Abstract

The tectonic and rheological evolution of the southern Alpine continental crust is reconstructed from structural, petrological, and radiometric studies in the Ivrea and Strona-Ceneri basement units. The deep crust of the southern Alps acquired its present compositional and metamorphic zonation during Paleozoic magmatism and amphibolite-to granulite-facies regional metamorphism. Inferred strength contrasts between lower crustal and upper mantle rocks in the Ivrea zone are low at the high temperatures of regional metamorphism. Late Paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early Mesozoic time. Extensional uplift and cooling of the basement section is coupled with crustal-scale trends of increasing rheological stratification, grain size reduction, and strain localization. Mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the Ivrea zone are inferred to have controlled the high-strain rheology of the attenuating southern Alpine crust. The deep crust in the thinned Ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust. Final uplift and exposure of the deep southern Alpine crust occurred during Late Cretaceous to Tertiary Alpine thrusting.

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