Presentation Information
[T1-P-6]Role of earthquake cycles and solution transfer in lower crustal deformation: Observations from Napier Complex, East Antarctica
*Ryo MURAYAMA1, Lakshmanan Sreehari 1 (1. Shimane Univ.)
Keywords:
pseudotachylyte,mylonite,UHT metamorphic rock
The Napier Complex in East Antarctica is one of the few region on Earth that exhibits extensive exposure of ultra-high temperature (UHT) metamorphic rocks. The timing of this UHT metamorphism has been constrained to 2550-2480 Ma (Horie et al., 2012).
The Riier-Larsen Main Shear Zone (RLMSZ) divides the UHT region of the Napier Complex into western and main blocks (Hokada. 1999). The RLMSZ is interpreted as a retrograde shear zone (Sheraton et al., 1987), with its activity inferred to have occurred either between 2400-700 Ma or more likely between 2500-2450 Ma ( Hokada et al., 2008). The latter age range overlaps with the timing of UHT metamorphism, suggesting a possible tectonometamorphic link. Presence of pseudotachylyte in RLMSZ is reported in previous studies (Ishikawa et al., 2000), but detailed studies about their origin are not documented yet.
Recently, pseudotachylytes generated in the lower crust have been increasingly recognized as being associated with fluid infiltration and as precursors to ductile shear zones (Menegon et al., 2017; Michalchuk et al., 2023). Lower crustal pseudotachylytes have been reported, for example, from Tonagh Island (Toyoshima et al., 1999) in the Napier Complex, yet the possibility of lower crustal earthquakes along the RLMSZ has not been thoroughly investigated.
This study aims to investigate the field relationships, microstructures, and chemical compositions of pseudotachylyte and mylonite derived from the RLMSZ, in order to evaluate the tectonic evolution of the RLMSZ and its link to UHT metamorphism.
The major lithounit in the RLMSZ is a felsic gneiss composed of quartz, feldspar, pyroxene, amphibole, and opaque minerals. This felsic gneiss has been strongly mylonitized, with pyroxene porphyroclasts showing evidence of a top-to-the-west sense of shear. These pyroxene porphyroclasts are partially altered to amphibole along cleavage planes, suggesting syn-tectonic fluid activity under upper amphibolite facies condition.
Pseudotachylytes observed in the RLMSZ can be classified into four types (one undeformed and three deformed) based on field relationships and microstructural characteristics. Deformed pseudotachylyte veins contain porphyroclastic clasts, primarily recrystallized quartz or feldspar, set within a fine-grained matrix. This matrix is dominated by amphibole, with minor biotite, ilmenite, and traces of pyroxene, all aligned parallel to the shear plane, indicating ductile overprinting.
In mylonitic rocks associated with the deformed pseudotachylyte, short black veins–interpreted as pressure-solution seams–are observed parallel to foliation. These seams are composed mainly of ilmenite with minor biotite and are restricted to the mylonitic basement and deformed pseudotachylyte. They are absent in the undeformed pseudotachylyte and the associated undeformed host rock.
Chemical compositions and temperature estimates using two-pyroxene thermometry indicate that the pyroxene porphyroclasts in the mylonite deformed under ultra-high temperature (UHT) conditions. This implies that the RLMSZ initially developed during or shortly after peak UHT metamorphism. The microstructures of the deformed pseudotachylyte reflect an evolution from brittle failure to ductile deformation, with amphibole-rich matrices suggesting fluid infiltration following seismic slip. The amphibole alteration of pyroxene and its presence in the mylonitic matrix further supports the role of fluids in the deformation history. Moreover, the occurrence of pressure-solution seams points to an interseismic deformation mechanism involving solution transfer.
Our observations suggest a multi-stage evolution of the RLMSZ, beginning with the formation of a lower crustal shear zone during UHT metamorphism. This was followed by the generation of lower crustal earthquakes under upper amphibolite-facies conditions, which triggered fluid infiltration and the development of a ductile shear zone. Pressure-solution features indicate interseismic deformation linked to fluid presence, while the presence of multiple brittle–ductile overprints suggest repeated earthquake cycles, possibly related to shear zone reactivation or exhumation. This study highlights the processes through which UHT lower crust is deformed, emphasizing how fluid infiltration is facilitated and how deformation becomes localized, leading to zones of intense strain. We conclude that lower crustal earthquakes, coupled with fluid-assisted ductile deformation, play a critical role in the tectonic evolution of deep crustal shear zones in the Napier Complex and similar high-grade terranes.
The Riier-Larsen Main Shear Zone (RLMSZ) divides the UHT region of the Napier Complex into western and main blocks (Hokada. 1999). The RLMSZ is interpreted as a retrograde shear zone (Sheraton et al., 1987), with its activity inferred to have occurred either between 2400-700 Ma or more likely between 2500-2450 Ma ( Hokada et al., 2008). The latter age range overlaps with the timing of UHT metamorphism, suggesting a possible tectonometamorphic link. Presence of pseudotachylyte in RLMSZ is reported in previous studies (Ishikawa et al., 2000), but detailed studies about their origin are not documented yet.
Recently, pseudotachylytes generated in the lower crust have been increasingly recognized as being associated with fluid infiltration and as precursors to ductile shear zones (Menegon et al., 2017; Michalchuk et al., 2023). Lower crustal pseudotachylytes have been reported, for example, from Tonagh Island (Toyoshima et al., 1999) in the Napier Complex, yet the possibility of lower crustal earthquakes along the RLMSZ has not been thoroughly investigated.
This study aims to investigate the field relationships, microstructures, and chemical compositions of pseudotachylyte and mylonite derived from the RLMSZ, in order to evaluate the tectonic evolution of the RLMSZ and its link to UHT metamorphism.
The major lithounit in the RLMSZ is a felsic gneiss composed of quartz, feldspar, pyroxene, amphibole, and opaque minerals. This felsic gneiss has been strongly mylonitized, with pyroxene porphyroclasts showing evidence of a top-to-the-west sense of shear. These pyroxene porphyroclasts are partially altered to amphibole along cleavage planes, suggesting syn-tectonic fluid activity under upper amphibolite facies condition.
Pseudotachylytes observed in the RLMSZ can be classified into four types (one undeformed and three deformed) based on field relationships and microstructural characteristics. Deformed pseudotachylyte veins contain porphyroclastic clasts, primarily recrystallized quartz or feldspar, set within a fine-grained matrix. This matrix is dominated by amphibole, with minor biotite, ilmenite, and traces of pyroxene, all aligned parallel to the shear plane, indicating ductile overprinting.
In mylonitic rocks associated with the deformed pseudotachylyte, short black veins–interpreted as pressure-solution seams–are observed parallel to foliation. These seams are composed mainly of ilmenite with minor biotite and are restricted to the mylonitic basement and deformed pseudotachylyte. They are absent in the undeformed pseudotachylyte and the associated undeformed host rock.
Chemical compositions and temperature estimates using two-pyroxene thermometry indicate that the pyroxene porphyroclasts in the mylonite deformed under ultra-high temperature (UHT) conditions. This implies that the RLMSZ initially developed during or shortly after peak UHT metamorphism. The microstructures of the deformed pseudotachylyte reflect an evolution from brittle failure to ductile deformation, with amphibole-rich matrices suggesting fluid infiltration following seismic slip. The amphibole alteration of pyroxene and its presence in the mylonitic matrix further supports the role of fluids in the deformation history. Moreover, the occurrence of pressure-solution seams points to an interseismic deformation mechanism involving solution transfer.
Our observations suggest a multi-stage evolution of the RLMSZ, beginning with the formation of a lower crustal shear zone during UHT metamorphism. This was followed by the generation of lower crustal earthquakes under upper amphibolite-facies conditions, which triggered fluid infiltration and the development of a ductile shear zone. Pressure-solution features indicate interseismic deformation linked to fluid presence, while the presence of multiple brittle–ductile overprints suggest repeated earthquake cycles, possibly related to shear zone reactivation or exhumation. This study highlights the processes through which UHT lower crust is deformed, emphasizing how fluid infiltration is facilitated and how deformation becomes localized, leading to zones of intense strain. We conclude that lower crustal earthquakes, coupled with fluid-assisted ductile deformation, play a critical role in the tectonic evolution of deep crustal shear zones in the Napier Complex and similar high-grade terranes.