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

Arjan H. Dijkstra*, Martyn R. Drury, Reinoud L.M. Vissers, Julie Newman, Herman L.M. Van Roermund

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

29 Citations (Scopus)

Abstract

There is abundant field and microstructural evidence for localization of deformation in alpine- and ophiolite-type mantle massifs. On the basis of field relationships and microstructures we recognize two types of tectonite shear zones (medium- to coarse- and fine-grained), as well as two types of mylonitic shear zones (anhydrous and hydrous peridotite mylonites). In tectonite shear zones, softening processes responsible for localization are probably melt-related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine-grained tectonites. In peridotite mylonites, the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep. Microstructural and petrological study of mylonite rocks reveals that reactions, either continuous net-transfer reactions (anhydrous and hydrous) or melt-rock reactions, play a key role in the formation of fine-grained material that promotes grain size sensitive creep. These reactions occur over a broad range of pressure-temperature conditions encompassing a large part of the lithospheric upper mantle. We conclude that mantle shear zones are widespread and that they reduce the (bulk) strength of the lithosphere significantly.

Original languageEnglish
Pages (from-to)11-24
Number of pages14
JournalGeological Society Special Publication
Volume224
DOIs
Publication statusPublished - 23 Jun 2004
Externally publishedYes

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