Development of a deep-crustal shear zone in response to syntectonic intrusion of mafic magma into the lower crust, Ivrea-Verbano zone, Italy

A 1 to 1.5 km-thick, high-temperature sheer zone is localized in wall rocks subparallel to the eastern intrusive contact of the Permian Mafic Complex of the Ivrea-Verbano zone (IVZ), Italy. The shear zone is characterized by concentrated ductile deformation manifested by a penetrative foliation subp arallel to the intrusive contact and a northeast-plunging sillimanite linea tion. Evidence of noncoaxial strain and transposition is widespread in the shear zone including such features as rootless isoclinal folds, dismemberme nt of competent layers, and scattered kinematic indicators. The metasedimen tary rocks in the shear zone are migmatitic, and the accumulation of leucos ome is variable within the shear zone. Near the intrusive contact with the Mafic Complex leucosome forms similar to 20 vol% of the wall rock, whereas leucosome concentrations may locally reach similar to 60 vol% of the wall r ock near the outer limits of the shear zone. This variation in vol% leucoso me suggests melt/magma migration from the inferred site of anatexis along t he intrusive contact to lower-strain regions within and near the margins of the shear zone. The leucosome accumulations chiefly occur as layer-paralle l concentrations, but are also folded and boudined, and locally are associa ted with tension gashes and fracture arrays. Networks of granitic dikes and small plutons in the eastern IVZ suggest that some magmas migrated out of the high-temperature shear zone. Some magma apparently migrated laterally a long the strike of the shear zone and concentrated in areas of lower strain where the intrusive contact takes a major westward bend. The high-temperat ure shear zone is interpreted as a "stretching fault" (or stretching shear zone) after Means [W.D. Means, Stretching faults, Geology 17 (1989) 893-896 ], whereupon the metasedimentary wail rocks and associated leucosome deform ed synchronously with the multistage emplacement and deformation flow of th e Mafic Complex. The recognition of a high-temperature shear zone associate d with the emplacement of mafic igneous rocks into the deep crust is an exa mple of the progressive stratification of the lower crust during magmatic u nder- or intraplating that has consequences for seismic imaging and its int erpretation. (C) 1999 Elsevier Science B.V. All rights reserved.