Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan

Bull Volcanol (1989) 51:41-50 Volc~ä'n°ölogy © Springer-Verlag 1989

Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan

Hiroki Kamata*

Geological Survey of Japan, Geothermal Research Department, Higashi 1-1-3, Tsukuba, Ibaraki 305, Japan

Abstract. Drill-hole, geochronologic, and gravity data identify the buried Shishimuta caldera beneath post-caldera lava domes and lacustrine deposits in the center of the Hohi volcanic zone. The caldera is the source of the Yabakei pyroclastic flow, which erupted 1.0 Ma ago with a bulk volume of 110 k m 3. The caldera is a breccia-filled funnel- shaped depression 8 km wide and > 3 km deep with a V-shaped negative Bouguer gravity anomaly up to 36 mgal. Neither ring vents nor resurgence was recognized; instead, post-caldera monogenetic volcanism in an extensional setting dom- inated the area. The andesitic breccia has a relatively low density and fills the caldera; it possibly formed by fragmentation of disrupted roof rock during the violent Yabakei eruption and related collapse. Fewer normal faults and shallow microearthquakes occur inside the caldera than around it, possibly because rocks beneath the caldera are structurally incoherent. A profile of Shishimuta caldera may be more elongated vertically, and have a more intensely fractured zone, than that of a Valles-type caldera.

Introduction

The Hohi volcanic zone (HVZ) in central Kyushu, southwest Japan, is a volcano-tectonic depression 70 km long and 45 km wide, elongated in an E-W direction, in which Plio-Pleistocene volcanic rocks are widely distributed (Kamata 1989). These volcanic rocks are composed primarily of ande-

* Presen t address : Cascades Volcano Observatory, US Geolog- ical Survey, 5400 MacArthur Blvd., Vancouver, Washington 98661, USA

sitic lava flows and dacitic pyroclastic-flow deposits, with a small volume of volcaniclastic sediment. The HVZ gives clear evidence that volcanic activity and subsidence took place alternately under a regional extensional stress field from about 5 Ma to the present, so that the depression was mostly compensated by filling of an equivalent volume of volcanic material (Kamata 1989).

Several large-scale, middle and late Pleisto- cene pyroclastic-flow deposits occur in the HVZ (Kamata 1987). Some clearly result from caldera formation (e.g. the pyroclastic-flow deposits of Aso caldera), but others lack obvious caldera sources and are interpreted to have issued from calderas now buried by products of later eruptions.

The Yabakei pyroclastic flow is widely distributed in the center of the HVZ (Fig. 1), where it forms a thick plateau of densely welded tuff of more than 110 k m 3 volume. No related caldera is recognized at the surface. I investigated the buried distribution of the Yabakei and recognized its source caldera using geology, drill-hole information, and geochronologic and gravity data. The subsurface structure of the buried caldera is defined from gravity, microearthquake and drill- hole data. The evidence suggests a funnel-shaped caldera, which is common in the Japanese island arc and is structurally different from the Valles- type caldera (Aramaki 1984).

Geology and age of the Yabakei pyroclastic flow

The Yabakei pyroclastic flow is a pumice-flow deposit of hornblende dacite (Matumoto 1933). It has a present outcrop area of about 480 km 2, a maximum thickness of about 150 m, and an aver- age thickness of 90 m. Its maximum volume, cal-

42 Kamata: Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohl volcanic zone, Japan

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Fig. 1. Distribution and elevation of the top of the Yabakei pyroclastic flow. Stip- pled area, surface distribution of Yabakei pyroclastic flow. Solid contours show elevation in meters of top of flow. Distribu- tion compiled from Matumoto (1933), Re- search Group for Geological Map of Hohl Geothermal Area (1982) and Kamata (1985a). Cirded dots, drill holes where Ya- bakei is present; numbers show elevation in meters of top of flow. Open circles, drill holes where Yabakei is absent. Halfclosed eiteles, drill holes where volcanic deposits older than 1.0 Ma are recognized at relatively high elevation shown by figure in parenthesis, as diseussed in text. Dashed contours, Bouguer anomaly of Komazawa and Kamata (1985) in milligals, y, z, locations of 3000-m-deep drill holes with spot coring. Inset, location of studied area. Ss, Shishimuta; Mi, Miyanoharu; Ha, Ha- neyama, Bin, Bungomori; He, Heikeya- ma; Ka, Karutoyama; Yb, Yabakei; M, Makinoharu

culated from surface exposures only, is 40 km 3 (Kamata 1989).

The Yabakei pyroclastic flow is a gray, densely welded tuff containing black, glassy, es- sential lenses. It generally consists of a single cooling unit. Devitrification is common and re- sults in compact, hard welded tuff. Where it is en- tirely devitrified, once-glassy lenses are difficult to recognize. The upper part of the deposit is lo- cally non-welded and commonly contains a va- por-phase zone.

Phenocrysts of the Yabakei pyroclastic flow are plagioclase, hornblende, hypersthene and augite with microphenocrysts of zircon, apatite and opaque minerals. Vitroclastic texture character- izes the groundmass. Glassy lenses and non- welded pumice fragments generally contain hornblende and platy plagioclase 2-3 mm long. These

features easily distinguish the Yabakei from the nearby Aso-4 pyroclastic flow, which has a similar mineral assemblage but contains needle- shaped hornblende and non-platy plagioclase. Natural remanent magnetization (NRM) of the Yabakei pyroclastic flow shows normal polarity (T. Soya personal communication 1984; Kamata 1985a; Suto 1985).

Uto and Suto (1985) determined the K-Ar age of the Yabakei pyroclastic flow as 0.99 ___ 0.03 Ma after removing accidental lithic fragments from densely welded specimens. The Yabakei underlies the Hanabira welded tuff (0.76+0.17 Ma; Uto and Suto 1985) and the Itsumaichi rhyolite lava flow (0.97+0.05 Ma; Kamata 1987). Both the Ha- nabira and the Itsumaichi have reversed magnetic polarity, so they probably erupted during late Ma- tuyama time. If so, then the Yabakei pyroclastic

Kamata: Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan 43

flow was probably erupted during the Jaramillo event (0.90-0.97 Ma; Mankinen and Dalrymple 1979) (Uto and Suto 1985).

Delineation of buried caldera for Yabakei pyroclastic flow

The Yabakei is commonly recognized in drill cores in the HVZ (Fig. 1) on the basis of: (1) lithologic features that resemble those of known Yaba- kei, including mineralogy, phenocryst ratios and abundance, shape of shards and NRM; (2) radiometric ages and magnetic polarities of volcanic rocks, below and above the Yabakei, that are con- sistent with its estimated eruption age (1.0 Ma).

Drill-core data indicate that the Yabakei pyroclastic flow occurs in the subsurface over an area 60 km wide (Fig. 1). These data combined with outcrop information indicate that the original distribution and bulk volume of the Yabakei were approximately 1200 km 2 and 110 km 3, respec- tively (Kamata 1989). Aramaki (1957) pointed out

that pyroclastic flows with bulk volumes exceed- ing 10 km 3 generally have caldera sources. The eruption of the Yabakei was probably accompanied by the formation of a caldera, because all other pyroclastic flows in Japan whose exposed volumes exceed 40 km 3 have associated calderas.

The elevation of the top of the Yabakei pyroclastic flow is highest (>800 m) at Heikeyama (He of Fig. 1) east of the Kusu basin. Top elevations north of the Kusu basin are 500-600 m and decrease northward and northwestward. The elevation south of Haneyama (Ha of Fig. 1) is 680 m, and that west of Miyanoharu (Mi of Fig. 1) is 610 m. Therefore, the elevation and slope of the upper surface of the Yabakei suggest that its source caldera is south of the Kusu basin and east of Miya- noharu.

The Yabakei pyroclastic flow does not outcrop near the Kusu basin and Shishimuta (Fig. 2), where andesite lava flows form several buttes whose summits are 600-800 m above sea level (Iwao 1979; Kamata 1985a, b). K-Ar ages of these flows are 0.9-2.2 Ma (Kamata 1987), almost all of

~ Sediments younger than 1.0 Ma

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[ ] Sediments older than 1.0 Ma

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~ Volcanics older than 1.0 Ma

t~~.) Buried caldera for Yabakei p.f.

Fig. 2. Geologic map of the Kusu basin and Shishimuta area. Solid squares, K-Ar age of lava flows in Ma. Circled dots, locations of drill holes of Fig. 6. Open circles, locations of 3000- m-deep drill holes with spot coring. Dashed line, outline of interpreted Shishimuta caldera. Location of map shown in Fig. 1. Ta, Taka- rayama; Da, Diagansenyama; Si, Shouganse- nyama; Ka, Karutoyama; Yo, Yokoyama; He, Heikeyarna; Ki, Kärikabuyama; Ts, Tsunomur- eyama

44 Kamata: Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan

which are older than the Yabakei. Clearly, the source caldera for the Yabakei cannot lie beneath these older lava flows, but should instead occur beneath rocks younger than 1 Ma near Shishi- muta (Fig. 2).

Locations of source calderas can be estimated by the variation in lithic size (Yokoyama 1974) and nature (Aramaki and Ui 1966). Flow linea- tions defined by grain orientation (Suzuki and Ui 1982), log orientation (Froggatt et al. 1981), and imbricate structure (Kamata and Mimura 1983) also help. However, these methods are not useful for the Yabakei pyroclastic flow, because most of its proximal distribution is buried by younger rocks. Therefore, I used gravity and drill-hole data to delineate the caldera.

A subcircular negative Bouguer anomaly of 36 mgal is centered on the Shishimuta area (Ss of Fig. 3). A gravity model of the depth to pre-Ter- tiary basement defines a V-shaped depression up to 3.8 km below sea level (Komazawa and Ka- mata 1985; Kamata submitted). The Yabakei pyroclastic flow occurs in drill cores outside, but not inside, the - 3 0 mgal contour (Fig. 1). Volcanic rocks older than 1 Ma occur at relatively high elevations (506, 368, 218 m above sea level) along the - 3 0 mgal contour north and northwest of Kuju

Fig. 3. Bouguer gravity map of the central part of Hohi volcanic zone (after Kornazawa and Kamata 1985). Assumed density, 2.3 g/cm 3. Contour interval, 2 mgals. Dotted line, location of Shishimuta caldera. Location of map shown in Fig. 1. Ss, Shishimuta; Ha, Haneyama; Mc, Machida; Bin, Bungomori; Wa, Waitasan; Mi, Miyanoharu; Mz, Mizuwake-toge; Kj, Ku- jusan; Hm, Hanamureyama; He, Heikeyama; Ic, Ichimoku- s a n

volcano (Fig. 1). These relations strongly suggest that subsidence occurred about 1.0 Ma within the - 30 mgal contour. Therefore, I interpret the margin of an 8-km-wide caldera, the Shishimuta caldera, along the - 3 0 mgal contour.

Subsurface lithology of Shishimuta caldera

Outside Shishimuta caldera, drill holes encounter pre-Tertiary rocks at 2.0-2.5 km depth (Sasada 1984; Tamanyu 1985). However, drill holes up to 3 km deep within the caldera (y, z in Fig. 1) pene- trate Plio-Pleistocene volcanic rocks rather than pre-Tertiary rocks (MITI 1986). Modeling of the depth to gravity basement in the caldera suggests that pre-Tertiary rocks are about 3.8 km below the surface (Komazawa and Kamata 1985; Kamata 1989). Therefore, pre-Tertiary rocks inside Shishi- muta caldera are at least 1-2 km lower than outside.

Samples from spot cores a few meters thick, taken at 300 m intervals from 0 to 2200 m below sea level within Shishimuta caldera, are composed primarily of volcanic breccia containing clasts of pyroxene andesite and hornblende andesite lava flows and dacite welded tuff that petro- graphically resemble pre-Yabakei rocks. Typical- ly, poorly sorted andesite fragments, as much as several tens of centimeters across, lie in a tuffa- ceous matrix. No blocks of the Yabakei occur in the breccia, but in places the matrix contains eu- hedral hornblende phenocrysts resembling those in the Yabakei. Siltstone fragments rarely occur in the matrix. The breccia occurs about 1-2 km lower than do the pre-Tertiary rocks outside the caldera, and no similar breccia occurs outside Shishimuta caldera.

I interpret the breccia to have formed during the explosive eruption that created the caldera. Lithologic features of the breccia suggest that it was derived from broken wall rock or roof rock, consisting mainly of andesite flows and rubble between the magma reservoir and the old ground surface. The hornblende crystals are evidence that Yabakei magma was erupting during the collapse. Ando (1983) reached a similar conclusion for the intra-caldera deposit of Nigorikawa caldera, which shows a funnel-shaped structure. The breccia may be interpreted as a megabreccia (Lipman 1976).

The Bouguer anomaly of Shishimuta caldera is subcircular in plan view, V-shaped in profile, and resembles that of a Krakatoan or funnel- shaped caldera (Yokoyama and Aota 1964). The

Kamata: Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohl volcanic zone, Japan 45

~ L a v a dornes younger than 0.1 Ma

Pyroxene andesite "] Hornblende andes|te / 0.1Ma_<Age<0.TMa

~ Biotite rhyolite J Yabakei

[ ] Volcanic rocks (0.TM <a~_Age<O.9Ma) ~ Pre-Tertlary basement and Miocene volcanic rocks

~ Shonal

Fig. 4. Regional distribution of volcanic rocks younger than 1 Ma around Shishimuta caldera (circle). Star, vent location of Yufugawa pyroclastic flow. Diamond, vent loca- ~ion of Imaichi pyroclastic flow. Triangle, summit of mountain

anomaly is not bowl-shaped, as are Bouguer ano- malies associated with Valles-type calderas, (e.g. Eaton et al. 1975). Shishimuta caldera differs in other ways from Valles-type structures: it is only 8 km in diameter, has no ring vents, and lacks resurgent activity, and post-caldera monogenetic volcanism continued for 0.6 million years after the caldera-forming eruption (Kamata et al. 1988).

This evidence suggests that Shishimuta caldera formed not by the sinking of a coherent cylindrical block, as exemplified by Valles-type calderas, but rather by the collapse of roof rocks in piecemeal fashion to make a funnel-shaped caldera, such as commonly accompanied eruption of large-scale Quaternary pyroclastic flows in Japan (Aramaki 1984).

Post-caldera deposits around Shishimuta caldera

Figure 4 shows the distribution of volcanic rocks younger than the Yabakei pyroclastic flow (1.0 Ma), classified by radiometric age and mineral assemblage. Volcanic rocks younger than 1 Ma clus-

ter around Shishimuta caldera, as well as around Yufu-Tsurumi and Aso volcanoes. These three clusters contain source vents of large-scale pyroclastic flows, such as the Yabakei, Yufugawa and Aso pyroclastic flows. Thus, volcanism after 1 Ma in the HVZ is limited to broad areas in which large-scale pyroclastic flows were previously erupted.

Near Shishimuta caldera, volcanic activity re- sumed at about 0.96 Ma, just after eruption of the Yabakei pyroclastic flow (Kamata et al. 1988). Vents of these lava flows and dornes are scattered around the Shishimuta cluster. Each vent pro- duced a small volume of lava and erupted only once, as is typical of monogenetic activity (Naka- mura 1977).

The Shishimuta area is in an extensional stress field, as shown by the prevalence of E-W normal faulting (Ikeda 1979; Muraoka and Kamata 1983), focal mechanisms of earthquakes (Yama- shina and Murai 1975), and geodetic measure- ments (Tada 1984). I interpret the lava dornes and flows around Shishimuta caldera as products of monogenetic volcanism typical on an extensional


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Fig. 5. Distribution and surface elevations of lacustrine deposits in and around Shishimuta caldera. Circled dots, locations of drill holes shown in Fig. 6, and elevation of top of upper sedimentary unit. Open circles, locations of spot-cored drill holes to 3000 m depth. Bh, Bungonakamura; Me, Machida; Kk, Kusuikei; Ok, Okunameshi; O9, Oguradake; Kb, Kabushidake

stress field, rather than as products of resurgence of Shishimuta caldera.

The deposits of post-caldera activity comprise hypersthene-augite andesite lava flows (the Shi- bayakata-toge lava of Kamata 1985b), volcanic breccia of similar lithology, and intercalated sedimentary deposits of siltstone, sandstone and pu- miceous beds (the Kusu Group; Matsumoto et al. 1973).

The lacustrine deposits typically occur 500- 600 m above sea level (Fig. 5). They are important in understanding the development of the caldera, because they indicate a closed depression. The deposit at Kusuikei (Kk in Fig. 5) interfingers with the Shibayakata-toge lava flows (0.6-0.7 Ma). The deposit at Bungonakamura (Bn in Fig. 5) is intercalated with the Nakamura pyroclastic flow, dated at 0.5 Ma (Kamata and Muraoka 1982). Lake deposits at Machida (Mc in Fig. 5) underlie the Oguradake lava flow (0.55 Ma; Ka- mata 1985b) and overlie the Machida lava flow (0.7 Ma; Kamata and Muraoka 1982). The lacustrine deposit at Okunameshi (Ok in Fig. 5) underlies the Kabushidake lava flow (0.3 Ma; Kamata 1985b). Therefore, the exposed lacustrine deposits formed 0.7-0.3 Ma ago.

Two main units of lacustrine deposits occur in drill cores (Fig. 6). The upper unit, about 500 m above sea level, overlies the Imaichi (0.8-0.9 Ma) and the Yabakei pyroclastic flows and a biotite- hornblende andesite lava flow (1.3 Ma), and underlies hornblende andesite (0.6 Ma) and pyroxene andesite (0.7 Ma) lava flows. Therefore, the upper unit is of post-caldera age and was deposi- ted 0.9-0.6 Ma ago. The origin of the lower unit of the deposit (<200 m above sea level) is un- known; it possibly formed before the Yabakei eruption. The lacustrine deposits in outcrops and in the upper unit in the drill holes are at similar elevations and have similar ages; hence, they are probably correlative.

Figure 5 shows that the distribution of lacustrine deposits exceeds the structural boundary (at a depth of about 1 km) of Shishimuta caldera, as delineated by the subsurface distribution of volcanic rocks older than 1 Ma and by the Bouguer anomaly. Smith and Bailey (1968) noted that the present margin of Valles Caldera is outside the ring dikes that define the area of collapse; they concluded that the present caldera margin at Valles retreated from its initial position owing to erosion. The distribution of lacustrine deposits


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similarly suggests that the surface expression of Shishimuta caldera widened by erosion after initial collapse.

Interpretive profile of Shishimuta ealdera

The Shishimuta-caldera area has patterns of normal faulting and microearthquakes that probably reflect the subsurface structure of the caldera. Figure 7 shows the distribution of active faults cutting volcanic rocks younger than 1 Ma. The faults, defined by landforms visible on aerial pho- tographs and during field-work (Kamata 1987; Kamata et al. 1988), have predominantly east- west trends and normal displacement. No strike- slip movement has been recognized. The highest concentrations of faults are south of Haneyama and near Kuenohirayama (Ha and Ku in Fig. 7). Faults are less common inside Shishimuta caldera, although volcanic rocks younger than 1 Ma are uniformly distributed in and around the caldera. These observations suggest that Shishimuta caldera forms a barrier to normal faulting.

Figure 8 shows the distribution of microearthquake epicenters in the HVZ. Hypocenters are shallower than 15 km (Ito and Sugihara 1985). These seismologic data indicate that epicenters, just as normal faults, are more scarce within the caldera than outside. Rocks down to 15 km depth beneath the caldera are possibly so highly fractured that little stress can accumulate.

Figure 9 shows an interpretive profile of Shi- shimuta caldera based on available evidence. On the surface, post-caldera lava domes and flows buried the caldera topography without resurgence. At 500 m elevation, lacustrine deposits ex- tend about 3 km beyond the subsurface structural boundary of the caldera. Beneath these deposits is 3.8 km of megabreccia (Lipman 1976) that formed during piecemeal collapse of older andesite flows. Below the breccia, fractures are numerous in pre- Tertiary basement rocks. A residual magma chamber for the Yabakei pyroclastic flow is assumed at a depth of 6-10 km, on the basis of studies at Aira caldera (Mogi 1958; Aramaki 1971). This model, including the fractured zone and the magma chamber, is deduced from the available evidence

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Fig. 7. Distribution of active faults and volcanic rocks younger than 1 Ma near Shishimuta caldera. Distribution of volcanic rocks from Kamata (1987; 1989). Radiometric ages from Kamata (1987) and Kamata et al. (1988). Dashed line, structural margin of Shishimuta caldera. Ss, Shishimuta; Ha, Haneyama; Bin, Bungomori; He, Heikeyama; Ku, Kuenohirayama; Il/a, Waitasan; As, Asono basin; Sn, Shonai; Yf, Yufudake; Ts, Tsurumidake; Kj, Kujusan

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Fig. 9. Interpretive N-S profile of Shishimuta caldera, 1, post- caldera lava domes and flows; 2, lacustrine sediment with intercalated autobrecciated lava flows; 3, volcanic breccia formed contemporaneously with eruption of Yabakei pyroclastic flow; 4, residual magma chamber; 5, volcanic: rocks erupted prior to Yabakei pyroclastic flow; 6, pre-Tertiary basement rocks, y, z, locations of spot-cored drill holes to 3000 m depth

to be funnel-shaped and more highly fractured and vertically elongated than models of Valles- type calderas.

Conclusions

The Yabakei pyroclastic flow, which erupted 1.0 Ma ago with 110 km 3 bulk volume, issued from the buried Shishimuta caldera in the center of the Hohi volcanic zone. The caldera is a breccia-filled funnel-shaped depression 8 km wide and > 3 km deep with a V-shaped negative Bouguer gravity anomaly of 36 mgal. Post-Yabakei activity shows no resurgence, but instead shows features of monogenetic volcanism typical of an extensional stress field. Post-caldera lacustrine deposits ex- tend about 3 km beyond the structural boundary

of Shishimuta caldera, probably due to widening by erosion.

An andesitic breccia of relatively low density, which comprises a mixture of pre-caldera rocks and the Yabakei, fills the depression, as interpreted from drill-core data. I believe the breccia formed by explosion and collapse during the violent caldera-forming eruption and settled into the deepening depression as a megabreccia.

Fewer normal faults and microearthquakes shallower than 15 km occur inside the caldera than in surrounding areas. This distribution suggests that the subsurface beneath the caldera is so highly fractured that little stress can accumulate.

I interpret Shishimuta caldera to have formed not by sinking of a coherent cylindrical block, as exemplified by a Valles-type caldera, but instead by piecemeal collapse of roof rock that resulted in a funnel-shaped caldera with a deep subjacent fractured zone.

Acknowledgements. This study was carried out as partial ful- fillment of a Ph. D. thesis presented to the University of Tokyo in 1987. Valuable comments and discussions for this study by K. Ono, T. Soya, D. A. Swanson, T. Ui, K. Uto, K. Watanabe, Y. Kobayashi, K. Nakamura, S. Aramaki, H. Hoshizumi, Y. Hase, T. Kobayashi, K. Suzuki-Kamata, K. Ogawa, A. Kubo- tera, K. Mizuno, T. J. Iiyama and K. Hirasawa are greatly appreciated. Critical reviews of various stages of drafts by D. A. Swanson, R. A. Bailey, T. Ui, K. Nakamura and H. Hoshizumi are appreciated. A. Kubotera and M. Sugihara informed the author of earthquake data detected in the Hohi volcanic zone. This study also benefits from data obtained by geothermal ex- plorations sponsored by the Ministry of International Trade and Industry, Japan.

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Received February 5, 1988/Accepted March 29, 1988

Documents

Shishimuta caldera, the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone, Japan