Structural characteristics of the Bayonnaise Knoll caldera as revealed by a high-resolution seismic reflection survey
© Yamashita et al.; licensee Springer. 2015
Received: 1 December 2014
Accepted: 17 March 2015
Published: 1 April 2015
The Bayonnaise Knoll caldera is a conical silicic caldera located on the eastern part of the back-arc rift zone of the Izu-Ogasawara arc. Many geological and geophysical surveys have shown that the Bayonnaise Knoll caldera contains hydrothermal sulfide deposits. The Japan Agency for Marine-Earth Science and Technology conducted high-resolution multi-channel seismic reflection surveys across the Bayonnaise Knoll caldera to ascertain details of the crustal structure, such as the configuration of faults around the caldera. A reflection profile of excellent quality was obtained by high-density velocity analysis at about 150-m intervals. We applied prestack depth migration by using the results of the high-density velocity analysis and further analyzed this region. The depth-migrated profile shows many faults, which correspond to bathymetric lineations, on the eastern side of the Bayonnaise Knoll caldera. The velocity structure of the Bayonnaise Knoll caldera resembles that of the Myojin Knoll caldera, which has been well surveyed and is associated with the hydrothermal deposit. The depth-migrated profile shows a clear reflective zone that is distributed asymmetrically to the Bayonnaise Knoll caldera center. These data suggest that caldera formation was controlled by back-arc rifting activity in the Izu-Ogasawara arc. The hydrothermal fluid migration path in the Bayonnaise Knoll caldera is estimated to be the result of faulting and magmatic intrusion on the eastern side of the structure. It is assumed that these fluids formed the Kuroko-type sulfide deposit in the eastern part of the caldera structure.
The Izu-Ogasawara (Bonin) arc extends over 1,200 km from the Honshu island of Japan to Guam on the northeastern margin of the Philippine Sea plate. The Izu-Ogasawara arc is a region that is beneficial for understanding intra-oceanic evolution, and it has been investigated well in many geological and geophysical studies (e.g., Taylor 1992; Suyehiro et al. 1996; Kodaira et al. 2007). Not only large-scale surveys, but also detailed analyses and surveys have specifically investigated its structural characteristics (e.g., Tsuru et al. 2008; Fujiwara et al. 2009).
Paleo-volcanic arrangements are known to exist along the north-south direction; these are known as the outer-arc high or frontal-arc high by geographic characteristics in the forearc basin. Since the spreading suspension of the Shikoku Basin in 15 Ma (Okino et al. 1994), many active volcanoes have developed in the Izu-Ogasawara region (e.g., Ishizuka et al. 2002). Additionally, several basins developed in the so-called back-arc rift zone between the current volcanic front and the paleo-volcanic arrangement in the west (Murakami 1996).
A large Kuroko-type polymetallic sulfide deposit was discovered on the caldera floor of the Myojin Knoll caldera (Iizasa et al. 1999) along the back-arc rift. Kuroko, which is a black ore produced in submarine volcanoes, includes minerals such as copper, lead, silver, and gold that are useful for industrial activities. The Kuroko deposit distribution is known to be restricted in island arcs. New hydrothermal deposits on the Bayonnaise Knoll caldera located west of the Myojin Knoll caldera have been reported from many surveys (Tanahashi et al. 2006). The configuration of the Myojin Knoll and Bayonnaise Knoll caldera region resembles that of formerly submarine onshore large deposits of the Hokuroku region in the Akita Prefecture (Tanahashi et al. 2008). Many ongoing hydrothermal activities have been recognized along the Izu-Ogasawara volcanic front (e.g., Nagaoka et al. 1992). Understanding the model of hydrothermal deposit formation is crucially important for the exploration of new hydrothermal deposits. Specifically, a better understanding of the internal structures of these calderas will give a new perspective as to the formation of Kuroko-type hydrothermal deposits. Because investigating the internal structure is necessary to ascertain the formation processes related to hydrothermal sulfide deposits of the Bayonnaise Knoll caldera, we conducted a high-resolution multi-channel seismic reflection (MCS) survey to find the relation between the hydrothermal deposit and crustal structure. As described in this paper, we investigated the fault configuration and the crustal structure of the Bayonnaise Knoll caldera with a high-density velocity analysis of newly obtained MCS data. Based on our analyses and interpretations of the results, we discuss the relationship between hydrothermal deposit formation and fault configuration in and around the Bayonnaise Knoll caldera.
Geological setting of the Bayonnaise Knoll caldera
Data acquisition and processing
The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) conducted a new multi-channel seismic reflection survey (standard fold number 55.5) across from the current volcanic front to the Eocene volcanic arrangement (Taylor 1992). For this study, we extracted data from part of the Bayonnaise Knoll caldera (Figure 1) to investigate the detailed crustal structures with a high-density velocity analysis. The seismic source used was an annular port array of 32 air guns with a total volume of 7,800 in3 (130 L) operating at a standard air pressure of 2,000 psi (14 MPa). The frequency spectrum was flat at the frequency range of 5 to 80 Hz. The hydrophone streamer cable used as a receiver was ca. 5,700 m long, and it had 444 channels spaced at intervals of 12.5 m. The air gun was fired every 50 m at a time interval of ca. 20 s along the survey line. Seismic reflection records of 15-s length with a 1-ms sampling interval were obtained for deep crustal imaging. The MCS data were processed through the standard seismic data processing flow of JAMSTEC, which consists of noisy-trace editing, 30- to 80-Hz band-pass filtering, velocity analysis, normal moveout (NMO) correction, common depth point (CDP) stacking, and poststack time migration. Especially, we applied high-density picking to the reflectors at every 25 CDP intervals (156.25 m) for velocity analysis to resolve the internal structure of the caldera.
It is difficult to image the precise crustal structure of the Bayonnaise Knoll caldera using the CDP stacking method assuming horizontally layered media because submarine calderas such as the Myojin Knoll caldera and Bayonnaise Knoll caldera have steep slopes, as evidenced by their rough sea floor topography (Tsuru et al. 2008). Therefore, we applied high-resolution imaging using prestack depth migration (PSDM; Sattlegger and Stiller 1974) with a high-density velocity model derived from the detailed velocity analysis. Additionally, we optimized the PSDM method to produce images of steep structures such as calderas or faults (Yamashita et al. 2007). The PSDM method, which is a powerful tool for imaging complicated structures, improved the precision of the reflector identification data. The imaging quality of crustal structures using reflection survey data depends on the reliability of applied velocity structures (Lines et al. 1993). For conventional reflection survey data processing, velocity information is observed using NMO correction and CDP stacking; it cannot address lateral variations of the structure because the algorithm defines the subsurface structure as a flat multilayered structure (e.g., Al-Yahya 1989; Stork and Clayton 1992). Back-arc basins are known to have widely variable normal faults, folds, and deformations in a shallow crustal structure. Therefore, the PSDM technique is useful to improve imaging in these settings. We used the smoothed velocity model of high-density picking for the initial velocity model in the PSDM analysis. The final velocity model was updated two times by iteration through repeated tomographic inversion.
Characteristics of the Bayonnaise Knoll caldera and the Myojin Knoll caldera
Summit water depth
15 to 22 km
Caldera rim diameter
2.5 to 3 km
5 to 7 km
1,570 to 2,400 m/s
1,590 to 2,460 m/s
Based on our results, we now propose a strategy for how to locate the hydrothermal deposit in the Bayonnaise Knoll caldera. The hydrothermal activity implies that there is intrusion of high-temperature materials such as hot water or magma beneath the caldera. Input of water may also occur around the caldera. Honsho et al. (2010, 2012) present the distribution of the magnetic anomaly, and these data likely correspond to the hydrothermal deposit distribution in the Bayonnaise Knoll caldera. A high-magnetization zone exists in the crater of the Bayonnaise Knoll caldera according to their results (Figure 4). They also suggest that the magnetic anomalies are distributed along the inferred fault with a north-south trend. According to our results, not only is there a fault in the center of the Bayonnaise Knoll caldera, but major faults also exist outside of the caldera rim. These faults are assumed to have strongly affected the formation process of the hydrothermal deposit. We now discuss the circulation of the hydrothermal fluid that influenced the growth process of the hydrothermal deposit in the Bayonnaise Knoll caldera. The locations of the hydrothermal deposit are concentrated along the caldera wall (Tanahashi et al. 2006). Regarding the hydrothermal activity in the Suiyo Seamount, Watanabe and Kajimura (1994) proposed a model of hydrothermal fluid circulation. The model shows that seawater is injected inside of the Suiyo caldera wall and is heated by either the dyke or magma intrusion at the caldera center; this has resulted in the formation of hydrothermal mounds and chimneys at present. Tsuji et al. (2012) proposed a hydrothermal fluid circulation model around the Iheya North Knoll of the Okinawa Trough, which is located at the continental back-arc basin. Their model shows the water input from the normal fault and ridge at the seafloor and fluid migration driven by the heat source beneath the hydrothermal field. Figure 4 depicts the hydrothermal fluid circulation model in the Bayonnaise Knoll caldera as inferred from the interpretation of data obtained from seismic time and depth sections and the velocity model. The reflective zone in the eastern part of the caldera is believed to comprise porous and permeable volcanic sediments that have low velocity. The high-velocity layer below the reflective zone is interpreted to be the result of magma intrusion. The hydrothermal fluid circulation model is estimated as follows. 1. The water intrudes along the major fault on the eastern outside edge of the Bayonnaise Knoll caldera. 2. The water passes through a porous and a permeable layer. 3. The fluid is heated by magma intrusion and forms a hydrothermal field in the Bayonnaise Knoll caldera. A similar type of volcanic activity seems to be distributed along the east side of the back-arc rift zone in the Izu-Ogasawara arc.
Faults along the bathymetric lineation have developed on the east side of the Bayonnaise Knoll caldera, and their locations correspond to the distribution of hydrothermal sulfide deposits.
The asymmetric structure of reflective zones beneath the Bayonnaise Knoll caldera was identified.
The velocity structure obtained by prestack depth migration of the Bayonnaise Knoll caldera data resembles that of the Myojin Knoll caldera.
The hydrothermal migration path is estimated to be the result of faulting and magma intrusion on the east side of the Bayonnaise Knoll caldera.
The authors are grateful to the anonymous reviewers for their valuable comments on the manuscript. We acknowledge the captain and crew of the R/V Kairei and marine technicians of the Nippon Marine Enterprise Ltd. for their support.
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