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

Abstract

[1] 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.

Subcontinental lithosphere reactivation beneath the Hoggar swell (Algeria): Localized deformation, melt channeling and heat advection

“…Rather than solid-state amphibole deformation, this correlation likely records amphibole crystallization with an orientation that mimics the preexisting, strain-induced CPO of clinopyroxene. Such epitaxial relationship between amphibole and clinopyroxene has been observed by Kaczmarek and Tommasi (2011) in the Lanzo orogenic peridotite (Italy), for instance. It is consistent with microstructural observations suggesting that amphibole crystallized belatedly, compared with secondary clinopyroxene.…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…Mt Avic, Aosta Valley, and Breithorn, Zermatt-Saas area, in the Northwestern Alps (Dal Piaz, 1969;Fontana et al, 2008); 3. Lanzo, to the south of the Sesia-Lanzo HP terrane in the Southwestern Alps (Bodinier, 1988;Bodinier et al, 1986Bodinier et al, , 1991Boudier and Nicolas, 1972;Ernst, 1978;Kaczmarek and Mü ntener, 2005, 2008, 2010Kaczmarek and Tommasi, 2011;Mü ntener et al, 2005;Nicolas, 1969;Piccardo et al, 2004bPiccardo et al, , 2005Piccardo et al, , 2007aPognante et al, 1985); 4. Erro-Tobbio in the Voltri group, western Liguria (Bezzi and Piccardo, 1971;Borghini et al, 2007;Chiesa et al, 1975;Hoogerduijn-Strating et al, 1990;Messiga and Piccardo, 1974;Piccardo and Vissers, 2007;Piccardo et al, 1980Piccardo et al, , 1988aRampone and Borghini, 2008;Rampone et al, 2005;Vissers et al, 1991); and 5.…”

Erratum to: Crystal preferred orientations of olivine, orthopyroxene, serpentine, chlorite, and amphibole, and implications for seismic anisotropy in subduction zones: a review

“…CPOs of olivine in natural shear zones were studied in Josephine Peridotite in southeastern Oregon, US (Warren et al, 2008;Skemer et al, 2010;Hansen and Warren, 2015), in the Red Hills, New Zealand (Webber et al, 2008), in the Hilti massif, Semail ophiolite, Oman (Linckens et al, 2011), in an extensional shear zone in the mantle, Lanzo massif, Italy (Kaczmarek and Tommasi, 2011), in an oblique-slip low-angle shear zone in the Beni Bousera peridotite (Rif Belt, Morocco) (Frets et al, 2014), in the mylonitic peridotites in the Ronda massif, Southern Spain (Precigout et al, 2017), and in Yugu peridotites in South Korea (Fig. 7) (Park and Jung, 2017).…”

“…In addition, one of the important minerals in the middle and lower crust is amphibole, which is also elastically anisotropic (Aleksandrov and Ryzhova, 1961a;Siegesmund et al, 1989;Weiss et al, 1999;Kaczmarek and Tommasi, 2011;Lloyd et al, 2011;Ji et al, 2013;Ko and Jung, 2015;Brown and Abramson, 2016;Almqvist and Mainprice, 2017). A recent experimental study under simple shear at high pressure and temperature showed that the CPO of amphibole (hornblende) might cause large seismic anisotropy (Ko and Jung, 2015).…”