MECHANISMS OF SHEAR LOCALIZATION IN THE CONTINENTAL LITHOSPHERE - INFERENCE FROM THE DEFORMATION MICROSTRUCTURES OF PERIDOTITES FROM THE IVREA ZONE, NORTHWESTERN ITALY

The ultramafic massif of Balmuccia, northwestern Italy, shows a variet y of deformation fabrics including some localized shear zones that res ulted in nearly complete melting (pseudotachylyte). A series of perido tite specimens were collected near one of the pseudotachylyte fault ve ins to investigate the mechanisms of shear localization. The microstru ctural analyses show at least three deformation stages. The first (sta ge I) is nearly homogeneous deformation at low stress (similar to 3 MP a) and high temperature(similar to 1300-1500 K), followed by localized deformation (stage II) at a higher stress (similar to 60 MPa) and mod erate temperature (similar to 1000-1150 K) and, finally, semibrittle d eformation (stage III) at a very high stress (similar to 400 MPa) that resulted in the formation of pseudotachylyte. The stage II deformatio n resulted in relatively small strains in most areas bur significant l ocalized deformation leading to dynamic recrystallization occurred, th e degree of which increases systematically toward the fault zone. Base d on this observation, together with the observation that the brittle deformation post-dated dynamic recrystallization, we conclude that the shear localization in this locality occurred as a result of ductile d eformation rather than brittle deformation. Various mechanisms of shea r localization in the ductile regime are examined and we conclude that the grain-size reduction due to dynamic recrystallization at relative ly high stresses and moderate temperatures is a probable mechanism of shear localization. The intermediate regime between dislocation and di ffusion creep, where rheology is grain-size-sensitive yet continuous r ecrystallization occurs due to dislocation creep, is suggested to play an important role in shear localization in the upper mantle. The cond itions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization. It is s hown that shear localization occurs al relatively low temperatures and /or high stresses. Localized deformation in the continental lithospher e will occur well below the brittle-ductile transition (similar to 10 km depth), down to similar to 40-80 km depth depending upon the geothe rmal gradient and water fugacity. (C) 1998 Elsevier Science Ltd.