EXPLORATION GEOLOGY OF CLUSTERED CALDERAS IN THE HAKKODA VOLCANIC FIELD, JAPAN

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1 101 Roc. 1lth New Zealand Geothermal Workshop 1989 EXPLORATION GEOLOGY OF CLUSTERED CALDERAS IN THE HAKKODA VOLCANIC FIELD, JAPAN Hirof u mi Muraoka Geothermal Research Dept., Geological Survey of Japan, Higashi, Tsukuba, 305 Japan (Present: Dept. of Geol., Univ. of Auckland) Abstract The Hakkoda volcanic field occurs in northern Honshu and contains five calderas of which two, the Hakkoda and Towada, were recognised prior to this study. Geologic sensing, geological mapping, age dating and petrochemical studies were out on Pliocene and early Pleistocenerocks. Three new calderaswere found within a 40 km by 40 km area and that have formed during the last 3.5 Ma. The five calderas are classified into the Valles-type and Crater Lake-type; they differ according to their eruption volume, collapsestructure and evolution. The five calderas may have formed under the Chilean-type and Northeast Japan-type subduction regimes. The major hydrothermal systems in the volcanic field are controlled by these calderas. Introduction The volcanic geology of the Northeast Japan arc has been studied by two groups of geologists: volcanologists and mining geologists. The former group preferred to study the physiographically prominent volcanoes, of late Pleistocene age, while the latterpreferred the Miocene submarine volcanic rocks related to the ore deposits. As a result, Pliocene and early Pleistocene vulcanism has not been studied in detail. However, geothermal geologists (a third p u p)have studied the Pliocene and early Pleistocene vulcanism. The Northeast Japan arc contains two zones of late Quaternary volcanoes: the Nasu Volcanic Zone on the fore-arc side and Chokai Volcanic Zone on the backarc side. Clustered volcanoesoccur at a 70 or 80 spacing along the Nasu Volcanic Zone and each cluster a major geothermal area. The Hakkoda volcanic field is one of the clustered volcano areas which contains known volcanoes, such as the and Towada calderas(fig. 1). The author has studied the Hakkoda volcanic field during the last decade, in particular the older volcanic centres, in order to understand their geothermal setting (Muraoka and Hase, 1981; Muraoka et al., 1985; Muraoka and Takakura, 1988, Muraoka, Three new calderas of Pliocene and Pleistocene age have been identified in addition to the known Hakkoda and Towada calderas. The available data indicate that the activity of the Pliocene and early Pleistocene volcanoes was episodic through the late Cenozoic. The recognitionof the clustered calderas is also critically important to understand the geothermal framework. This paper summarizes the geology and vulcanism of the five calderas in the Hakkoda volcanic field, and discussesthe heat source of the geothermal resource. Geologic remote sensing Remote sensing imagery, such as the Landsat MSS, Landsat TM and airborne radar, was used initially to interpret the geology of the volcanoes. The five calderas, shown in Fig. 2a, are described in order of decreasing age, oldest first and youngest last. The Towada caldera, embracing Lake Towada, is a square-shaped depression whose width is 11 km. Its has long been known and can easily be recognized even without using remote sensing The Hakkoda caldera was also known prior to this study from its prominent northeastern wall. The southwestern caldera wall was thought to have been concealed by younger volcanic edifices of the North and South Hakkoda Volcanoes. But my interpretation of the airborne radar imagery suggests that the SouthHakkoda Volcano itself is the southern of the Hakkoda caldera. This possibility was proved during subsequent field surveys (Muraoka et al., 1983). The Hakkoda caldera is an ellipticaldepression which is 13 by 9 km. Many hot and boiling springs have been known in the Okiura area but the volcanic centres in the area have not been described. New information has recently been provided from interpretation of the Landsat satellite imagery. Hase (1978) remarked on a circular topographic feature in the area. Muraoka and Hase (1981) proposed that it is a dissected Valles-type caldera and named it the Okiura caldera. The Okiura caldera is a semicirculardepression and is 17 km in diameter. The basin topography and central in the Ikarigaseki area have also been interpreted from Landsat imagery. Topographic elementsof the Ikarigaseki basin are similar to those in the Hakkoda caldera although they are more dissected. Field surveys have demonstrated that the Ikaxigaseki basin is 2 dissected Crater Lake-type caldera. It is an ellipticaldepression whose dimensions 12 by 8 km. The Yunosawa caldera is also a circular depression, 15 km in diameter, where the caldera-forming tuffs were deposited. The caldera topography has been partly obscured on its northern side by the younger Ikarigaseki caldera since the southern dissected rim is all that can be seen on the Landsat MSS and imagery. Age and volume ob caldera-forming tuffs The distribution of the five caldera-forming tuff units is shown on Fig. 2. The extent of the tuffs decreases with age because the older tuffsare more eroded. Figure 1. Locality of the Hakkoda volcanic field.

2 102 J Figure 2.Five calderas and distribution of their caldera-formingtuff. a: Index map. b: Yunosawa caldera. c: Ikarigaseki caldera. d caldera. e: Hakkoda caldera. Towada caldera. The oldest Obirakiyama Tuff from the Yunosawa caldera does not follow this trend which suggests that the eruption volume was large. The volume of the Tuff is estimated to km3 and is probably the largest deposit of the five calderas studied. The K-Arage of the Obirakiyama Tuff is on average 3.5 Ma. The Tuff surrounds the Ikarigasekicaldera so it is likely to its tion source. The is estimated to be 35 The K-Ar age tuff is about 2.5 Ma. The intracaldera tuffs have rarely been found in Crater Lake-type calderas but they are found in the eroded caldera. The intracaldera Tuff is silicified in the central part of the caldera which demonstrates that activity has occurred. Thick deposits of the Aoni Tuff accumulated with deposits in the Okiura caldera. The extracaldera Aoni Tuff has been detected only in limited It has been difficult to distinguish the submarine Aoni Tuff from the thick coastal sandstone beds in the horizon. The K-Ar age of the Aoni Tuff is on average 1.5 Ma (Muraoka, 1986). The volume of the Aoni Tuff is estimated to be 200 km3 within the intracaldera itself and may be close to 300 if deposits outside the caldera arc included. The Hakkoda Pyroclastic Flow Deposits from the caldera the relatively fresh pyroclastic plateaus; they arc divided into two units based on hypersthene chemistry (Muraoka and Takakura, 1988). The K-Ar ages of the deposits are 0.65 Ma for the earlier unit and 0.40Ma for the later unit (Muraoka, The total volume of the both units is estimated to be 50

3 103 The Towada Pumice Flow Deposits from the Towada caldera are classified into the three major units. Their ages were determined by the method and are 55 Ka, 25 Ka and 13 Ka (Hayakawa, 1985). The total volume of the three units is estimated to be Structure and type of calderas Large-scalecollapse calderas that produce voluminous ash flow tuffs are divided into two categories: the Valles-type calderas (Smith and Bailey, 1968) and Crater Lake-type calderas (Williams, 1941). Although a model of the caldera type is currently preferred by some workers (Bacon, 1984; Lipman et al., a comparison of the collapse structure of the five calderas in the volcanic field with the types already described suggests that there arc two types of caldera. Figure 3. Bouguer anomaly map of the Aomori district (Hiroshimaet al., 1989).Density assumption is 2.3. Fig. 3. shows Bouguer anomaly map assuming the rock density is 2.3 (Hiroshima et (1989). Each of the five calderas coincide with low gravity anomalies. The classified into two categories: one is piston-shaped, such as the Yunosawa and Okiura calderas, and the other is funnel-shaped, the Ikarigaseki, Hakkoda and Towada calderas. The is surrounded by steep topography which correspond to a ring fault system. The ring faults can be observedon the Yunosawa and Okiura calderas. They play a key role not only in the (Fig. 4a) but also in the discharge of fluids. These calderas can be called the Figure 4. Collapse models of the Valles-type (upper side) and Crater

4 104 Muraoka Ab Ab Mole per cent Or Figure 5. plot of the five caldera-forming tuffs. The latter caldera type does not have a ring fault system; its chaotically caldera is likely tohave subsided piecemeal (Fig. 4b). This idea is compatible with the fact that the calderaforming tuffs from the Hakkoda and Towada calderas contain more lithic fragments than those from the Yunosawa and Okiura calderas. The post-caldera cones and the geothermal manifestations tend to be concentrated in the part of these calderas. They can be called the Crater calderas. volumes of the tuffs am different in each of the two types of calderas. The tuffs of the Valles-type calderas almost ten times larger than those of the Crater Lake-type calderas in this area. Petrochemical trends A number of major element analyses have been performed on the volcanic rocks from the five calderas (Muraoka, and (1986). The results are summarized The eruption units of the Valles-type such as the Yunosawa and Okiura calderas, consist of the tuffs and postcaldera lavas or dikes. They are mostly rhyolites of the calc-alkalic rock (Miyashiro, 1974); basaltic magmas also erupted simultaneously with the and post-calderadeposits of the Okiura caldera, and mixed with the rhyolite magma (Muraoka,1985). This mixing produced dacite magmas. Figure 6. Distribution of hot springs in the volcanic field.

5 105 Throughout the Ikarigaseki, Hakkoda and Towada Crater Lake-type calderas, a general magma cycle was detected. of these calderas was preceded by the formation of composite tholeiitic andesite volcanoes. Detailed mapping indicates that the precaldera volcanoes consist of scattered cones and domes which extend over about times the radius of the host caldera. The caldera-forming tuffs and post-caldera eruptives are mostly calc-alkalinedacites. The and post-caldera calc-alkaliceruptives areprobably derived from the caldera magma chamber. However, the precaldera tholeiitic eruptives may not be directly connected with the caldera magma chamber but may be enerated at the deeper sourceregions. In other words, large-scale magmatism is a prerequisite for the generation of the Crater Lake-typecaldera magmas A comparison of the Valles-type tuffs with the Lake-type tuffs (Fig. 5) demonstratesthat the former group is related to the Andean calc-alkalineseries and the latter to the island arc calc-alkaline series (Jake's and White, 1972). Although the tuffs of the Okiura caldera lie between these two types, the silicic end members are less affected by magma mixing and close to the Andean calc-alkaline series. To conclude, there is a petrochemical difference between the Valles-type and Crater Lake-typecaldera Discussion Following an interpretationof the geologic remote sensing, geological mapping, gravimetric, K-Ar age and major element petrochemical data, three new Pliocene and late Pleistocene calderas have been identified in the Hakkoda volcanic field. The Hakkuda and Towada calderas were already known. These calderas within a km by area and have formed during the last 3.5 Ma. The calderas tend to change from the to the Crater Lake-type with increasing age. This trend probably reflects a change in the magma composition from the Andean calc-alkalineseries to the island series. Such a petrochemicalchange with is also recognized in other areas of the Northeast Japan arc and may have been caused by the temporal change in subduction tectonics et 1988). The Northeast Japan arc has been subject to tectonics and by the subduction since the Miocene. Early Andean calc-alkalinemagmatism may have occurred in to the Chilean-type low angle The later change of magma composition may reflect the gradual change of the subduction Fromthe Chilean-type to the Northeast Japan-type. From the view point of geothermal potential, the older calderas are not necessarily a good prospect for power generation Smith and Shaw, 1975). However, the clustered calderas are linked to the major hydrothennal systems in the Hakkoda volcanic field. Fig. 6 shows the distribution of the hot springs in Hakkoda volcanic field. Most of high temperature hot springs are concentrated in the clustered calderas. More detailed geothermal aspects will be presented by Muraoka (1989, this volume). Acknowledgements I thank Drs Hirokazu Hase, Kenzo Baba Yasushi Yamaguchi, Yasukuni Okubo, Tamanyu and Katsuro Ogawa, Geological Survey of Japan, for their help in the course of this study. References Bacon, C.R. (1983): Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, U.S.A. J. Volcanol. Geotherm. Res. 18, Hase, H. (1978): Geothermal exploration and remote sensing. Geothermal (Chinetsu) Energy (in Japanese). Hayakawa, 985): geology of Towada Volcano. Bull. Earthquake Res. Inst. Univ. Tokyo Muraoka Hiroshima, T., Komazawa, M. and Nakatsuka T. (1989): Gravity map of Aomori district (Bouguer anomalies), scale Gravity Map Series Geol. Surv. Japan. Jakes, P. and White, A.J.R. (1972): Major and trace element abundances in volcanic rocks of orogenic areas. Geol. Am. Bull. 83, Lipman, P.W., Self, S. and Heiken, G. (1984): Introduction to caldera special issue. J. Geophys. Res. A. (1974): Volcanic rock series in island arcs and active continental margins. Am. Jour. Muraoka, H. (1985): Coexistence of bimodal magmas inferred from volcanic ejecta in the Okiura caldera. Memorial Issue of Yoshida Retirement, Hiroshima University, (in Japanese with English abstract). Muraoka, H. (1986): Geochronology of the Okiura caldera, Northeast Japan. Bull. Geol. Surv.Japan 37, (in Japanese with English abstract). Muraoka, H. Geologic and magmatic evolution of clustered calderas in the Hakkoda regional volcanic field, Japan. Unpublished thesis Thesis, Hiroshima University, Muraoka, H. (1 Exploration geology of hydrothermal systems in the Hakkoda volcanic field, Japan. 11th N.Z. Geothermal Workshop (this volume). Muraoka, H.and Hase, H. (1981): Okiura caldera, discovery of a Valles-type caldera in the Honshu, Japan. In abstract 1981 IAVCEI Symposium, Tokyo, Muraoka, H., M. and Takagi, 5. (1985): Nationwide geothermal exploration survey project (2nd step) with special reference to the Hakkoda area, Japan. InternationalVol., Geothermal Resources Council, 48 Muraoka, H. and Takakura, 5. (1988): Explanato text of the geological of Hakkoda Geothermal Area, Miscellaneous Map Series Gcol. Japan, (in Japanese with English abstract). Muraoka, H.,Yamaguchi, Y. and Nakazawa, 5. (1983): Mutual relationship between the southern Hakkoda Volcano and Hakkoda Caldera, Jour. Geol. Japan (in Japanese). Muraoka, H., Yamaguchi, Y. and Sakaguchi, (1988): Central Andean-type vulcanism in the late Cenozoic Northeast Japan arc. de la Argentina, (in press). (New Energy Development Organization) (1986): Report on the distribution and geochronology of volcanic fiscal year 1985 of Nationwide Geothermal Exploration Survey Project (2nd step) district". (in Japanese). Smith, R.L. and Bailey R.A. (1968): Resurgent cauldrons, in Coats, and others, Studies in volcanology: Geol. Amer. Memoir 1 Smith, R.L. and Shaw, H.R.(1975): geothermal systems. in White, D.E.and Williams, D.L., eds., Assessment of geothermal resources of the United States U.S. Geol. Surv. Circular Williams, H. (1941): Calderas and their origin. Univ. California Publ., Geol. 25,

6 Photos: WORKS; September, 1989 E.