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[HQR04-01] Post-caldera Landform Development on Akaigawa Caldera, Southwestern Hokkaido, Japan.
Keywords:Akaigawa Caldera, Akaigawa Fault, Fluvial terrace, Lacustrine terrace, Tephra
1. Introduction
The Akaigawa Caldera, which is located on southwestern Hokkaido, had collapsed in 1.6 Ma [1]. The Akaigawa fault runs from the western to northern part of the caldera, and displaces the Late Pleistocene terraces [2, 3]. However, the basis of ages of terraces and tectonic geomorphological features of the fault are unclear. In this research, we discuss the post-caldera landform development of the Akaigawa Caldera through clarifying these points.
2. Methods
Terraces and faults were mapped by using aerial photographs. Terrace deposits and cover deposits were observed on the field, and fine deposits were sampled to detect volcanic glass sherds. They were correlated to known tephras through measuring the major-element compositions by SEM-EDS.
3. Results and discussions
3.1 Classification and chronology of terrace surfaces
Terrace surfaces in the Akaigawa Caldera and along the Yoichi River were classified as below. Distributions of terraces and outcrops are shown in Fig. 1.
i) Lacustrine terraces
Lacustrine terraces were divided into the lacustrine terrace I and II (LTI and LTII respectively) according to their heights from the present caldera floor. At Loc. 1, LTII consists of >9 m thick alternating layers of silt, sand and gravel.
ii) Fluvial terraces
In the caldera, a terrace surface which buries LTII in the upper reach and incises it in the lower is developed along tributaries. Considering the relative height, degree of dissection and slope, the terrace correlates to the fluvial terrace II (FTII, described below) which develops along the Yoichi River.
Fluvial terraces along the Yoichi River and its tributaries were classified as the fluvial terrace I (FTI), fluvial terrace II (FTII) and lower terraces. At Loc. 2, a pyroclastic flow deposit crops out, and is correlated to the Toya tephra (106 ka) [4] according to its chemical composition. A terrace deposit of FTII at Loc. 3 contains volcanic glass shards of the Spfa-1 tephra (46 ka) [4]. Hence, ages of FTI and FTII are >106 ka and <46 ka respectively.
iii) Debris flow fan
Debris flow fan which is steeper than other terrace surfaces develops along the foot of the somma. Boundaries between the fan and LTII or FTII are concave break lines or scarps that have been considered as the Akaigawa fault [2, 3].
3.2 Tectonic geomorphological features of the Akaigawa fault
The Akaigawa fault consists of two lines of arcuate fault which runs from the western to northern part of the caldera in parallel. While the inner fault deforms LTII, the outer one deforms FTII or limits the boundary between FTII and the debris flow fan. On both faults, the inner side of the caldera drops down relative to the other.
Characteristics of the Akaigawa fault are similar to caldera boundary faults that develop during caldera collapse. Therefore, the Akaigawa fault is thought to be formed during collapse of the Akaigawa Caldera in 1.6 Ma. The last activity age of the fault, however, is younger than 46 ka because it deforms LTII. For the reason, the Akaigawa fault is thought to have reactivated during the post-caldera period. NW-SE trending regional stress [5] and the post-caldera volcanism can be pointed out as causes of this reactivation.
4. Conclusions
The Akaigawa fault which runs in the Akaigawa Caldera is considered to be a caldera boundary fault developed during collapse of the caldera in 1.6 Ma. On the other hand, since the fluvial terrace whose age is younger than 46 ka is displaced by the Akaigawa fault, its last activity age is younger than 46 ka too. This indicates that the fault has reactivated during the post-caldera period.
References
[1] Yokoyama et al. (2003) Jour. Mineral. Petro. Sci., 32.
[2] The Research Group for Active Faults of Japan (1991) Active Faults in Japan, Univ. of Tokyo Press
[3] HEPCO (2015) Tomari Plant Ground (Geology and Geological Structure around the Site)
[4] Nakamura et al. (2013) Jour. Japanese Associ. Petro. Tech., 78.
[5] Watanabe (1993) Jour. Geo. Soc. Japan, 99.
The Akaigawa Caldera, which is located on southwestern Hokkaido, had collapsed in 1.6 Ma [1]. The Akaigawa fault runs from the western to northern part of the caldera, and displaces the Late Pleistocene terraces [2, 3]. However, the basis of ages of terraces and tectonic geomorphological features of the fault are unclear. In this research, we discuss the post-caldera landform development of the Akaigawa Caldera through clarifying these points.
2. Methods
Terraces and faults were mapped by using aerial photographs. Terrace deposits and cover deposits were observed on the field, and fine deposits were sampled to detect volcanic glass sherds. They were correlated to known tephras through measuring the major-element compositions by SEM-EDS.
3. Results and discussions
3.1 Classification and chronology of terrace surfaces
Terrace surfaces in the Akaigawa Caldera and along the Yoichi River were classified as below. Distributions of terraces and outcrops are shown in Fig. 1.
i) Lacustrine terraces
Lacustrine terraces were divided into the lacustrine terrace I and II (LTI and LTII respectively) according to their heights from the present caldera floor. At Loc. 1, LTII consists of >9 m thick alternating layers of silt, sand and gravel.
ii) Fluvial terraces
In the caldera, a terrace surface which buries LTII in the upper reach and incises it in the lower is developed along tributaries. Considering the relative height, degree of dissection and slope, the terrace correlates to the fluvial terrace II (FTII, described below) which develops along the Yoichi River.
Fluvial terraces along the Yoichi River and its tributaries were classified as the fluvial terrace I (FTI), fluvial terrace II (FTII) and lower terraces. At Loc. 2, a pyroclastic flow deposit crops out, and is correlated to the Toya tephra (106 ka) [4] according to its chemical composition. A terrace deposit of FTII at Loc. 3 contains volcanic glass shards of the Spfa-1 tephra (46 ka) [4]. Hence, ages of FTI and FTII are >106 ka and <46 ka respectively.
iii) Debris flow fan
Debris flow fan which is steeper than other terrace surfaces develops along the foot of the somma. Boundaries between the fan and LTII or FTII are concave break lines or scarps that have been considered as the Akaigawa fault [2, 3].
3.2 Tectonic geomorphological features of the Akaigawa fault
The Akaigawa fault consists of two lines of arcuate fault which runs from the western to northern part of the caldera in parallel. While the inner fault deforms LTII, the outer one deforms FTII or limits the boundary between FTII and the debris flow fan. On both faults, the inner side of the caldera drops down relative to the other.
Characteristics of the Akaigawa fault are similar to caldera boundary faults that develop during caldera collapse. Therefore, the Akaigawa fault is thought to be formed during collapse of the Akaigawa Caldera in 1.6 Ma. The last activity age of the fault, however, is younger than 46 ka because it deforms LTII. For the reason, the Akaigawa fault is thought to have reactivated during the post-caldera period. NW-SE trending regional stress [5] and the post-caldera volcanism can be pointed out as causes of this reactivation.
4. Conclusions
The Akaigawa fault which runs in the Akaigawa Caldera is considered to be a caldera boundary fault developed during collapse of the caldera in 1.6 Ma. On the other hand, since the fluvial terrace whose age is younger than 46 ka is displaced by the Akaigawa fault, its last activity age is younger than 46 ka too. This indicates that the fault has reactivated during the post-caldera period.
References
[1] Yokoyama et al. (2003) Jour. Mineral. Petro. Sci., 32.
[2] The Research Group for Active Faults of Japan (1991) Active Faults in Japan, Univ. of Tokyo Press
[3] HEPCO (2015) Tomari Plant Ground (Geology and Geological Structure around the Site)
[4] Nakamura et al. (2013) Jour. Japanese Associ. Petro. Tech., 78.
[5] Watanabe (1993) Jour. Geo. Soc. Japan, 99.