A theoretical model for the evolution of microstructure in lithospheric shear zones

Elvira Mulyukova, David Bercovici

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

Abstract

The strength of the lithosphere plays a key role in the formation and evolution of tectonic plate boundaries. Localized lithospheric deformation associated with plate tectonics requires a mechanism for weakening across the entire width of the lithosphere, including the strongest cold ductile region.We explore the microphysics of weakening of lithospheric materials, and in particular the coupled evolution ofmineral grain size and intragranular defects and their control on lithospheric strength.We propose a model for the interaction between grain-boundaries and dislocation density to reduce the net free energy of grains during dynamic recrystallization (DRX). The driving forces for DRX arise from heterogeneity in dislocation density and grain boundary curvature. Our model shows that grain growth driven by variation in grain boundary curvature can be impeded by variation in dislocation density; this occurs because as the grains grow, to minimize their surface energy, their dislocation density and associated internal energy may increase and offset the driving forces for grain growth. The correlation between grain size and dislocation density can, for example, arise because the dislocation accumulation in smaller grains is suppressed due to the large stress that is needed to bend and elongate a short dislocation (as dictated by the small grain size), while the larger grains can have long dislocations and reach a steady-state dislocation density dictated by the applied stress. In a lithospheric setting, slower grain growth means that it would require less mechanical work to establish weak localized shear zones through grain damage, and retard the healing of previously damaged zones. Furthermore, the competition of two different timescales-that of grain growth and the dislocation kinetics-can lead to oscillating behaviour over 1-10 yr as the grain size and dislocation density advance towards their steady states. These oscillations are likely to have an effect on the rheology of lithospheric rocks, for example, their strengthening and weakening through time, and have a potential application to geological processes such as post-seismic creep in ductile shear zones.

Original languageEnglish (US)
Pages (from-to)803-819
Number of pages17
JournalGeophysical Journal International
Volume216
Issue number2
DOIs
StatePublished - Feb 1 2019

Keywords

  • Creep and deformation
  • Crust and lithosphere
  • Defects
  • Microstructure
  • Rheology

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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