Analysis of the microstructures in the km-scale mantle shear zone that
separates the northern and the central parts of the Lanzo peridotite
massif provides evidence of an evolution in time and space of
deformation processes accommodating shearing in the shallow mantle
within an extensional setting. This shear zone displays an asymmetric
distribution of deformation facies. From south to north, gradual
reorientation of the foliation of coarse porphyroclastic
plagioclase-bearing peridotites is followed by development of
protomylonites, mylonites, and mm-scale ultramylonite bands. A sharp
grain size gradient marks the northern boundary. Early deformation under
near-solidus conditions in the south is recorded by preservation of
weakly deformed interstitial plagioclase and almost random clinopyroxene
and plagioclase crystal orientations. Feedback between deformation and
melt transport probably led to melt focusing and strain weakening in the
shear zone. Overprint of melt-rock reaction microstructures by
solid-state deformation and decrease in recrystallized grain size in the
protomylonites and mylonites indicate continued deformation under
decreasing temperature. Less enriched peridotite compositions and
absence of ultramafic dykes or widespread melt-impregnation
microstructures north of the shear zone and clinopyroxene and amphibole
enrichment in the mylonites and ultramylonites suggest that the shear
zone acted as both a thermal barrier and a high-permeability channel for
late crystallizing fluids. These observations, together with chemical
data indicating faster cooling of central Lanzo relative to the northern
body, corroborate that this shear zone is a mantle detachment fault. All
deformation facies have crystal preferred orientations consistent with
deformation by dislocation creep with dominant activation of the
(010)[100] and (100)[001] systems in olivine and orthopyroxene,
respectively. Dynamic recrystallization produces dispersion of olivine
CPO but not a change of dominant deformation mechanism. Evidence for
activation of grain boundary sliding is limited to mm-scale
ultramylonite bands, where solid-state reactions produced very fine
grained polymineralic aggregates. Except for these latest stages of
deformation, strain localization does not result from the
microstructural evolution; the grain size decrease is a consequence of
the need to deform a rock volume whose strength continuously increases
because of decreasing temperature conditions. Strain localization in the
intermediate levels thus essentially results from the more localizing
behavior of both the deep, partially molten, and shallow parts of this
extensional shear zone distribution.