformation by high-frequency seismic waves generated by earthquakes along the Insubric fault, and exerting oscillatory compression, dilation and torsion in a ≤. 10 km wide zone along the footwall of the Insubric line. The in-situ post-Hercynian cataclasis, absence of lower-crustal extensional shear zones and lack of sideways rotation of the IVZ and SdL call for a new and simpler crustal model of the region.