[SCG58-21] Serpentinized shear zones with low frictional strength by grain-boundary sliding of antigorite at the depth of deep SSEs
キーワード：B-type antigorite CPO、Shallow wedge mantle、Grain boundary sliding、Deep slow slip events、Serpentinized shear zones、Friction
Deep slow slip events (SSEs) commonly occur around the plate interface between the subducting slab and the shallow wedge mantle at depths of ~30–50km. At these depths it is thought to be difficult to sustain large porous channels capable of transporting large volumes of fluids derived from the subducting slab, the main exception being water released by metamorphic reactions along plate interfaces (Abers et al., 2017, Nat. Geosci.). In addition, it is proposed that slow-slip behaviours arise near the threshold between stable and unstable failure related to frictional instabilities in hydrous silicates (Leeman et al., 2016, Nat. Comm.). For these reasons, the frictional properties of hydrous silicates are likely to be important in understanding SSE phenomena.
The shallow forearc wedge mantle is thought to contain significant amounts of hydrated peridotite in the form of high-temperature serpentine, antigorite (Atg). However, previous experimental studies have focused on gouge samples of the low-temperature serpentine minerals lizardite and chrysotile, and proposed the importance of both low effective normal stresses developed due to influence of pore fluids and the low frictional strength of the low-temperature serpentine (μ values from 0.7–0.5) to explain the slow rupture velocities of the short-term SSEs observed in nature (Hirauchi & Muto, 2015, EPS). Therefore, there are the gaps between the understanding of slow slip based on experimental frictional studies and the expected distribution of serpentinite minerals based on their thermodynamic stabilities in the region of deep SSEs.
Here we focus on the frictional property of antigorite as a key to deepen our understanding of the material properties relevant to deep SSEs. Atg is a highly anisotropic sheet silicate mineral and alignment of this mineral can result in strong frictional and permeability anisotropies in the forearc wedge mantle. However, several different types of crystal preferred orientation (CPO) patterns of Atg and the different CPO formation mechanisms have been proposed and 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 EBSD analysis applied to strain gradients preserved in natural serpentinite shear zones. In addition, EBSD maps provided aspect ratios of individual grains and changes in shape preferred orientation. We also compared chemical composition with the observed geometric changes in Atg grains, using the results of EPMA analyses.
CPO analysis shows a clear rotation of the (001) basal plane of Atg towards parallel to the shear zone with increasing strain. The Atg CPO within the shear zone shows the B-type Atg CPOwhere a strong concentration of b-axes of Atg is parallel to the shear direction. There are only limited mis-orientations in individual Atg grains within the shear zone that could indicate internal plastic deformation. 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 zones. 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. This implies that serpentinized shear zones that develop B-type Atg CPO along the slab-mantle boundary in subduction zones have a lower frictional strength than expected from experimental studies using the gouge sample of Atg and may be comparable to low-temperature serpentines, and could develop aseismic frictional sliding in deep SSEs domains.
The shallow forearc wedge mantle is thought to contain significant amounts of hydrated peridotite in the form of high-temperature serpentine, antigorite (Atg). However, previous experimental studies have focused on gouge samples of the low-temperature serpentine minerals lizardite and chrysotile, and proposed the importance of both low effective normal stresses developed due to influence of pore fluids and the low frictional strength of the low-temperature serpentine (μ values from 0.7–0.5) to explain the slow rupture velocities of the short-term SSEs observed in nature (Hirauchi & Muto, 2015, EPS). Therefore, there are the gaps between the understanding of slow slip based on experimental frictional studies and the expected distribution of serpentinite minerals based on their thermodynamic stabilities in the region of deep SSEs.
Here we focus on the frictional property of antigorite as a key to deepen our understanding of the material properties relevant to deep SSEs. Atg is a highly anisotropic sheet silicate mineral and alignment of this mineral can result in strong frictional and permeability anisotropies in the forearc wedge mantle. However, several different types of crystal preferred orientation (CPO) patterns of Atg and the different CPO formation mechanisms have been proposed and 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 EBSD analysis applied to strain gradients preserved in natural serpentinite shear zones. In addition, EBSD maps provided aspect ratios of individual grains and changes in shape preferred orientation. We also compared chemical composition with the observed geometric changes in Atg grains, using the results of EPMA analyses.
CPO analysis shows a clear rotation of the (001) basal plane of Atg towards parallel to the shear zone with increasing strain. The Atg CPO within the shear zone shows the B-type Atg CPOwhere a strong concentration of b-axes of Atg is parallel to the shear direction. There are only limited mis-orientations in individual Atg grains within the shear zone that could indicate internal plastic deformation. 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 zones. 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. This implies that serpentinized shear zones that develop B-type Atg CPO along the slab-mantle boundary in subduction zones have a lower frictional strength than expected from experimental studies using the gouge sample of Atg and may be comparable to low-temperature serpentines, and could develop aseismic frictional sliding in deep SSEs domains.