Based on the 3-D orientations of intracrystalline healed, sealed and open extension microcracks in quartz grains in the Late Cretaceous Toki Granite, we discuss the paleostress field and the history of microcracking combining the microthermometry of fluid inclusions in healed microcracks and sealing material identification in sealed microcracks. Twenty one oriented samples are collected from the DH-15 core (240-1000 mabh) drilled by Japan Atomic Energy Agency (JAEA) and additionally five oriented samples from outcrops in the Tono region. 3-D orientations of healed microcracks indicate the σHmax orientation of N-S to NW-SE in almost all sites, whereas those of sealed and open microcracks indicate the dominant σHmax orientation of E-W. Two or three orthogonal sets of microcracks are common in both healed and sealed (+open) microcracks. The formation of these sets can be attributed to the switch of principal stress axis due to stress release just after the crack formation. Healed microcracks probably reflect more regional paleostress field because of consistency of the orientations than the case of sealed and open microcracks that show rather scattering orientations. N-S to NW-SE trending healed microcracks are formed around 60 Ma on the basis of K-Ar biotite ages of the Toki Granite and formation temperature (c. 300-400°C) of fluid inclusions estimated from microthermometry in the case of intrusion depth (3.5 km=100 MPa) of the Toki Granite. Thus the σHmax orientation trended NW-SE after the restoration of clockwise rotation of SW Japan at c. 15 Ma. Following the healed microcrack formation, E-W trending high-angle sealed microcracks filled with carbonate mineral are formed. Open microcracks presumably formed at near-surface at the last stage of sealed microcrack formation after c. 20 Ma when the Mizunami Group deposited unconformably on the granite.
We propose a new 40 thousand year-old eruption history of Hokkaido-Komagatake volcano, northern Japan, from new field observations, radiocarbon dating of charcoal woods and whole-rock chemistry of juvenile materials. In previous studies, eight distinct plinian eruption units have been recognized in the last 40 thousand years (Ko-a, 1929 AD; Ko-c1, 1856 AD; Ko-c2, 1694 AD; Ko-d, 1640 AD; Ko-f, 6.3 cal ka; Ko-g, 6.8 cal ka; Ko-h, 20 cal ka; Ko-i, 39 cal ka). Our study has revealed seven new eruption units between Ko-d and Ko-i in the northern flank area of the volcano. Eruption ages of the new units are estimated to be: P1 (6.5−6.3 cal ka); P2 (6.5−6.3 cal ka); P3 (ca. 12.8 cal ka); P4 (ca. 14.8 cal ka); P5 (ca. 17.4 cal ka); P6 (ca. 17.7 cal ka); and P7 (ca. 19 cal ka). The eruption history can be divided into four stages, separated by long periods of dormancy: pre-39 cal ka, 20 cal ka to 12.8 cal ka, 6.8 cal ka to 6.3 cal ka, and post-1640 AD. Each of the four stages is characterized by distinct whole-rock chemistry. The last three stages are characterized by an initial large eruption and subsequent medium to smaller scale eruptions. Our new findings could provide important background information and context for understanding the present state of Hokkaido-Komagatake volcano.
We identified four pyroclastic flow deposits in central Hokkaido as belonging to the same flow deposit which erupted from the Tokachimitsumata basin with a circular topographic moat (10×14 km). A dense network survey of gravimetric data revealed a negative depression profile of a Bouguer anomaly in the basin. The surface elevation and thickness of the Muka pyroclastic flow deposit adjacent to the basin gradually increase toward the basin moat. These findings suggest that the Muka deposit was derived from the basin, even though the volume of this deposit is substantially less than that estimated for the caldera. Field surveys, petrographic analyses, and K-Ar dating were also conducted on the other three deposits (i.e. the Meto tuff bed, Kuttari pyroclastic flow deposit, and Biotite dacite tuff-breccia), which had similar petrographic characteristics to those of the Muka deposit. The strong correlation between the four deposits can be inferred by the following: the K-Ar ages of feldspar minerals from pumices of the four deposits are identical (1 Ma), the pumices of the four deposits exhibit very similar mineral assemblages and volcanic-glass and mineral chemistry, and, like the Muka deposit, the surface elevation, thickness and welding degree of the three deposits appear to increase toward the basin. The Muka, Meto, and Kuttari deposits began forming a pumice fall deposit, indicating a plinian phase for the first eruption. It can thus be inferred that these four deposits have arisen from a large-scale eruption that occurred 1 Ma, which formed the caldera. The total volume of ejecta was approximately >130 km3. We have named the fall deposit, flow deposit, and basin the Tokachimitsumata pyroclastic fall deposit (Tkm-pfa), Tokachimitsumata pyroclastic flow deposit (Tkm-pfl), and Tokachimitsumata caldera, respectively.
Six distal rhyolitic ash layers (HR-1∼HR-6 in descending order) are interbedded in the pile of large-scale pyroclastic flow and pumice fall deposits derived from Akan volcano, eastern Hokkaido, during Pleistocene. These layers characteristically contain hydrous minerals such as hornblende and biotite, which are not common in the rocks of Quaternary volcanoes in eastern Hokkaido. Based on mineral assemblage, glass chemistry, and stratigraphy, these can be correlated with the early to middle Pleistocene large-scale pyroclastic flow deposits (pfld) distributed in central Hokkaido. The HR-5 and -6 layers have been already correlated with the Tokachi pfld (1.3∼1.46 Ma) in the central Hokkaido. In addition, we have newly correlated these ash layers to pfld in the central Hokkaido, as follows: HR-4 to the Tokachimitsumata pfld (1 Ma), HR-2 to the Kamishikaribetsu pfld and, HR-1 to the Kamiasahigaoka pfld, respectively. K-Ar ages for plagioclase in pumice of Kamiasahigaoka pfld is determined to be 0.51±0.14 Ma. These dates suggest that the large-scale felsic explosive volcanism had continued in both of central and eastern Hokkaido for at least eight hundred thousand years long during the early to middle Pleistocene. Especially, there is no evidence for dormancy, such as paleosol and erosional gap, between Ak14 and intercalating HR-5, indicating that large explosive eruptions simultaneously occurred in central and eastern Hokkaido. HR-1∼HR-6 could be Quaternary good time markers not only in eastern Hokkaido but also in Pacific Ocean and Kurile Islands because these tephras are more than 50 cm in thickness around Akan volcano.