Grain boundary sliding as an antigorite CPO formation mechanism and implications for the slab-mantle boundary rheology: Example of antigorite schist from the Sanbagawa belt, SW Japan
Abstract
Antigorite (Atg) is predicted to be in large parts of the shallow hydrated forearc mantle. Atg shows strong anisotropies related to the elasticity, friction and permeability in the domain. Although several different types of Atg crystal preferred orientation (CPO) patterns and the different CPO formation mechanisms have been proposed, the conditions under which these different types are likely to develop are not well known.
In this study, we examine progressive development of Atg CPO by using strain gradients preserved in natural serpentinite shear zones. Atg CPO were determined by EBSD analysis. In addition, aspect ratios of indivudual grains and changes in shape preferred orientation were determined using EBSD mapping. The observed geometric changes were compared with chemical characteristics determined using EPMA analyses. CPO analysis shows a clear rotation of the c-axes of Atg towards the normal to the shear zone with increasing strain. The Atg CPO within the shear zone shows the b-axis of Atg aligned parallel to the shear direction referred to as B-type CPO. There is only limited evidence for internal plastic deformation of individual Atg grains within the shear zone. In addition, there are no significant differences seen in the size, aspect ratios and major element chemical compositions of the Atg grains within and outside of the shear zone. Finite strain ellipses estimated using March's model for passive rotation of elongate shapes show increasing strain ratios associated with progressive rotation of the maximum stretching direction towards the shear plane of the shear zone. The above observations suggest reorientation of Atg grains occurred with increasing strain but without internal deformation or grain size reduction. Therefore, we propose the B-type Atg CPO formed by mechanical rotation of grains associated with grain-boundary sliding. The association of grain boundary sliding with the formation of B-type Atg CPO in the present shear zones is in agreement with the experimental observation that the low-friction direction of Atg is parallel to the b-axis. The development of similar Atg CPOs along the slab-mantle boundary in subduction zones could play a significant role in developing aseismic deformation associated with the slow slip events in the shallow mantle wedge reported at the depth of 30-50 km.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T31G0393N
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8045 Role of fluids;
- STRUCTURAL GEOLOGYDE: 8170 Subduction zone processes;
- TECTONOPHYSICS