Deep crustal extensional faulting in the Ivrea Zone of Northern Italy

Abstract

The Ivrea Zone in Northern Italy consists of amphibolite and granulite facies metasedimentary, metabasic, and ultrabasic rocks, and is thought to represent a section through the deep continental crust. Detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones, which, after the effects of later low temperature faulting and folding have been removed, appears to accommodate extension of the sequence. The shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks, and locally show evidence for development under prograde conditions. These observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting. In this model the effects of shear heating and displacement of cool hanging-wall rocks against hotter footwall rocks are calculated. These observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust, especially when basic rocks are the dominant components of the rock pile. A restored section through the extended lower crust as observed in the Ivrea Zone is presented, and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed.