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Submarine lava fields in French Polynesia
Marine Geology 373 (2016) 39–48
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Marine Geology journal homepage: www.elsevier.com/locate/margo
Submarine lava fields in French Polynesia Naoto Hirano a,⁎, Masao Nakanishi b, Natsue Abe c, Shiki Machida c a b c
Center for Northeast Asian Studies, Tohoku University, Kawauchi 41, Aoba-ku, Sendai 980-8576, Japan Graduate School of Science, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Japan Agency for Marine-Earth Science and Technology: Natsushima-cho 2-15, Yokosuka 237-0061, Japan
a r t i c l e
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Article history: Received 16 April 2015 Received in revised form 16 December 2015 Accepted 1 January 2016 Available online 6 January 2016 Keywords: Hotspot Petit-spot French Polynesia Flood lava Volcanic cone Submarine volcano
a b s t r a c t Shipboard multibeam survey is powerful tool to locate submarine volcanoes especially having small volume. Small submarine volcanoes may represent the initial stages of hotspot activity, but they may also form via lithospheric flexing, regional convection of the mantle, and the presence of fracture zones. Here we describe several volcanoes, flood lavas, and volcanic clusters in French Polynesia using data from archives of multibeam data. The clusters of small volcanoes are similar to petit-spots, and they are not considered to represent the initial stages of a hotspot as they are composed of both young and old edifices, and because the sites are located far from any known hotspot. These newly discovered submarine volcanoes are located in areas with low-velocity seismic shear waves at depths of 60 and 100 km. These lava fields will therefore facilitate geochemical mapping of the mantle in areas unrelated to hotspots, because these lavas may have developed from melts in the shallow mantle beneath French Polynesia. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Hotspots are believed to develop over stationary plumes that rise from the deep mantle beneath tectonic plates (Morgan, 1971), although other explanations for their origin have been proposed, such as antiplumes from the deep mantle. These hotspots lead to the formation of lines of seamounts and/or oceanic islands with a progressive increase in age along the direction of plate motion. However, the bathymetric characteristics and radiometric dating of seamounts are not always consistent with this simple model of a hotspot trail (Clouard and Bonneville, 2001). The Hawaiian hotspot on the Pacific plate has existed for more than 70 Myr, but its position below the plate has not remained stationary in the mantle (Tarduno et al., 2003), whereas the Louisville hotspot has been nearly stationary for 70 Myr (Koppers et al., 2012). Most seamount trails that formed in the western Pacific during the Cretaceous show short-lived progressions in age (e.g., a few millions or a few tens of millions of years; Koppers et al., 2003). The initial phase of a hotspot volcano may be represented by small submarine volcanoes that form prior to the development of a large, possibly subaerial volcano over a hotspot, and indeed, such activity has been observed on the Loihi Seamount in the Hawaiian chain (e.g., Moore et al., 1979), the Macdonald Seamount in the Austral chain (e.g., Johnson, 1970), the Adams Seamount in the Pitcairn chain (Devey et al., 2003), and the Vailulu'u Seamount in the Samoan chain (e.g., Hart et al., 2000). Determining the location of submarine volcanoes ⁎ Corresponding author. E-mail address: [email protected] (N. Hirano).
http://dx.doi.org/10.1016/j.margeo.2016.01.002 0025-3227/© 2016 Elsevier B.V. All rights reserved.
is therefore critical to identifying possible hotspots. In addition to hotspot-related volcanoes, other new and unexpected kinds of volcano, such as petit-spot volcanoes and arch lavas, have been reported from the submarine environment, and these were first discovered on the subducting Pacific plate off northeastern Japan and on the flexural Hawaiian arch 300–500 km from the Hawaiian Islands, respectively (Holcomb et al., 1988; Hirano et al., 2006). These discoveries relied on the use of submarine acoustic surveys, which are vital for such work. Hotspot geochronology and the distribution of volcanic activity have been recorded from subaerial lavas on the islands of French Polynesia, as well as from a few submarine lavas. Further exploration of the submarine volcanoes in the French Polynesia region is vital if we are to elucidate the hotspot geology and mantle structure below this area. 2. Regional setting In French Polynesia, active hotspots have been identified around the Society, Marquesas, Macdonald, and Pitcairn islands (Fig. 1). The hotspots correspond to the sites of zero-aged volcanoes which may be seated on hotspot trails arising from the deep mantle (McNutt and Fischer, 1987). The age progression characteristics of both the Society and Pitcairn hotspots (along the Society Islands and the Pitcairn– Gambier island axes, respectively) are well known (Gripp and Gordon, 2002). However, along the Cook–Austral seamount trail, the age progression is complicated, because several zero-aged volcanoes occur at loci of magmatism that are thousands of kilometers from the reconstructed positions of the Macdonald Seamount hotspot (Johnson, 1970). Some seamounts along the chain were either rejuvenated or
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Fig. 1. Bathymetric map of French Polynesia (left) and shipboard bathymetry (right) for the region indicated by the large square in the left panel. Bathymetric data based on satellite gravity measurements are from Smith and Sandwell (1997). Pink stars correspond to sites of stationary hotspots identified from zero-aged (active) volcanoes (Clouard and Bonneville, 2001). Major fracture zones shown by solid lines in the right panel are truncated in the French Polynesian region (Matthews et al., 2011). The red lines in the right panel show the paleomagnetic isochron (Cande et al., 1989). The yellow circle is the potential site of the submarine lavas described by Searle et al. (1995).
resumed volcanism as a result of multiple overlapping volcanoes that erupted during the Quaternary (Dickinson, 1998; Bonneville et al., 2002). Most studies on the geochronological characteristics of these chains have been restricted to subaerial lavas, even though the hotspots are actually located below the ocean at the southeastern extensions of their associated seamount chains (e.g., Cheminee et al., 1989; Binard et al., 1991; Hekinian et al., 1991; Bonneville et al., 2002; Hekinian et al., 2003). However, dating of submarine samples from the Nelson Bank has not helped to explain the hotspot model along the Austral Islands (Bonneville et al., 2006). We note, too, that tiny submarine eruptions of lava do exist in submarine French Polynesia, as described by Searle et al. (1995) on the basis of data from the GLORIA towed sidescan (the location of the survey is indicated by the small solid circle in Fig. 1). 3. Methods The spatial resolution of the bathymetric information derived from satellite gravity data is too low to use in the detection of very small volcanic edifices less than 2–3 km in diameter. In addition, the extent of the areas covered by sonar surveys of the ocean floor is limited. The multibeam data shown on the right of Fig. 1 were obtained mainly from the databases of the Scripps Institution of Oceanography (http:// siox.sdsc.edu/search.php) and the National Centers for Environmental Information, National Oceanic and Atmospheric Administration (NCEI/ NOAA, http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html). Other data in Fig. 1 were obtained from two cruises of the R/V Mirai (MR08-06 and MR09-01), run by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We selected data from four cruises (MR08-06, PANR06MV, WEST13MV, and AMAT03RR; Figs. 2–6) from all of the available databases (Fig. 1) in order to make discussions in this paper. To focus on small submarine volcanoes, we excluded data obtained by multibeam echo sounder system without the acoustic backscatter function. During detailed multibeam surveys of the ocean floor, areas identified by acoustic backscatter as having a high acoustic reflectivity typically indicate
areas of hard ocean floor that are overlain by just a thin layer of soft pelagic sediment. Although it is not possible to compare the absolute and relative values in the backscatter data (Masetti et al., 2011), this technique can be used to identify areas of volcanic activity because the reflectivity of a volcano is typically more than three times that of the surrounding abyssal plain covered by a thick layer of fine-grained pelagic sediment in approximately the same angle of swath (e.g., Hirano et al., 2008). Sidescan data in multibeam echo sounder (MBES) are obtained by using the differing amplitudes of the echoes returned from the bottom by the various proprietary systems. However, it is common for this information not to be clearly documented by the system manufacturers. Sidescan data of most MBESs are not generally calibrated. Therefore, it is difficult to make a common scale of sidescan data recorded by different MBESs. We focus here on the relative magnitude of the sidescan data in a single track to find potential submarine volcanoes. Consequently, we did not make a common scale of sidescan data obtained by different MBESs. In this study, we used multibeam bathymetry data from French Polynesia obtained during four research cruises. The R/V Mirai cruise (MR08-06 Leg 1) covered the area from south of the Tuamotu Islands to the eastern Austral Islands in the southern Pacific Ocean, and was undertaken by JAMSTEC. The survey conditions during collection of the bathymetry data during MR08-06 Leg 1 were reported by Abe et al. (2013). Data from around the Marquesas Islands in northern French Polynesia were collected during cruises by the R/V Melville (PANR06MV and WEST13MV) and the R/V Roger Revelle (AMAT03RR), and are archived in the Geological Data Center at the Scripps Institution of Oceanography and NCEI/NOAA, respectively. The R/V Mirai and R/V Melville were equipped with a SeaBeam 2112 multibeam system with 151 beams and a SeaBeam 2000 multibeam system with 121 beams. The swath width and beam width of both systems were 120° from 4000 to 4500 m and 2° × 2° (beam interval = 1°), respectively. Their footprint of 2° × 2° at a depth of 4000 m covers an area of roughly 140 × 140 m (4000 m × tan 2°). The R/V Roger Revelle, on the other hand, was equipped with a Kongsberg EM120 multibeam system with 191 beams. The swath width and beam width were 150° and 1° × 2°
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Fig. 2. Shipboard a) bathymetric and b) reflectivity maps for the area to the east of the Marquesas Islands, in the northeastern part of Fig. 1. The fracture zone is shown by solid lines (Matthews et al., 2011). Young/old edifices are indicated by pink and black dotted circles, respectively.
(beam interval = 1°), respectively. The footprint of a 1° × 2° beam at a depth of 4000 m is roughly 70 × 140 m. The individual beam widths for each cruise were dependent on the depth and swath angle. The software used to process the swath data was the MB-System (Caress and Chayes, 1996), and the bathymetric and sidescan data were processed according to the recommendations in the MB-System manual. We reconstructed the bathymetry directly from the travel time data by full ray-tracing through a sound velocity profile calculated from the temperature and salinity data contained in the 1982 Climatological Atlas of the World Ocean (Levitus, 1982) and using the Del Grosso equations (Del Grosso, 1974). We then conducted a manual bathymetry editing process using an interactive graphical utility (mbedit) within the MB-System. After manual editing, bathymetric grids were produced at a resolution of 300 × 300 m. To correct the sidescan data for the variation of the grazing angle, we calculated the grazing angles for each sidescan pixel taking into account seafloor slopes and then generated tables of the average sidescan values in the swath sonar data as a
function of the grazing angle. Finally, the sidescan grids were produced with a cell interval of 50 m. Next, we generated tables of the average sidescan values in the swath sonar data as a function of the grazing angle with the seafloor using the mbbackangle command of MBsystem. The tables were used to correct the sidescan data for the variations in grazing angle. We used Generic Mapping Tools (Wessel and Smith, 1998) to plot the datasets and produce wide-swath contour maps and acoustic backscatter images of the seafloor. 4. Results The bathymetric data and acoustic reflectivities are shown in Figs. 2–6. The center of the ships' tracks in the sidescan images is all significantly distorted because the survey coincides with the artificially high-intensity center beam that is always observed in the middle of a ship's track, and which represents the most direct reflections collected vertically from beneath the ship (Figs. 2b, 3a, c, 4b,c, 5c,d,e,f, and 6b,c).
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Fig. 3. Shipboard a), c) reflectivity and b) bathymetric maps obtained for the area to the south of the Marquesas Islands, in the northern part of the area shown in Fig. 1. The locations of old and young edifices are shown by black-dotted and pink-dotted circles, respectively, and these were identified from the acoustic reflectivity data. Solid lines indicate a fracture zone (Matthews et al., 2011). Some small knolls of 1.6–2.4 km in diameter are potential young volcanoes in potential flood lavas. The numbering of the knolls is listed in Table 1.
The data show some bathymetric highs related to ridges, seamounts, and knolls. The elongated ridge and/or seamounts are surrounded by steep slopes that are more than 1000 m high, many of which seem to be distributed along the azimuth of fracture zones (Figs. 2–4). The knolls described in this study are of various sizes; i.e., 0.8 to 11 km in diameter and a few tens of meters to 600 m in height (Figs. 2–6), some of which
form clusters (Figs. 5 and 6). The sizes of representative knolls are listed in Table 1. The multibeam surveys allowed us to identify the regionally high backscatter images from the south and east of the Marquesas hotspot (Figs. 2–4), the eastern tip of the Pukapuka Ridges (Fig. 5), and southeast and offshore from the Macdonald hotspot (Fig. 6), although the
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Fig. 4. Shipboard a) bathymetric and b) reflectivity maps for the area south of the Marquesas Islands (northern part of the area shown in Fig. 1). The area highlighted in blue in b) is enlarged in c) showing representative ranges of absolute reflectivity values. The locations of old and young edifices are shown by black-dotted and pink-dotted circles, respectively, and these were identified using the acoustic reflectivity data. Solid lines indicate the fracture zone (Matthews et al., 2011). Potential young knolls are 3.2–4.8 km in diameter. The numbering of the knolls is shown in Table 1.
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Fig. 5. Shipboard a), b) bathymetric and c), d) reflectivity maps for the area at the western tip of Pukapuka Ridges, in the northwestern part of the area shown in Fig. 1. Bathymetric maps, a) and b), are same figures. The area highlighted by the blue square in c) and d) is enlarged in e) and f), and shows representative ranges of absolute reflectivity values. Typical examples of old edifices showing low reflectivities on the flat tops of knolls are indicated as A–F in c) and f). The locations of old and young edifices are shown by the black-dotted and pink-dotted circles, respectively. Potential young knolls are 1.9–5.7 km in diameter. The numbering of the knolls is listed in Table 1.
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Fig. 6. Shipboard a) bathymetric and b) reflectivity maps for the area to the southeast of the Macdonald hotspot, in southern part of the area shown in Fig. 1. The area outlined by the blue square in b) is enlarged in c), and shows representative ranges of absolute reflectivity values. The representative locations of old and young edifices are shown by black and pink circles and ellipses, respectively. Some high reflectivities are evident on steep slopes and mounds in areas outside of the circles. Some knolls might be piled up at the center of the map. Potential young knolls are 0.8–3.9 km in diameter. The young and old edifices are indicated by pink and black dotted circles, respectively. The numbering of the knolls is listed in Table 1.
acoustic reflectivity data did not always correspond to the bathymetry, particularly in Figs. 2 and 6. Most of the high acoustic reflectivities corresponded to knolls and other topographic highs. However, Figs. 2–4 show high acoustic reflectivities that are widely distributed and appear to paint out the seafloor and avoid any topographic highs. The rest of the backscatter images along the cruise tracks of MR08-06,
Table 1 Sizes of representative knolls. Map
Knoll#
Diameter (km)
Height (km)
Judgement
Fig. 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
9.3 1.5 4.7 4.4 5.0 3.7 4.0 3.7 11 1.6 1.3 5.1 2.7 2.4 3.5 8.3 4.0 1.3 3.0 5.6 6.0 1.7 2.5 0.83 2.1 2.9 3.5 3.1 2.5
0.6 0.2 0.2 0.3 0.3 0.2 0.2 0.4 0.3 0.05 0.05 0.2 0.1 0.1 0.2 0.7 0.2 0.05 0.08 0.3 0.1 0.04 0.1 0.04 0.03 0.07 0.09 0.1 0.2
Old Young Old Young Old Young Young Old Old Young Young Young Young Young Young Old ? Old Young Young Old Young ? Young Young Old Old ? Old
Fig. 4
Fig. 5
Fig. 6
PANR06MV, WEST13MV, and AMAT03RR in French Polynesia showed low reflectivities on the abyssal plain or gentle slopes.
5. Discussion The sidescan images of some of the knolls show high reflectivity because they are covered by a much thinner layer of soft pelagic sediment than on the surrounding abyssal plain. However, such a highly reflective area of seafloor does not necessarily indicate a “hard” seafloor (such as lava), because a slope towards the center of a ship's track would generate artificially high acoustic reflectivity despite being covered with a thick layer of pelagic sediment. As a steep slope is usually covered with a smaller thickness of sediment than the surrounding abyssal plain, it is impossible to directly compare the acoustic reflectivities of both the steep slope and the abyssal plain. Excluding areas with steep slopes, the data that mainly show high acoustic reflectivity from the ocean floor or knolls can be explained in terms of three geological possibilities: the presence of lava flows (Hirano et al., 2008; 2013), a submarine landslide (Moore et al., 1989; Moore and Clague, 2002; Smith et al., 2002a, 2002b), or a field of manganese nodules (Machida et al., submitted for publication). In the case of manganese nodules, the reflectivity would be around three times as high as the normal range of reflectivity values expected from an ocean floor covered with pelagic sediment (de Moustier, 1985), but it would still be lower than the reflectivity from a petitspot lava field (Hirano et al., 2008). Submarine landslides found off the Hawaiian Islands are fine examples of the other possible causes of high acoustic reflectivity (Moore et al., 1989; Moore and Clague, 2002; Smith et al., 2002a, 2002b). The field of debris at the eastern base of the submarine Hana volcanic ridge is acoustically weaker than the volcanic field on Hana ridge (Smith et al., 2002a). Huge blocks of debris, up to a maximum of 40 km in diameter, characterize the Nuuanu and Wailau landslides offshore from northern Molokai Island (Moore and Clague, 2002), and these blocks have a variety of sizes, as well as irregular and angular shapes, and have been carried as far as 170 km to the north of the island.
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Although the blocks in the South Kauai debris avalanche are smaller (maximum of 8 km in diameter) than those north of Molokai, they are widely spread over more than 100 km to the south of Kauai Island. In both cases, the landslide areas are characterized by an undulating abyssal plain (Smith et al., 2002a). As for the study areas in French Polynesia, it is impossible to recognize homogeneously distributed high reflectivities on the seafloor (Figs. 2–4), as submarine landslide fields show highly speckled reflectivity in the field (Smith et al., 2002a). Furthermore, many of the knolls showing high acoustic reflectivity in Figs. 2–6 can reasonably be interpreted as submarine lava fields because they are smaller (most are a few kilometers in diameter; Table 1) and rounder in shape than the submarine landslide blocks around the Hawaiian Islands. To determine whether the volcanic knolls are relatively young or old, we used the reflectivity data from the knolls as an index of the submarine exposure of the lavas (Figs. 3–6). Although it is impossible to compare both the steep slope and the abyssal plain as mentioned before, we assessed whether lava is exposed by comparing the data of the flat tops of knolls with the data of the surrounding abyssal plain. We then assumed that the rates of sedimentation are fairly similar on the top terrace of the knolls and also on the surrounding abyssal plain in this area. In the absence of flat tops or gentle slopes on the knolls, it is impossible to determine the duration of times for which the lavas have been exposed. We found some good examples of young and old edifices (marked with pink and black dotted circles, respectively, in Figs. 3–6). All of the acoustically young knolls are small (b5 km in diameter), which is in contrast to some of the larger and older knolls that are ~10 km in diameter (Table 1).
5.2. Hotspot or petit-spot?
5.1. Distributions of submarine volcanoes
5.3. Implications for mantle structures unrelated to hotspots
We classified the knolls at 138°13′W in Fig. 4 and knolls A–E in Fig. 5e and f as “old” edifices because the top terrace of the knolls showed low reflectivities. Although the data does not show their formation ages, we were only able to tentatively identify the oldest knolls in the area (e.g., at 138°13′W in Fig. 4 and knoll B in Fig. 5e), as the reflectivity values of their top terraces are similar to those of the surrounding abyssal plain (Figs. 4c and 5e), and the knolls have a similar thickness of pelagic sediment as the surrounding abyssal plain. In the case of knoll 17 at 13°35′S and 141°53′W, it was difficult to determine whether this was a young or old edifice because the distribution of high reflectivity does not correspond to bathymetry (Fig. 5b and d). A volcanic cluster at the eastern tip of the Pukapuka Ridges comprises both old and young volcanic cones (Fig. 5). Furthermore, in the southeastern area of the Macdonald hotspot (Fig. 6), a few young volcanic cones have erupted in clusters of pre-existing knolls, indicating the presence of older edifices here as well as in the area depicted in Fig. 5. It would be geologically impossible to explain the edifices in terms of a single landslide event. Despite having high acoustic reflectivities, some of these areas are not associated with any gradients in topography; they are sparsely distributed, and form a smear on the abyssal plain around terraces and knolls to the south and east of the Marquesas hotspot (Figs. 2–4), and their homogeneously distributed high reflectivity is fairly similar to those of areas of submarine flood lavas rather than debris fields showing heterogeneous reflectivities in the fields (e.g., the North Arch Volcanic Field; Clague et al., 2002; the debris fields; Smith et al., 2002a). This possibly provides evidence for flood lavas on the abyssal plain near the Marquesas Fracture Zone, now covered by a thin layer of pelagic sediment. Lava flows appear to extend to the east of the trough along the fracture zone, given the gentle upslope (~3.5‰) to the west, which distribute along the topographic lows surrounding topographic highs in Fig. 2. Likewise, some of the abyssal plains have high acoustic reflectivities to the south of the Marquesas Fracture Zone (Figs. 3 and 4), indicating the presence of flood lavas; the adjacent topographic swells and terraces correspond to the relatively low-intensity acoustic reflectivity data.
The SeaBeam survey of the N.O. Jean Charcot and the GRORIA survey of the R.R.S. Charles Darwin in the late 1980s both reported areas of high acoustic reflectivity east-southeast off Tahiti Island at around 20°S, 143°E (small circle on Fig. 1), indicating the presence of lava flows that are some distance from any hotspot (Searle et al., 1995). Elevated temperatures, due to the combination of a mantle plume and a thin lithosphere, may have been responsible for an eruption of sublithospheric magma (Searle et al., 1995). Recently, Suetsugu et al. (2009) described the tomography of the mantle below French Polynesia. They described narrow areas of slow anomalies in the shallow mantle that ascend from the mantle transition zone (410 to 660 km depth) to the surfaces of the hotspots. The hotspots would not necessarily be connected to the South Pacific Superplume, detected in the lower mantle (e.g., Marquesas, Pitcarn, and Arago). The areas of possible submarine volcanoes, detected in the present study and also by Searle et al. (1995), are located between the French Polynesian hotspots, and they are mostly found in areas with low seismic velocities at depths of 60 and 100 km (Suetsugu et al., 2009). As mentioned above, submarine intraplate volcanoes that are not associated with a hotspot are considered to form as a result of regional mantle convection around a plume (Bianco et al., 2005), lithospheric deformation near an island or trench (Bianco et al., 2005; Hirano et al., 2006), “leaky” volcanism along a fracture zone (Searle et al., 1995), or in a fracture zone that passes near a plume (Yamamoto and Morgan, 2009). All these mechanisms might possibly be responsible for the intraplate volcanoes of the French Polynesian region. For example, marine gravity surveys using satellite altimetry show remarkable “moats” due to seamount loading around the Marquesas and Tuamotu islands (Sandwell and Smith, 2009), and the French Polynesian region contains three major fracture zones (Fig. 1). We still have very limited multibeam data as shown in Fig. 1. Future studies, including research cruises, are needed to examine more closely the distribution and geochemistry of the submarine lavas, and to clarify and map the geochemical characteristics of the mantle that overlies the South Pacific Superplume in the French Polynesian region.
All of the young knolls identified in this study were between 0.8 and 5.7 km in diameter (Figs. 3–6 and Table 1) except for the flood lavas distributed over several tens of kilometers in Figs. 2–4. The sizes are small when compared with other recently discovered submarine volcanoes around French Polynesia (e.g., 50 km in diameter for the Vailulu'u seamount, and ~18 km in diameter for the Adams and Bounty seamounts; Searle, 1983; Hart et al., 2000; Hekinian et al., 2003). Submarine flood lavas and volcanic cones have been reported over an area of approximately 25,000 km2 in the North Arch volcanic field on the flexural arch north of Oahu Island, Hawaii (Clague et al., 1990). The clusters of young submarine volcanoes reported by Hirano et al. (2006) on a flexural outer-rise erupted as monogenetic volcanoes over the period from 0 to 9 Ma (Hirano et al., 2008; Hirano, 2011; Machida et al., 2015). It seems clear that the volcanic cones did not erupt simultaneously because the acoustic reflectivities of individual cones vary markedly (Figs. 4–6), even within a single cluster, implying a complex of young and old volcanic edifices. The sizes of these volcanic edifices are on the same order of magnitude as the sizes of petit-spot volcanoes, but differ to the hotspot seamounts and off-ridge volcanoes near the East Pacific Rise (Hirano et al., 2008). As they are dissimilar to active and zero-aged hotspots, it is intriguing to speculate about their origins and eruptions, but we must wait for future sampling and surveys to determine their detailed distribution, lava composition, and absolute ages.
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6. Summary and conclusions In the French Polynesian region, newly discovered potential submarine volcanoes take the form of individual volcanic cones, clustered volcanic cones, and flood lavas. A number of possible young volcanic cones and flood lavas were detected to the south and east of the Marquesas hotspot, western tip of the Pukapuka Ridges, and southeast of the Macdonald seamount. The sidescan imagery of some volcanic edifices shows a high reflectivity where these young lavas are covered with much thinner layer of soft pelagic sediment than that of the normal Pacific Plate. In fact, the reflectivity values are more than three times as high as in the surrounding abyssal plain, excluding those areas that have steep slopes (Hirano et al., 2008). Some areas of high acoustic reflectivity do not contain obvious volcanic edifices, and these may represent areas of flood lava, sparsely distributed around terraces and knolls near the Marquesas Fracture Zone. The volcanic clusters are made up of both young and old volcanic cones, and they may be similar to the tiny submarine volcanoes (i.e., petit-spot volcanoes; Hirano et al., 2006). Otherwise, newly detected young lavas and cones may possibly have developed from melts that rose from the low-velocity part of the shallow mantle (Suetsugu et al., 2009), which occupies some of the space between the hotspots in the French Polynesian region. Our understanding of the distribution of these small volcanoes would be enhanced by further investigations and rock sampling, and such work should provide us with information about the geochemical structure of the Southern Pacific Superplume.
Acknowledgments We thank Prof A.A.P. Koppers and Prof H. Staudigel, and Mrs. J. Perez and B. Miyahara from the Scripps Institution of Oceanography for their assistance with processing the shipboard data stored at the Geological Data Center at Scripps Institution of Oceanography (http://gdc.ucsd. edu/). The crew and scientific party of the R/V Mirai cruise MR08-06 are also thanked for providing multibeam data. This study was funded in part by the Toray Science and Technology Grant (#11-5208) and by the Japan Society for the Promotion of Science (#24654180).
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Contents lists available at ScienceDirect
Marine Geology journal homepage: www.elsevier.com/locate/margo
Submarine lava fields in French Polynesia Naoto Hirano a,⁎, Masao Nakanishi b, Natsue Abe c, Shiki Machida c a b c
Center for Northeast Asian Studies, Tohoku University, Kawauchi 41, Aoba-ku, Sendai 980-8576, Japan Graduate School of Science, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Japan Agency for Marine-Earth Science and Technology: Natsushima-cho 2-15, Yokosuka 237-0061, Japan
a r t i c l e
i n f o
Article history: Received 16 April 2015 Received in revised form 16 December 2015 Accepted 1 January 2016 Available online 6 January 2016 Keywords: Hotspot Petit-spot French Polynesia Flood lava Volcanic cone Submarine volcano
a b s t r a c t Shipboard multibeam survey is powerful tool to locate submarine volcanoes especially having small volume. Small submarine volcanoes may represent the initial stages of hotspot activity, but they may also form via lithospheric flexing, regional convection of the mantle, and the presence of fracture zones. Here we describe several volcanoes, flood lavas, and volcanic clusters in French Polynesia using data from archives of multibeam data. The clusters of small volcanoes are similar to petit-spots, and they are not considered to represent the initial stages of a hotspot as they are composed of both young and old edifices, and because the sites are located far from any known hotspot. These newly discovered submarine volcanoes are located in areas with low-velocity seismic shear waves at depths of 60 and 100 km. These lava fields will therefore facilitate geochemical mapping of the mantle in areas unrelated to hotspots, because these lavas may have developed from melts in the shallow mantle beneath French Polynesia. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Hotspots are believed to develop over stationary plumes that rise from the deep mantle beneath tectonic plates (Morgan, 1971), although other explanations for their origin have been proposed, such as antiplumes from the deep mantle. These hotspots lead to the formation of lines of seamounts and/or oceanic islands with a progressive increase in age along the direction of plate motion. However, the bathymetric characteristics and radiometric dating of seamounts are not always consistent with this simple model of a hotspot trail (Clouard and Bonneville, 2001). The Hawaiian hotspot on the Pacific plate has existed for more than 70 Myr, but its position below the plate has not remained stationary in the mantle (Tarduno et al., 2003), whereas the Louisville hotspot has been nearly stationary for 70 Myr (Koppers et al., 2012). Most seamount trails that formed in the western Pacific during the Cretaceous show short-lived progressions in age (e.g., a few millions or a few tens of millions of years; Koppers et al., 2003). The initial phase of a hotspot volcano may be represented by small submarine volcanoes that form prior to the development of a large, possibly subaerial volcano over a hotspot, and indeed, such activity has been observed on the Loihi Seamount in the Hawaiian chain (e.g., Moore et al., 1979), the Macdonald Seamount in the Austral chain (e.g., Johnson, 1970), the Adams Seamount in the Pitcairn chain (Devey et al., 2003), and the Vailulu'u Seamount in the Samoan chain (e.g., Hart et al., 2000). Determining the location of submarine volcanoes ⁎ Corresponding author. E-mail address: [email protected] (N. Hirano).
http://dx.doi.org/10.1016/j.margeo.2016.01.002 0025-3227/© 2016 Elsevier B.V. All rights reserved.
is therefore critical to identifying possible hotspots. In addition to hotspot-related volcanoes, other new and unexpected kinds of volcano, such as petit-spot volcanoes and arch lavas, have been reported from the submarine environment, and these were first discovered on the subducting Pacific plate off northeastern Japan and on the flexural Hawaiian arch 300–500 km from the Hawaiian Islands, respectively (Holcomb et al., 1988; Hirano et al., 2006). These discoveries relied on the use of submarine acoustic surveys, which are vital for such work. Hotspot geochronology and the distribution of volcanic activity have been recorded from subaerial lavas on the islands of French Polynesia, as well as from a few submarine lavas. Further exploration of the submarine volcanoes in the French Polynesia region is vital if we are to elucidate the hotspot geology and mantle structure below this area. 2. Regional setting In French Polynesia, active hotspots have been identified around the Society, Marquesas, Macdonald, and Pitcairn islands (Fig. 1). The hotspots correspond to the sites of zero-aged volcanoes which may be seated on hotspot trails arising from the deep mantle (McNutt and Fischer, 1987). The age progression characteristics of both the Society and Pitcairn hotspots (along the Society Islands and the Pitcairn– Gambier island axes, respectively) are well known (Gripp and Gordon, 2002). However, along the Cook–Austral seamount trail, the age progression is complicated, because several zero-aged volcanoes occur at loci of magmatism that are thousands of kilometers from the reconstructed positions of the Macdonald Seamount hotspot (Johnson, 1970). Some seamounts along the chain were either rejuvenated or
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Fig. 1. Bathymetric map of French Polynesia (left) and shipboard bathymetry (right) for the region indicated by the large square in the left panel. Bathymetric data based on satellite gravity measurements are from Smith and Sandwell (1997). Pink stars correspond to sites of stationary hotspots identified from zero-aged (active) volcanoes (Clouard and Bonneville, 2001). Major fracture zones shown by solid lines in the right panel are truncated in the French Polynesian region (Matthews et al., 2011). The red lines in the right panel show the paleomagnetic isochron (Cande et al., 1989). The yellow circle is the potential site of the submarine lavas described by Searle et al. (1995).
resumed volcanism as a result of multiple overlapping volcanoes that erupted during the Quaternary (Dickinson, 1998; Bonneville et al., 2002). Most studies on the geochronological characteristics of these chains have been restricted to subaerial lavas, even though the hotspots are actually located below the ocean at the southeastern extensions of their associated seamount chains (e.g., Cheminee et al., 1989; Binard et al., 1991; Hekinian et al., 1991; Bonneville et al., 2002; Hekinian et al., 2003). However, dating of submarine samples from the Nelson Bank has not helped to explain the hotspot model along the Austral Islands (Bonneville et al., 2006). We note, too, that tiny submarine eruptions of lava do exist in submarine French Polynesia, as described by Searle et al. (1995) on the basis of data from the GLORIA towed sidescan (the location of the survey is indicated by the small solid circle in Fig. 1). 3. Methods The spatial resolution of the bathymetric information derived from satellite gravity data is too low to use in the detection of very small volcanic edifices less than 2–3 km in diameter. In addition, the extent of the areas covered by sonar surveys of the ocean floor is limited. The multibeam data shown on the right of Fig. 1 were obtained mainly from the databases of the Scripps Institution of Oceanography (http:// siox.sdsc.edu/search.php) and the National Centers for Environmental Information, National Oceanic and Atmospheric Administration (NCEI/ NOAA, http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html). Other data in Fig. 1 were obtained from two cruises of the R/V Mirai (MR08-06 and MR09-01), run by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We selected data from four cruises (MR08-06, PANR06MV, WEST13MV, and AMAT03RR; Figs. 2–6) from all of the available databases (Fig. 1) in order to make discussions in this paper. To focus on small submarine volcanoes, we excluded data obtained by multibeam echo sounder system without the acoustic backscatter function. During detailed multibeam surveys of the ocean floor, areas identified by acoustic backscatter as having a high acoustic reflectivity typically indicate
areas of hard ocean floor that are overlain by just a thin layer of soft pelagic sediment. Although it is not possible to compare the absolute and relative values in the backscatter data (Masetti et al., 2011), this technique can be used to identify areas of volcanic activity because the reflectivity of a volcano is typically more than three times that of the surrounding abyssal plain covered by a thick layer of fine-grained pelagic sediment in approximately the same angle of swath (e.g., Hirano et al., 2008). Sidescan data in multibeam echo sounder (MBES) are obtained by using the differing amplitudes of the echoes returned from the bottom by the various proprietary systems. However, it is common for this information not to be clearly documented by the system manufacturers. Sidescan data of most MBESs are not generally calibrated. Therefore, it is difficult to make a common scale of sidescan data recorded by different MBESs. We focus here on the relative magnitude of the sidescan data in a single track to find potential submarine volcanoes. Consequently, we did not make a common scale of sidescan data obtained by different MBESs. In this study, we used multibeam bathymetry data from French Polynesia obtained during four research cruises. The R/V Mirai cruise (MR08-06 Leg 1) covered the area from south of the Tuamotu Islands to the eastern Austral Islands in the southern Pacific Ocean, and was undertaken by JAMSTEC. The survey conditions during collection of the bathymetry data during MR08-06 Leg 1 were reported by Abe et al. (2013). Data from around the Marquesas Islands in northern French Polynesia were collected during cruises by the R/V Melville (PANR06MV and WEST13MV) and the R/V Roger Revelle (AMAT03RR), and are archived in the Geological Data Center at the Scripps Institution of Oceanography and NCEI/NOAA, respectively. The R/V Mirai and R/V Melville were equipped with a SeaBeam 2112 multibeam system with 151 beams and a SeaBeam 2000 multibeam system with 121 beams. The swath width and beam width of both systems were 120° from 4000 to 4500 m and 2° × 2° (beam interval = 1°), respectively. Their footprint of 2° × 2° at a depth of 4000 m covers an area of roughly 140 × 140 m (4000 m × tan 2°). The R/V Roger Revelle, on the other hand, was equipped with a Kongsberg EM120 multibeam system with 191 beams. The swath width and beam width were 150° and 1° × 2°
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Fig. 2. Shipboard a) bathymetric and b) reflectivity maps for the area to the east of the Marquesas Islands, in the northeastern part of Fig. 1. The fracture zone is shown by solid lines (Matthews et al., 2011). Young/old edifices are indicated by pink and black dotted circles, respectively.
(beam interval = 1°), respectively. The footprint of a 1° × 2° beam at a depth of 4000 m is roughly 70 × 140 m. The individual beam widths for each cruise were dependent on the depth and swath angle. The software used to process the swath data was the MB-System (Caress and Chayes, 1996), and the bathymetric and sidescan data were processed according to the recommendations in the MB-System manual. We reconstructed the bathymetry directly from the travel time data by full ray-tracing through a sound velocity profile calculated from the temperature and salinity data contained in the 1982 Climatological Atlas of the World Ocean (Levitus, 1982) and using the Del Grosso equations (Del Grosso, 1974). We then conducted a manual bathymetry editing process using an interactive graphical utility (mbedit) within the MB-System. After manual editing, bathymetric grids were produced at a resolution of 300 × 300 m. To correct the sidescan data for the variation of the grazing angle, we calculated the grazing angles for each sidescan pixel taking into account seafloor slopes and then generated tables of the average sidescan values in the swath sonar data as a
function of the grazing angle. Finally, the sidescan grids were produced with a cell interval of 50 m. Next, we generated tables of the average sidescan values in the swath sonar data as a function of the grazing angle with the seafloor using the mbbackangle command of MBsystem. The tables were used to correct the sidescan data for the variations in grazing angle. We used Generic Mapping Tools (Wessel and Smith, 1998) to plot the datasets and produce wide-swath contour maps and acoustic backscatter images of the seafloor. 4. Results The bathymetric data and acoustic reflectivities are shown in Figs. 2–6. The center of the ships' tracks in the sidescan images is all significantly distorted because the survey coincides with the artificially high-intensity center beam that is always observed in the middle of a ship's track, and which represents the most direct reflections collected vertically from beneath the ship (Figs. 2b, 3a, c, 4b,c, 5c,d,e,f, and 6b,c).
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Fig. 3. Shipboard a), c) reflectivity and b) bathymetric maps obtained for the area to the south of the Marquesas Islands, in the northern part of the area shown in Fig. 1. The locations of old and young edifices are shown by black-dotted and pink-dotted circles, respectively, and these were identified from the acoustic reflectivity data. Solid lines indicate a fracture zone (Matthews et al., 2011). Some small knolls of 1.6–2.4 km in diameter are potential young volcanoes in potential flood lavas. The numbering of the knolls is listed in Table 1.
The data show some bathymetric highs related to ridges, seamounts, and knolls. The elongated ridge and/or seamounts are surrounded by steep slopes that are more than 1000 m high, many of which seem to be distributed along the azimuth of fracture zones (Figs. 2–4). The knolls described in this study are of various sizes; i.e., 0.8 to 11 km in diameter and a few tens of meters to 600 m in height (Figs. 2–6), some of which
form clusters (Figs. 5 and 6). The sizes of representative knolls are listed in Table 1. The multibeam surveys allowed us to identify the regionally high backscatter images from the south and east of the Marquesas hotspot (Figs. 2–4), the eastern tip of the Pukapuka Ridges (Fig. 5), and southeast and offshore from the Macdonald hotspot (Fig. 6), although the
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Fig. 4. Shipboard a) bathymetric and b) reflectivity maps for the area south of the Marquesas Islands (northern part of the area shown in Fig. 1). The area highlighted in blue in b) is enlarged in c) showing representative ranges of absolute reflectivity values. The locations of old and young edifices are shown by black-dotted and pink-dotted circles, respectively, and these were identified using the acoustic reflectivity data. Solid lines indicate the fracture zone (Matthews et al., 2011). Potential young knolls are 3.2–4.8 km in diameter. The numbering of the knolls is shown in Table 1.
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Fig. 5. Shipboard a), b) bathymetric and c), d) reflectivity maps for the area at the western tip of Pukapuka Ridges, in the northwestern part of the area shown in Fig. 1. Bathymetric maps, a) and b), are same figures. The area highlighted by the blue square in c) and d) is enlarged in e) and f), and shows representative ranges of absolute reflectivity values. Typical examples of old edifices showing low reflectivities on the flat tops of knolls are indicated as A–F in c) and f). The locations of old and young edifices are shown by the black-dotted and pink-dotted circles, respectively. Potential young knolls are 1.9–5.7 km in diameter. The numbering of the knolls is listed in Table 1.
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Fig. 6. Shipboard a) bathymetric and b) reflectivity maps for the area to the southeast of the Macdonald hotspot, in southern part of the area shown in Fig. 1. The area outlined by the blue square in b) is enlarged in c), and shows representative ranges of absolute reflectivity values. The representative locations of old and young edifices are shown by black and pink circles and ellipses, respectively. Some high reflectivities are evident on steep slopes and mounds in areas outside of the circles. Some knolls might be piled up at the center of the map. Potential young knolls are 0.8–3.9 km in diameter. The young and old edifices are indicated by pink and black dotted circles, respectively. The numbering of the knolls is listed in Table 1.
acoustic reflectivity data did not always correspond to the bathymetry, particularly in Figs. 2 and 6. Most of the high acoustic reflectivities corresponded to knolls and other topographic highs. However, Figs. 2–4 show high acoustic reflectivities that are widely distributed and appear to paint out the seafloor and avoid any topographic highs. The rest of the backscatter images along the cruise tracks of MR08-06,
Table 1 Sizes of representative knolls. Map
Knoll#
Diameter (km)
Height (km)
Judgement
Fig. 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
9.3 1.5 4.7 4.4 5.0 3.7 4.0 3.7 11 1.6 1.3 5.1 2.7 2.4 3.5 8.3 4.0 1.3 3.0 5.6 6.0 1.7 2.5 0.83 2.1 2.9 3.5 3.1 2.5
0.6 0.2 0.2 0.3 0.3 0.2 0.2 0.4 0.3 0.05 0.05 0.2 0.1 0.1 0.2 0.7 0.2 0.05 0.08 0.3 0.1 0.04 0.1 0.04 0.03 0.07 0.09 0.1 0.2
Old Young Old Young Old Young Young Old Old Young Young Young Young Young Young Old ? Old Young Young Old Young ? Young Young Old Old ? Old
Fig. 4
Fig. 5
Fig. 6
PANR06MV, WEST13MV, and AMAT03RR in French Polynesia showed low reflectivities on the abyssal plain or gentle slopes.
5. Discussion The sidescan images of some of the knolls show high reflectivity because they are covered by a much thinner layer of soft pelagic sediment than on the surrounding abyssal plain. However, such a highly reflective area of seafloor does not necessarily indicate a “hard” seafloor (such as lava), because a slope towards the center of a ship's track would generate artificially high acoustic reflectivity despite being covered with a thick layer of pelagic sediment. As a steep slope is usually covered with a smaller thickness of sediment than the surrounding abyssal plain, it is impossible to directly compare the acoustic reflectivities of both the steep slope and the abyssal plain. Excluding areas with steep slopes, the data that mainly show high acoustic reflectivity from the ocean floor or knolls can be explained in terms of three geological possibilities: the presence of lava flows (Hirano et al., 2008; 2013), a submarine landslide (Moore et al., 1989; Moore and Clague, 2002; Smith et al., 2002a, 2002b), or a field of manganese nodules (Machida et al., submitted for publication). In the case of manganese nodules, the reflectivity would be around three times as high as the normal range of reflectivity values expected from an ocean floor covered with pelagic sediment (de Moustier, 1985), but it would still be lower than the reflectivity from a petitspot lava field (Hirano et al., 2008). Submarine landslides found off the Hawaiian Islands are fine examples of the other possible causes of high acoustic reflectivity (Moore et al., 1989; Moore and Clague, 2002; Smith et al., 2002a, 2002b). The field of debris at the eastern base of the submarine Hana volcanic ridge is acoustically weaker than the volcanic field on Hana ridge (Smith et al., 2002a). Huge blocks of debris, up to a maximum of 40 km in diameter, characterize the Nuuanu and Wailau landslides offshore from northern Molokai Island (Moore and Clague, 2002), and these blocks have a variety of sizes, as well as irregular and angular shapes, and have been carried as far as 170 km to the north of the island.
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Although the blocks in the South Kauai debris avalanche are smaller (maximum of 8 km in diameter) than those north of Molokai, they are widely spread over more than 100 km to the south of Kauai Island. In both cases, the landslide areas are characterized by an undulating abyssal plain (Smith et al., 2002a). As for the study areas in French Polynesia, it is impossible to recognize homogeneously distributed high reflectivities on the seafloor (Figs. 2–4), as submarine landslide fields show highly speckled reflectivity in the field (Smith et al., 2002a). Furthermore, many of the knolls showing high acoustic reflectivity in Figs. 2–6 can reasonably be interpreted as submarine lava fields because they are smaller (most are a few kilometers in diameter; Table 1) and rounder in shape than the submarine landslide blocks around the Hawaiian Islands. To determine whether the volcanic knolls are relatively young or old, we used the reflectivity data from the knolls as an index of the submarine exposure of the lavas (Figs. 3–6). Although it is impossible to compare both the steep slope and the abyssal plain as mentioned before, we assessed whether lava is exposed by comparing the data of the flat tops of knolls with the data of the surrounding abyssal plain. We then assumed that the rates of sedimentation are fairly similar on the top terrace of the knolls and also on the surrounding abyssal plain in this area. In the absence of flat tops or gentle slopes on the knolls, it is impossible to determine the duration of times for which the lavas have been exposed. We found some good examples of young and old edifices (marked with pink and black dotted circles, respectively, in Figs. 3–6). All of the acoustically young knolls are small (b5 km in diameter), which is in contrast to some of the larger and older knolls that are ~10 km in diameter (Table 1).
5.2. Hotspot or petit-spot?
5.1. Distributions of submarine volcanoes
5.3. Implications for mantle structures unrelated to hotspots
We classified the knolls at 138°13′W in Fig. 4 and knolls A–E in Fig. 5e and f as “old” edifices because the top terrace of the knolls showed low reflectivities. Although the data does not show their formation ages, we were only able to tentatively identify the oldest knolls in the area (e.g., at 138°13′W in Fig. 4 and knoll B in Fig. 5e), as the reflectivity values of their top terraces are similar to those of the surrounding abyssal plain (Figs. 4c and 5e), and the knolls have a similar thickness of pelagic sediment as the surrounding abyssal plain. In the case of knoll 17 at 13°35′S and 141°53′W, it was difficult to determine whether this was a young or old edifice because the distribution of high reflectivity does not correspond to bathymetry (Fig. 5b and d). A volcanic cluster at the eastern tip of the Pukapuka Ridges comprises both old and young volcanic cones (Fig. 5). Furthermore, in the southeastern area of the Macdonald hotspot (Fig. 6), a few young volcanic cones have erupted in clusters of pre-existing knolls, indicating the presence of older edifices here as well as in the area depicted in Fig. 5. It would be geologically impossible to explain the edifices in terms of a single landslide event. Despite having high acoustic reflectivities, some of these areas are not associated with any gradients in topography; they are sparsely distributed, and form a smear on the abyssal plain around terraces and knolls to the south and east of the Marquesas hotspot (Figs. 2–4), and their homogeneously distributed high reflectivity is fairly similar to those of areas of submarine flood lavas rather than debris fields showing heterogeneous reflectivities in the fields (e.g., the North Arch Volcanic Field; Clague et al., 2002; the debris fields; Smith et al., 2002a). This possibly provides evidence for flood lavas on the abyssal plain near the Marquesas Fracture Zone, now covered by a thin layer of pelagic sediment. Lava flows appear to extend to the east of the trough along the fracture zone, given the gentle upslope (~3.5‰) to the west, which distribute along the topographic lows surrounding topographic highs in Fig. 2. Likewise, some of the abyssal plains have high acoustic reflectivities to the south of the Marquesas Fracture Zone (Figs. 3 and 4), indicating the presence of flood lavas; the adjacent topographic swells and terraces correspond to the relatively low-intensity acoustic reflectivity data.
The SeaBeam survey of the N.O. Jean Charcot and the GRORIA survey of the R.R.S. Charles Darwin in the late 1980s both reported areas of high acoustic reflectivity east-southeast off Tahiti Island at around 20°S, 143°E (small circle on Fig. 1), indicating the presence of lava flows that are some distance from any hotspot (Searle et al., 1995). Elevated temperatures, due to the combination of a mantle plume and a thin lithosphere, may have been responsible for an eruption of sublithospheric magma (Searle et al., 1995). Recently, Suetsugu et al. (2009) described the tomography of the mantle below French Polynesia. They described narrow areas of slow anomalies in the shallow mantle that ascend from the mantle transition zone (410 to 660 km depth) to the surfaces of the hotspots. The hotspots would not necessarily be connected to the South Pacific Superplume, detected in the lower mantle (e.g., Marquesas, Pitcarn, and Arago). The areas of possible submarine volcanoes, detected in the present study and also by Searle et al. (1995), are located between the French Polynesian hotspots, and they are mostly found in areas with low seismic velocities at depths of 60 and 100 km (Suetsugu et al., 2009). As mentioned above, submarine intraplate volcanoes that are not associated with a hotspot are considered to form as a result of regional mantle convection around a plume (Bianco et al., 2005), lithospheric deformation near an island or trench (Bianco et al., 2005; Hirano et al., 2006), “leaky” volcanism along a fracture zone (Searle et al., 1995), or in a fracture zone that passes near a plume (Yamamoto and Morgan, 2009). All these mechanisms might possibly be responsible for the intraplate volcanoes of the French Polynesian region. For example, marine gravity surveys using satellite altimetry show remarkable “moats” due to seamount loading around the Marquesas and Tuamotu islands (Sandwell and Smith, 2009), and the French Polynesian region contains three major fracture zones (Fig. 1). We still have very limited multibeam data as shown in Fig. 1. Future studies, including research cruises, are needed to examine more closely the distribution and geochemistry of the submarine lavas, and to clarify and map the geochemical characteristics of the mantle that overlies the South Pacific Superplume in the French Polynesian region.
All of the young knolls identified in this study were between 0.8 and 5.7 km in diameter (Figs. 3–6 and Table 1) except for the flood lavas distributed over several tens of kilometers in Figs. 2–4. The sizes are small when compared with other recently discovered submarine volcanoes around French Polynesia (e.g., 50 km in diameter for the Vailulu'u seamount, and ~18 km in diameter for the Adams and Bounty seamounts; Searle, 1983; Hart et al., 2000; Hekinian et al., 2003). Submarine flood lavas and volcanic cones have been reported over an area of approximately 25,000 km2 in the North Arch volcanic field on the flexural arch north of Oahu Island, Hawaii (Clague et al., 1990). The clusters of young submarine volcanoes reported by Hirano et al. (2006) on a flexural outer-rise erupted as monogenetic volcanoes over the period from 0 to 9 Ma (Hirano et al., 2008; Hirano, 2011; Machida et al., 2015). It seems clear that the volcanic cones did not erupt simultaneously because the acoustic reflectivities of individual cones vary markedly (Figs. 4–6), even within a single cluster, implying a complex of young and old volcanic edifices. The sizes of these volcanic edifices are on the same order of magnitude as the sizes of petit-spot volcanoes, but differ to the hotspot seamounts and off-ridge volcanoes near the East Pacific Rise (Hirano et al., 2008). As they are dissimilar to active and zero-aged hotspots, it is intriguing to speculate about their origins and eruptions, but we must wait for future sampling and surveys to determine their detailed distribution, lava composition, and absolute ages.
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6. Summary and conclusions In the French Polynesian region, newly discovered potential submarine volcanoes take the form of individual volcanic cones, clustered volcanic cones, and flood lavas. A number of possible young volcanic cones and flood lavas were detected to the south and east of the Marquesas hotspot, western tip of the Pukapuka Ridges, and southeast of the Macdonald seamount. The sidescan imagery of some volcanic edifices shows a high reflectivity where these young lavas are covered with much thinner layer of soft pelagic sediment than that of the normal Pacific Plate. In fact, the reflectivity values are more than three times as high as in the surrounding abyssal plain, excluding those areas that have steep slopes (Hirano et al., 2008). Some areas of high acoustic reflectivity do not contain obvious volcanic edifices, and these may represent areas of flood lava, sparsely distributed around terraces and knolls near the Marquesas Fracture Zone. The volcanic clusters are made up of both young and old volcanic cones, and they may be similar to the tiny submarine volcanoes (i.e., petit-spot volcanoes; Hirano et al., 2006). Otherwise, newly detected young lavas and cones may possibly have developed from melts that rose from the low-velocity part of the shallow mantle (Suetsugu et al., 2009), which occupies some of the space between the hotspots in the French Polynesian region. Our understanding of the distribution of these small volcanoes would be enhanced by further investigations and rock sampling, and such work should provide us with information about the geochemical structure of the Southern Pacific Superplume.
Acknowledgments We thank Prof A.A.P. Koppers and Prof H. Staudigel, and Mrs. J. Perez and B. Miyahara from the Scripps Institution of Oceanography for their assistance with processing the shipboard data stored at the Geological Data Center at Scripps Institution of Oceanography (http://gdc.ucsd. edu/). The crew and scientific party of the R/V Mirai cruise MR08-06 are also thanked for providing multibeam data. This study was funded in part by the Toray Science and Technology Grant (#11-5208) and by the Japan Society for the Promotion of Science (#24654180).
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