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[SVC30-P07] Eruption history of Azuma-Jododaira Volcano based on the paleomagnetic directions and secular variation curve
Keywords:Azuma Volcano, Jododaira Volcano, Paleomagnetic direction, Pyroclastic cone, Lava flow
Introduction
A large depression called “Jododaira” is situated at the eastern part of the Azuma Volcano Group in Fukushima Prefecture, Eastern Japan. Several craters and lava flows are distributed inside the Jododaira, comprising “Azuma-Jododaira volcano” [1]. The activity of Azuma-Jododaira volcano began with the formation of Okenuma crater around 6.7 ka and has continued to the Present. Azuma-kofuji is one of the main craters of the volcano that formed during 5.9–4.8 ka [1]. Products from Azuma-kofuji are composed of pyroclastic cone, distal tephra and lava flows, all widely distributed in Jododaira [1]. Although the effusive center of the lava flows was initially thought to be Azuma-kofuji [1], the exact source remains unclear due to coverage by younger products. The age and stratigraphic relationships of the lava flows to Azuma-kofuji pyroclastic cone (AKPC) are also still unknown. In this study, we conducted paleomagnetic analysis to quantitatively evaluate the temporal relationship between AKPC and the lava flows.
Geological outline
AKPC consists of four units (C1–C4 from bottom to top), such as surge deposits (C1, C4), Strombolian eruption deposits (C2) and Vulcanian eruption deposits (C3) [2]. Three lobes of “rootless lava flow” are piled on the eastern slope of the pyroclastic cone. The main lava flows in Jododaira flowed down to the east for more than 5 km and are widely distributed to the east of AKPC. The C3 covers these lava flows suggesting that the lava flows effused prior to the formation of AKPC [3]. Block and ash flow deposits (BAF) are recognized at the edge of one of the lava flows [4].
Samples and analysis
We conducted oriented sampling from the agglutinate of C2 and the pyroclastic breccia of C4 that form AKPC (3 sites from C2 and C4: C2L, C2U, C4), the rootless lavas (2 sites: RL-01, 02), the lava flows (3 sites: L-01–03) and the BAF (1 site: Ak-BAF). This was done by using the electric drill and block sampling method. We performed stepwise thermal demagnetization to obtain characteristic remanent magnetization (ChRM) from each sample. The site-mean directions were calculated from the ChRM directions at each site (number of samples at one site = 6–11).
Results and discussion
We obtained coherent ChRM directions for each site with small 95% confidence limit (α95≦5.6°). Stereographic projections of these site-mean directions are separated into two main areas. C2L, C2U, C4, RL-01, and RL-02 from AKPC show an eastern declination (all-site mean of Cone group: Dec=9.7°, Inc=50.0°, α95=4.7°), while L-01–03 and Ak-BAF show a western declination (Lava group: Dec=341.3°, Inc=59.5°, α95=3.4°) (Fig.1). The clear difference of palaeomagnetic directions between Cone and Lava groups suggests that there was a significant time gap between the formation of AKPC and lava flows.
Paleomagnetic dating was performed by comparing our data with recorded Holocene paleosecular variation curve (PSVC) at Lake Biwa [5]. For this correlation, a dipole magnetic field was assumed, and our direction obtained in Jododaira was converted to that in Lake Biwa. The directions of Cone group overlap with the PSVC at the age range of 4.8–5.9 ka, showing good agreement with reported age of Azuma-kofuji (Fig.2). The changing rate of the PSVC during this period is too small for higher-resolution dating. The direction of Lava group matches with the PSVC in the three age bands of 6.8–7.2 ka, 7.4 ka, and 9.5–9.7 ka. All the three age ranges are remarkably (> ca. 1 ky) older than the well-determined age of Cone group, that is consistent with the geologically established relationships [3]. The age range of 6.8–7.2 ka is close to the age of Okenuma crater, suggesting that some of the lava flows may have originated from Okenuma crater.
Looking more closely at the Cone group, C2U and C4 show distinct mean directions (angular distance=11.3°) without overlapping of the α95 circles. Assuming a changing rate of 0.06°/yr [6], the time gap between the two units can be estimated as 190 years. Considering the smaller change rate during 4.8–5.9 ka in the PSVC of Lake Biwa, the time gap is more than 190 years. In conclusion, AKPC started the activity after a long time gap (> 1k yrs) from the lava effusions in Jododaira, and formed by multiple eruptions over a long period (> 200 yrs.).
References
[1] Yamamoto (2005) Jour. Geol. Soc. Japan; [2] Suzuki et al. (2022) Meet. Volcanol. Soc. Japan, Abrtr.; [3] Suzuki et al. (2022) Meet. Geol. Soc. Japan, Abstr.; [4] Hasegawa and Suzuki (2021) JpGU; [5] Ali et al. (1999) Geophys. J. Int.; [6] Hasegawa et al. (2018) Jour. Tokyo Geogr. Soc.
A large depression called “Jododaira” is situated at the eastern part of the Azuma Volcano Group in Fukushima Prefecture, Eastern Japan. Several craters and lava flows are distributed inside the Jododaira, comprising “Azuma-Jododaira volcano” [1]. The activity of Azuma-Jododaira volcano began with the formation of Okenuma crater around 6.7 ka and has continued to the Present. Azuma-kofuji is one of the main craters of the volcano that formed during 5.9–4.8 ka [1]. Products from Azuma-kofuji are composed of pyroclastic cone, distal tephra and lava flows, all widely distributed in Jododaira [1]. Although the effusive center of the lava flows was initially thought to be Azuma-kofuji [1], the exact source remains unclear due to coverage by younger products. The age and stratigraphic relationships of the lava flows to Azuma-kofuji pyroclastic cone (AKPC) are also still unknown. In this study, we conducted paleomagnetic analysis to quantitatively evaluate the temporal relationship between AKPC and the lava flows.
Geological outline
AKPC consists of four units (C1–C4 from bottom to top), such as surge deposits (C1, C4), Strombolian eruption deposits (C2) and Vulcanian eruption deposits (C3) [2]. Three lobes of “rootless lava flow” are piled on the eastern slope of the pyroclastic cone. The main lava flows in Jododaira flowed down to the east for more than 5 km and are widely distributed to the east of AKPC. The C3 covers these lava flows suggesting that the lava flows effused prior to the formation of AKPC [3]. Block and ash flow deposits (BAF) are recognized at the edge of one of the lava flows [4].
Samples and analysis
We conducted oriented sampling from the agglutinate of C2 and the pyroclastic breccia of C4 that form AKPC (3 sites from C2 and C4: C2L, C2U, C4), the rootless lavas (2 sites: RL-01, 02), the lava flows (3 sites: L-01–03) and the BAF (1 site: Ak-BAF). This was done by using the electric drill and block sampling method. We performed stepwise thermal demagnetization to obtain characteristic remanent magnetization (ChRM) from each sample. The site-mean directions were calculated from the ChRM directions at each site (number of samples at one site = 6–11).
Results and discussion
We obtained coherent ChRM directions for each site with small 95% confidence limit (α95≦5.6°). Stereographic projections of these site-mean directions are separated into two main areas. C2L, C2U, C4, RL-01, and RL-02 from AKPC show an eastern declination (all-site mean of Cone group: Dec=9.7°, Inc=50.0°, α95=4.7°), while L-01–03 and Ak-BAF show a western declination (Lava group: Dec=341.3°, Inc=59.5°, α95=3.4°) (Fig.1). The clear difference of palaeomagnetic directions between Cone and Lava groups suggests that there was a significant time gap between the formation of AKPC and lava flows.
Paleomagnetic dating was performed by comparing our data with recorded Holocene paleosecular variation curve (PSVC) at Lake Biwa [5]. For this correlation, a dipole magnetic field was assumed, and our direction obtained in Jododaira was converted to that in Lake Biwa. The directions of Cone group overlap with the PSVC at the age range of 4.8–5.9 ka, showing good agreement with reported age of Azuma-kofuji (Fig.2). The changing rate of the PSVC during this period is too small for higher-resolution dating. The direction of Lava group matches with the PSVC in the three age bands of 6.8–7.2 ka, 7.4 ka, and 9.5–9.7 ka. All the three age ranges are remarkably (> ca. 1 ky) older than the well-determined age of Cone group, that is consistent with the geologically established relationships [3]. The age range of 6.8–7.2 ka is close to the age of Okenuma crater, suggesting that some of the lava flows may have originated from Okenuma crater.
Looking more closely at the Cone group, C2U and C4 show distinct mean directions (angular distance=11.3°) without overlapping of the α95 circles. Assuming a changing rate of 0.06°/yr [6], the time gap between the two units can be estimated as 190 years. Considering the smaller change rate during 4.8–5.9 ka in the PSVC of Lake Biwa, the time gap is more than 190 years. In conclusion, AKPC started the activity after a long time gap (> 1k yrs) from the lava effusions in Jododaira, and formed by multiple eruptions over a long period (> 200 yrs.).
References
[1] Yamamoto (2005) Jour. Geol. Soc. Japan; [2] Suzuki et al. (2022) Meet. Volcanol. Soc. Japan, Abrtr.; [3] Suzuki et al. (2022) Meet. Geol. Soc. Japan, Abstr.; [4] Hasegawa and Suzuki (2021) JpGU; [5] Ali et al. (1999) Geophys. J. Int.; [6] Hasegawa et al. (2018) Jour. Tokyo Geogr. Soc.