5:15 PM - 7:15 PM
[SCG54-P05] Deformation mechanisms of chlorite-actinolite schist in the Nagasaki metamorphic rocks, Nomo Peninsula, Japan
Keywords:actinolite, Nagasaki metamorphic rocks, dislocation creep, dissolution-precipitation creep, subduction zone, rheology
Investigating the rheological properties of the shallow slab-mantle wedge interface is crucial for understanding the degree of viscous decoupling between the plate interface and the physical mechanisms of deep slow earthquakes such as slow slip (Tarling et al, 2019; Tulley et al, 2022; Nishiyama et al, 2023). Nishiyama et al. (2023) conducted structural and geochemical analyses of a tectonic mélange (Mie mélange) in the Nishisonogi metamorphic rocks, revealing that metasomatic alteration driven by a high chemical potential gradient between the mantle wedge (serpentinite) and the subducting oceanic crust (metabasite) led to the development of a shear zone consisting of chlorite-actinolite schist (CAS). Nishiyama et al. (2023) also suggested that dissolution-precipitation creep was the dominant deformation mechanism of CAS, based on the chemical zoning and structure that cuts zoning (truncation) observed for fine-grained actinolite aggregates, based on the presence of chemical zoning and truncation of the zoning. However, few structural petrological studies have been conducted to elucidate the deformation mechanisms of CAS, except for the Nishisonogi metamorphic rocks. Therefore, we conducted a microstructural analysis of CAS from the Nomo metamorphic rocks, Kyushu, Japan.
In the Nomo metamorphic rocks, ultramafic mélange zone (Nishiyama et al., 1997), with a thickness ranging from a few meters to tens of meters, is intercalated between the serpentinite and the mafic schist. CAS occurs as a vein-like filling within extensional fractures parallel to the schistosity of the ultramafic mélange. The schistosity observed in CAS is defined by the shape- preferred orientation of fine-grained (50-200 μm in length), acicular actinolite aggregates, which wrap around lenticular domains consisting of fine-grained, interpenetrating chlorite aggregates. The long axis of the lenticular domains defines the lineation of CAS. In addition, coarse-grained (>1 mm in length), which are randomly oriented or weakly aligned, occur as vein fillings within extensional fractures along the schistosity.
Crystallographic orientation analysis of the fine-grained actinolite aggregates using electron backscatter diffraction revealed a strong crystallographic orientation, with [100] axes oriented perpendicular to the schistosity plane, [010] axes perpendicular to the lineation within the foliation plane, and [001] axes parallel to the lineation. The inverse pole figure orientation map shows the presence of subgrain boundaries parallel to the long axes of the acicular actinolite, as well as undulose extinction. In addition, misorientation angle distributions between neighboring grain pairs show a high frequency of misorientation angles <15°. These microstructural features suggest that actinolite in CAS has undergone intracrystalline deformation through dynamic recrystallization via subgrain rotation. In future studies, elemental mapping of the fine-grained actinolite aggregates will be conducted to investigate whether microstructural evidence for dissolution-precipitation creep is present.
References: Tarling et al. (2019), Nature Geoscience, 12, 1034-1042. Tulley et al. (2022), Geophysical Research Letters, 49, e2022GL098945. Nishiyama et al. (2023), Lithos, 446-447, 107115. Nishiyama et al. (1997), Field Trip Information Form for the 104th Annual Meeting of the Geological Society of Japan, 131-162.
In the Nomo metamorphic rocks, ultramafic mélange zone (Nishiyama et al., 1997), with a thickness ranging from a few meters to tens of meters, is intercalated between the serpentinite and the mafic schist. CAS occurs as a vein-like filling within extensional fractures parallel to the schistosity of the ultramafic mélange. The schistosity observed in CAS is defined by the shape- preferred orientation of fine-grained (50-200 μm in length), acicular actinolite aggregates, which wrap around lenticular domains consisting of fine-grained, interpenetrating chlorite aggregates. The long axis of the lenticular domains defines the lineation of CAS. In addition, coarse-grained (>1 mm in length), which are randomly oriented or weakly aligned, occur as vein fillings within extensional fractures along the schistosity.
Crystallographic orientation analysis of the fine-grained actinolite aggregates using electron backscatter diffraction revealed a strong crystallographic orientation, with [100] axes oriented perpendicular to the schistosity plane, [010] axes perpendicular to the lineation within the foliation plane, and [001] axes parallel to the lineation. The inverse pole figure orientation map shows the presence of subgrain boundaries parallel to the long axes of the acicular actinolite, as well as undulose extinction. In addition, misorientation angle distributions between neighboring grain pairs show a high frequency of misorientation angles <15°. These microstructural features suggest that actinolite in CAS has undergone intracrystalline deformation through dynamic recrystallization via subgrain rotation. In future studies, elemental mapping of the fine-grained actinolite aggregates will be conducted to investigate whether microstructural evidence for dissolution-precipitation creep is present.
References: Tarling et al. (2019), Nature Geoscience, 12, 1034-1042. Tulley et al. (2022), Geophysical Research Letters, 49, e2022GL098945. Nishiyama et al. (2023), Lithos, 446-447, 107115. Nishiyama et al. (1997), Field Trip Information Form for the 104th Annual Meeting of the Geological Society of Japan, 131-162.