Anatomy of an extensional shear zone in the mantle, Lanzo massif, Italy

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