Structural geometry, lower crustal magmatic underplating and lithospheric stretching in the Ivrea-Verbano zone, northern Italy

Abstract

The Ivrea-Verbano zone is believed to represent an up-ended cross-section through the lower continental crust as it existed at the end of the Hercynian orogeny in the Southern Alps. Structurally-oriented geological mapping has been carried out in the central and northern parts of the Ivrea-Verbano zone, a region some 30 km along strike. The geology of the southern half of the zone is dominated by a large basic-ultrabasic complex (the Mafic Formation), which is in contact with a strongly banded metasedimentary + metabasic sequence to the north. Particular attention was paid to: (a) the structural relationship between the rocks of the Mafic Formation and their envelope of high-grade metasedimentary and metabasic rocks; (b) the geometrical configuration throughout the Ivrea-Verbano zone of high-temperature shear zones, which accommodated post-Hercynian crustal extension; and (c) the geometry of late, low-temperature faulting, the effects of which have been removed in order to produce a restoration of the structure as it existed during the post-Hercynian extensional phase. The intrusion of large volumes of basic magma (ca 50% of the outcrop area) to form the rocks of the Mafic Formation appears to be coeval with the onset of extension ( ca 280 Ma). The main basic body has a laccolithic form, which was originally more than 10 km thick. Overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot, immediately subsolidus or partially molten body, incorporating its envelope of hot metasediments into a large, originally recumbent fold. A geometrical association with a high-temperature, low-angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion. The Ivrea-Verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating, which may aid in the interpretation of present-day deep seismic profiles. It also demonstrates the geometry of a network of conjugate, high-temperature, low-angle shear zones in a well-layered lower crustal section.