MECHANISMS OF SHEAR LOCALIZATION IN THE CONTINENTAL LITHOSPHERE - INFERENCE FROM THE DEFORMATION MICROSTRUCTURES OF PERIDOTITES FROM THE IVREA ZONE, NORTHWESTERN ITALY
Dh. Jin et al., MECHANISMS OF SHEAR LOCALIZATION IN THE CONTINENTAL LITHOSPHERE - INFERENCE FROM THE DEFORMATION MICROSTRUCTURES OF PERIDOTITES FROM THE IVREA ZONE, NORTHWESTERN ITALY, Journal of structural geology, 20(2-3), 1998, pp. 195-209
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.