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

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. The Geological Society of London 2004.

published proceedings

  • Geological Society London Special Publications

author list (cited authors)

  • Dijkstra, A. H., Drury, M. R., Vissers, R., Newman, J., & Van Roermund, H.

citation count

  • 27

complete list of authors

  • Dijkstra, Arjan H||Drury, Martyn R||Vissers, Reinoud LM||Newman, Julie||Van Roermund, Herman LM

publication date

  • January 2004