A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan
Introduction
Many gravimetric studies in volcanic regions have been conducted to reveal caldera structures, their forming history and magma intrusion (e.g., Yokoyama, 1963; Koizumi et al., 1988; Okubo and Watanabe, 1989; Komazawa, 1995). These studies show that most of the calderas have a relative gravity low which indicates the lack of mass and/or depression structures (e.g., Yokoyama, 1963; Komazawa, 1995).
In a region where both volcanic and tectonic depressions exist, however, the gravity low does not always show the location of a buried caldera and/or a surface caldera because the tectonic basin may also make a relative gravity low. Accordingly, in such cases, a correct interpretation of a gravity low is difficult (e.g., Kobayashi et al., 1995). We usually need detailed geological surveys in the area to determine whether the gravity low is associated to a caldera or not (e.g., Kamata, 1989a). However, these works require long time and costs.
On the other hand, a gravity low of tectonic origin should be interpreted as a result of tectonic deformation in the past. An appropriate tectonic restoration can reveal the gravity low which is not originated by volcanic activity, and this kind of information can greatly help the interpretation of the gravity anomaly.
We use the following procedure to identify whether the gravity low is of volcanic origin or not:
(1) we restore the gravity lows of tectonic origin by numerical modeling,
(2) we compare the restored gravity anomalies with the observed gravity anomalies, and
(3) we identify the gravity low which is not restored by the 2nd procedure as a caldera.
Because the above procedure requires only a tectonic background of the study area, this technique is effective especially when we can not obtain detailed geological and geophysical information in the area.
To examine the applicability of this technique, we selected, as a test field, the Hohi volcanic zone (Kamata, 1989b), central Kyushu, Japan. In this area, a buried caldera has been already confirmed by detailed geological studies (Kamata, 1989a). We will use this example to see how a buried caldera can be detected by our method.
Section snippets
The Hohi volcanic zone
The Hohi volcanic zone (HVZ: Fig. 1) is known as a volcano–tectonic depression (Kamata, 1989b; Kamata and Kodama, 1994). Radiometric ages of volcanic rocks in the area are younger inward from the margin and show a systematic zonation (see fig. 5 in Kamata and Kodama, 1994). These facts suggest that successive volcanic events occurred in the central part. This area is also located at the intersection of three tectonic lines, i.e., Median Tectonic Line (MTL), Oita-Kumamoto Tectonic Line (OKTL)
Restoration of gravity anomaly
In the restoration modeling of gravity anomaly, we first removed the regional trend from the Bouguer gravity anomalies. We assumed that: (1) gravity anomalies reflect the result of accumulated crustal deformations in the past, and (2) effects of density change due to fault motions (Okubo, 1992) are very small and can be ignored in the modeling.
We used a simple two layers structure, i.e., the basement and the sedimentary layer, to model the gravity anomaly. We employed the Banerjee and Gupta's
Results
Fig. 5 shows the restored gravity anomaly map at the surface. Because the gravity anomalies were calculated from elastically deformed fields with giving a unit dislocation on the fault plane, the absolute values observed have no specific meaning. We only aim at the identification whether the gravity lows are of volcanic origin or not. The patterns of the restored gravity anomalies are important. Thus, we normalized the values by the minimum gravity value. It is noted that Fig. 5 should be
Discussion and conclusion
From geological viewpoints, the A4 gravity low is understood as a buried caldera called the Shishimuta caldera (Kamata, 1989a).
The Shishimuta caldera was identified by the existence of its inherent pyroclastic flow (Yabakei pyroclastic flow) and the depth distribution of old lava flows (older than 1 Ma) in the drilling cores in and around the A4 gravity low (Kamata, 1989a). Since most of the proximal Yabakei pyroclastic flow is covered by younger lava flow and lava dome (younger than 0.9 Ma),
Acknowledgements
We are thankful to Dr. M. Battaglia and an anonymous referee for their careful reviews of the paper and helpful comments on the manuscript. This work was partly supported by a Grant in Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (No. 07305048).
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Understanding caldera structure and development: An overview of analogue models compared to natural calderas
2007, Earth-Science ReviewsCitation Excerpt :Moreover, there is still fragmented or poor information on the location, geometry and kinematics of the main caldera structures and how these develop. Gravity data have been used to investigate the deeper parts of calderas, as at Long Valley (California; Carle, 1988), Valles (New Mexico; Self and Wolff, 2005, and references therein) Yellowstone (Wyoming; Smith and Braile, 1994, and references therein), Bolsena (Italy; Nappi et al., 1991), Coromandel Peninsula, Taupo, Reporoa, Rotorua and Okataina (New Zealand; Spinks et al., 2005, and references therein; Smith et al., 2006), Sao Miguel (Azores; Montesinos et al., 1999), Hohi Volcanic Zone (Japan; Kusumoto et al., 1999) and Las Canadas (Canary Islands; Camacho et al., 1991; Arana et al., 2000). In some cases, as Campi Flegrei (Italy; Barberi et al., 1991) or Guayabo (Costa Rica; Hallinan, 1993; Hallinan and Brown, 1995), gravity data suggest nested collapses.