The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone

Abstract

Schmid, SM., Zingg, A. and Handy, M., 1987. The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone. In: H.J. Zwart, M. Martens, I. van der Molen, C.W. Passchier, C. Spiers and R.L.M. Vissers (Editors), Tectonic and Structural Processes on a Macro-, Meso- and Micro-Scale. Tecta~a~hys~~, 135: 47-66. The Insubric Line west of Locamo is characterized by a 1 km thick greenschist facies mylonite belt. East of Locamo, these mylonites are overprinted by a discrete brittle fault. The mylonites are derived from basement units of the Central Alps (Sesia Zone) and the Southern Alps (Ivrea Zone) as well as from the Permo-Mesozoic cover of the Southern Alps (Canavese). Sense of shear criteria indicate that the mylonites accomm~ated backt~s~g followed by dextral strike-slip motion. Mylonitization during backtbrusting was synchronous with backfolding of the Central Alpine nappes under higher metamorphic conditions. A horizontal temperature gradient resulted from the rapid juxtaposition of the warm Central Alpine block against the cold Southern Alpine block. Mylonites formed during the later dextral strike-slip event are related to large transcurrent displacements in the Central Alps deduced from regional kinematic considerations. Thus, both mylo~t~ation events are contempor~eous with deformation to the north and south of the Insubric Line (Insubric phase) extensively modifying the pre-Insubric crustal configuration of the Alps. The Insubric phase post-dates the Bergell intrusion (30 m.y.). The emplacement of the geophysical Ivrea body is a combined effect of vertical uplift due to E-W directed crustal thinning during the Early Jurassic and underplating by continental crust associated with Late Cretaceous compression. A deep crustal normal fault (Pogallo Line), subsequently rotated during Tertiary Alpine orogenesis, separates deeper parts of the Southern Alpine crust (Ivrea Zone) from intermediate crustal levels (Strona-Ceneri Zone). The rigid Ivrea body localir.ed large strains within the Insubric mylonite belt and is responsible for the present curvature of the Insubric Line. In~~uetion