Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones

Linckens, Jolien; Herwegh, Marco; Müntener, Othmar (2015). Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones. Tectonophysics, 643, pp. 26-43. Elsevier 10.1016/j.tecto.2014.12.008

[img] Text
Herwegh_1.pdf - Published Version
Restricted to registered users only
Available under License Publisher holds Copyright.

Download (4MB) | Request a copy

Low viscosity domains such as localized shear zones exert an important control on the geodynamics of the uppermost mantle. Grain size reduction and subsequent strain localization related to a switch from dislocation to diffusion creep is one mechanism to form low viscosity domains. To sustain strain localization, the grain size of mantle minerals needs to be kept small over geological timescales. One way to keep olivine grain sizes small is by pinning of mobile grain boundaries during grain growth by other minerals (second phases). Detailed microstructural studies based on natural samples from three shear zones formed at different geodynamic settings, allowed the derivation of the olivine grain-size dependence on the second-phase content. The polymineralic olivine grain-size evolution with increasing strain is similar in the three shear zones. If the second phases are to pin the mobile olivine grain boundary the phases need to be well mixed before grain growth. We suggest that melt-rock and metamorphic reactions are crucial for the initial phase mixing in mantle rocks. With ongoing deformation and increasing strain, grain boundary sliding combined with mass transfer processes and nucleation of grains promotes phase mixing resulting in fine-grained polymineralic mixtures that deform by diffusion creep. Strain localization due to the presence of volumetrically minor minerals in polymineralic mantle rocks is only important at high strain deformation (ultramylonites) at low temperatures (<~800°C). At smaller strain and stress conditions and/or higher temperatures other parameters like overall energy available to deform a given rock volume, the inheritance of mechanical anisotropies or the presence of water or melts needs to be considered to explain strain localization in the upper mantle.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Other Institutions > Teaching Staff, Faculty of Science
08 Faculty of Science > Institute of Geological Sciences > Tectonics

UniBE Contributor:

Herwegh, Marco

Subjects:

500 Science > 550 Earth sciences & geology
500 Science

ISSN:

0040-1951

Publisher:

Elsevier

Language:

English

Submitter:

Marco Herwegh

Date Deposited:

01 May 2015 13:30

Last Modified:

06 Oct 2015 09:50

Publisher DOI:

10.1016/j.tecto.2014.12.008

BORIS DOI:

10.7892/boris.67724

URI:

https://boris.unibe.ch/id/eprint/67724

Actions (login required)

Edit item Edit item
Provide Feedback