11:30 〜 11:45
[SMP26-10] Structural geology of the Kasumi and Niban Rocks: Probing the possibility of regional-scale terrane accretion in the Lützow-Holm Complex, East Antarctica
キーワード：Terrane Accretion, East Antarctica, Neoproterozoic, Collision
The Lutzow-Holm Complex (LHC) in the East Antarctica preserves a metamorphic record ranging from amphibolite- to ultrahigh-temperature facies, making it a suitable location to investigate the tectono-thermal evolution of Precambrian terranes. As an important part of the East Antarctic Shield, the LHC offers insights into crustal evolution from the Archean to the Proterozoic.
Recent studies have identified two distinct metamorphic events recorded in its rocks at ~1000Ma and ~600Ma (Dunkley et al., 2020; Baba et al., 2022; Mori et al., 2023). Understanding the field and structural relationships between terranes of different age is crucial for interpreting the asocciated tectonic processes. In this project, we compare the structural relationships observed in two eastern LHC exposures-Kasumi Rock and Niban Rock-using direct field observations, aerial photos, and microstructural analyses.
Niban Rock is a ~3sq.km outcrop located about 15km northeast of the Japanese base, Syowa Station. It's major lithologies include, biotite gneiss, sillimanite-garnet biotite gneiss, amphibolite, granite, and granitic pegmatite. Amphibolite occurs as enclaves within the gneissic units, and granite or pegmatite intrudes these units. Detailed structural investigation reveals three main deformation events in Niban Rock: an early D1 phase with NW-SE to N-S trending axial planes and parallel reverse shear zones, a D2 phase characterized by folds with E-W trending axial planes, and a D3 phase marked by NW-SE trending strike-slip faults with predominantly dextral motion.
Kasumi Rock is a ~2sq.km outcrop located about 130km northeast of the Syowa station. Its major lithologies include biotite gneiss, pink gneiss, granitic pegmatite, amphibolite, marble, skarn, and minor ultramafic rocks. A notable feature is a granitic pegmatite with large K-feldspar megacrysts that intrudes parallel to the gneissic foliation. A later generation of granitic pegmatite crosscuts all units and represents the youngest magmatic activity in the region. Unlike Niban Rock, Kasumi Rock lacks N-S trending fold axes; its early deformation is dominated by E-W recumbent fold axes, which are later refolded by upright to plunging non-coaxial folds. Most mineral lineations are parallel to these fold axes, possibly indicating that folding in Kasumi Rock represents continuous deformation.
Previous geochronological studies indicate ~1000Ma monazite ages and minor population of ~600Ma monazite ages in Niban Rock (Mori et al., 2023), while Rb-Sr isochron ages for the granitic pegmatites in Kasumi Rock are ~491Ma (Ajishi et al., 2004). This suggests that the D2 deformation likely corresponds to the ~600Ma regional event, as indicated by the active folding of granitic pegmatites in Kasumi Rock. In contrast, the D1 deformation-with its upright folds, N-S trending axial planes, and reverse shear zones-likely represents an earlier fold-and-thrust event. Similar foliation trends in the nearby Hinode Block, which shows an exclusively ~1000Ma age, support this interpretation. E-W-trending folds are common in both the Kasumi and Niban Rock areas; however, field observations indicate that folding intensity in Niban Rock is relatively limited. This suggests that the ~600 Ma deformation was concentrated, at least in exposures west of Niban Rock, possibly within a shear or fold zone. Our observations, supported by previous geological maps, indicate that the E-W-trending fold axes extend continuously from Kasumi to Akarui Point. Additionally, NW-SE-trending strike-slip faults in Niban Rock likely represent post-Pan-African faults, as also identified in geophysical data (Nogi et al., 2013).
The structural comparison of map-scale structures in these two exposures in the eastern LHC highlights structural mismatches in the region. This structural variation in neighboring terranes probably represents terrane accretion to collisional process that took place between ~1000 and 600Ma in the LHC.
Recent studies have identified two distinct metamorphic events recorded in its rocks at ~1000Ma and ~600Ma (Dunkley et al., 2020; Baba et al., 2022; Mori et al., 2023). Understanding the field and structural relationships between terranes of different age is crucial for interpreting the asocciated tectonic processes. In this project, we compare the structural relationships observed in two eastern LHC exposures-Kasumi Rock and Niban Rock-using direct field observations, aerial photos, and microstructural analyses.
Niban Rock is a ~3sq.km outcrop located about 15km northeast of the Japanese base, Syowa Station. It's major lithologies include, biotite gneiss, sillimanite-garnet biotite gneiss, amphibolite, granite, and granitic pegmatite. Amphibolite occurs as enclaves within the gneissic units, and granite or pegmatite intrudes these units. Detailed structural investigation reveals three main deformation events in Niban Rock: an early D1 phase with NW-SE to N-S trending axial planes and parallel reverse shear zones, a D2 phase characterized by folds with E-W trending axial planes, and a D3 phase marked by NW-SE trending strike-slip faults with predominantly dextral motion.
Kasumi Rock is a ~2sq.km outcrop located about 130km northeast of the Syowa station. Its major lithologies include biotite gneiss, pink gneiss, granitic pegmatite, amphibolite, marble, skarn, and minor ultramafic rocks. A notable feature is a granitic pegmatite with large K-feldspar megacrysts that intrudes parallel to the gneissic foliation. A later generation of granitic pegmatite crosscuts all units and represents the youngest magmatic activity in the region. Unlike Niban Rock, Kasumi Rock lacks N-S trending fold axes; its early deformation is dominated by E-W recumbent fold axes, which are later refolded by upright to plunging non-coaxial folds. Most mineral lineations are parallel to these fold axes, possibly indicating that folding in Kasumi Rock represents continuous deformation.
Previous geochronological studies indicate ~1000Ma monazite ages and minor population of ~600Ma monazite ages in Niban Rock (Mori et al., 2023), while Rb-Sr isochron ages for the granitic pegmatites in Kasumi Rock are ~491Ma (Ajishi et al., 2004). This suggests that the D2 deformation likely corresponds to the ~600Ma regional event, as indicated by the active folding of granitic pegmatites in Kasumi Rock. In contrast, the D1 deformation-with its upright folds, N-S trending axial planes, and reverse shear zones-likely represents an earlier fold-and-thrust event. Similar foliation trends in the nearby Hinode Block, which shows an exclusively ~1000Ma age, support this interpretation. E-W-trending folds are common in both the Kasumi and Niban Rock areas; however, field observations indicate that folding intensity in Niban Rock is relatively limited. This suggests that the ~600 Ma deformation was concentrated, at least in exposures west of Niban Rock, possibly within a shear or fold zone. Our observations, supported by previous geological maps, indicate that the E-W-trending fold axes extend continuously from Kasumi to Akarui Point. Additionally, NW-SE-trending strike-slip faults in Niban Rock likely represent post-Pan-African faults, as also identified in geophysical data (Nogi et al., 2013).
The structural comparison of map-scale structures in these two exposures in the eastern LHC highlights structural mismatches in the region. This structural variation in neighboring terranes probably represents terrane accretion to collisional process that took place between ~1000 and 600Ma in the LHC.