Shear zones in the upper mantle: evidence from alpine- and ophiolite-type peridotite massifs

Dijkstra, Arjan H.; Drury, M. R.; Vissers, R. L. M.; Newman, Joseph W.; Roermund, H.L.M. van

doi:10.1144/gsl.sp.2004.224.01.02

Cited by 28 publications

(16 citation statements)

References 61 publications

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“…[]; Lanzo: Kaczmarek and Müntener [, ] and Kaczmarek and Tommasi []; Othris: Dijkstra et al . []; Lizard ophiolite; Allerton and Macleod []; and Josephine: Kelemen and Dick []), suggesting deformation in the vicinity of a thermal anomaly [ Vauchez et al ., ], a hypothesis which has recently been supported by geophysical surveys using SKS‐wave splitting in the active rift setting of the Afar (Ethiopia) region [ Hammond et al ., ]. As shown by experimental investigations, the presence of melt reduces the harzburgite's flow strength [ Hirth and Kohlstedt , ; Zimmerman and Kohlstedt , ] forcing deformation to localize.…”

Section: Discussionmentioning

confidence: 99%

Mechanical anisotropy control on strain localization in upper mantle shear zones

et al. 2016

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Mantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid-state ductile to brittle deformation with progressive cooling. Using the crustal-scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes from more evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900-800°C) to ultramylonite (<700°C) and a serpentine foliation (<500°C) from the SZ rim to the center. The spatial correlation between solid-state deformation fabrics and magmatic features indicates progressive strain localization in the SZ on the cooling path. Three stages can be discriminated: (i) Cycles of melt injection (dunite channels and websterite dikes) and solid-state deformation (protomylonites-mylonites; 1100-900°C), (ii) dominant solid-state deformation in harzburgite mylonites (900-800°C) with some last melt injections (gabbronorites) and ultramylonites (<700°C), and (iii) infiltration of seawater inducing a serpentine foliation (<500°C) followed by cataclasis during obduction. The change of these processes in space and time indicates that early dike-related ridge-parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.

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Section: Discussionmentioning

confidence: 99%

Mechanical anisotropy control on strain localization in upper mantle shear zones

et al. 2016

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“…Relatively to mantle shear zones mapped in other peridotite massifs (cf. review by Dijkstra et al [2004]), the northern Lanzo shear zone has two advantages: (1) it records the evolution of deformation under a large range of continuously varying temperature conditions, from near‐solidus conditions representative of the lithosphere‐asthenosphere boundary to fully lithospheric temperatures (800–900°) and (2) the massif is large enough allowing the exposure of the entire shear zone and of the protoliths on both sides of it.…”

Section: Discussionmentioning

confidence: 99%

“…Large‐scale mylonitic shear zones were nevertheless mapped in some orogenic massifs, like Ronda and Beni Bousera in the Betic‐Rifean belt (S Spain–N Morocco), Erro‐Tobio in the Alps, and Turon de Tecouere in the Pyrenees, and in the Othris, Voltri, and Oman ophiolites (cf. review by Dijkstra et al [2004]). A common characteristic of these mylonitic zones, shared by the northern Lanzo shear zone, is that they record an evolution under variable pressure and temperature conditions, associated to mantle exhumation in response to lithospheric thinning in extensional settings (orogenic massifs) or obduction (ophiolites).…”

Section: Discussionmentioning

confidence: 99%

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

Kaczmarek

Tommasi

2011

Geochem. Geophys. Geosyst.

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[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.

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“…Variations over more than one order of magnitude, from 0.01 cm to several cm, characterize mantle samples (Boyd & Meyer 1979). At a larger scale, it is generally found that areas that were subjected to major strains, such as collisional areas in the shallow lithospheric mantle, show, on average, smaller grain sizes (Dijkstra et al 2004). Overall, an average diameter of the rock matrix from available upper-mantle samples can be estimated at around 0.1 cm (Boyd & Meyer 1979), though larger diameters, for example, ∼1 cm and more, are not inconsistent with rheology.…”

Section: Grain Sizementioning

confidence: 99%

Radial profiles of seismic attenuation in the upper mantle based on physical models

Cammarano¹,

Romanowicz²

2008

Geophysical Journal International

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Thermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ∼1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. © 2008 The Authors Journal compilation © 2008 RAS

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Shear zones in the upper mantle: evidence from alpine- and ophiolite-type peridotite massifs

Cited by 28 publications

References 61 publications

Mechanical anisotropy control on strain localization in upper mantle shear zones

Mechanical anisotropy control on strain localization in upper mantle shear zones

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

Radial profiles of seismic attenuation in the upper mantle based on physical models

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