Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy
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....
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| Format: | Journal Article |
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American Geophysical Union
2011
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| Online Access: | http://hdl.handle.net/20.500.11937/19572 |
| _version_ | 1848750070636216320 |
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| author | Kaczmarek, M. Tommasi, Andréa |
| author_facet | Kaczmarek, M. Tommasi, Andréa |
| author_sort | Kaczmarek, M. |
| building | Curtin Institutional Repository |
| collection | Online Access |
| description | 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. |
| first_indexed | 2025-11-14T07:30:59Z |
| format | Journal Article |
| id | curtin-20.500.11937-19572 |
| institution | Curtin University Malaysia |
| institution_category | Local University |
| last_indexed | 2025-11-14T07:30:59Z |
| publishDate | 2011 |
| publisher | American Geophysical Union |
| recordtype | eprints |
| repository_type | Digital Repository |
| spelling | curtin-20.500.11937-195722017-09-13T16:08:22Z Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy Kaczmarek, M. Tommasi, Andréa extension grain boundary sliding lithospheric thinning ultramylonite bands dislocation creep melt 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. 2011 Journal Article http://hdl.handle.net/20.500.11937/19572 10.1029/2011GC003627 American Geophysical Union unknown |
| spellingShingle | extension grain boundary sliding lithospheric thinning ultramylonite bands dislocation creep melt Kaczmarek, M. Tommasi, Andréa Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title | Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title_full | Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title_fullStr | Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title_full_unstemmed | Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title_short | Anatomy of an extensional shear zone in the mantle Lanzo massif, Italy |
| title_sort | anatomy of an extensional shear zone in the mantle lanzo massif, italy |
| topic | extension grain boundary sliding lithospheric thinning ultramylonite bands dislocation creep melt |
| url | http://hdl.handle.net/20.500.11937/19572 |