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Active-Source Seismic Survey on the Northeastern Hawaiian Arch

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Active-Source Seismic Survey on the Northeastern Hawaiian Arch Ohira et al. Earth, Planets and Space (2018) 70:121 https://doi.org/10.1186/s40623-018-0891-8 FULL PAPER Open Access Active‑source seismic survey on the northeastern Hawaiian Arch: insights into crustal structure and mantle refectors Akane Ohira1,2* , Shuichi Kodaira1,2, Gregory F. Moore3, Mikiya Yamashita1, Toshiya Fujiwara4, Yuka Kaiho1, Seiichi Miura1 and Gou Fujie1 Abstract Seismic refraction and multi-channel seismic refection surveys were conducted on the northeastern Hawaiian Arch to examine the efect of hotspot volcanism on the seismic structure of the crust and uppermost mantle. The crustal thickness deduced from the refraction data was typical for oceanic crust, which suggests that magmatic underplating does not occur, at least in our survey area, although the crustal seismic velocity may be infuenced by fexure of the lithosphere on the arch. We identifed high P-wave velocities (~ 8.65 km/s) in the uppermost mantle parallel to the paleo-seafoor spreading direction, which indicates that the shallower mantle structure immediately below the Moho preserves the original structure formed at a mid-ocean ridge. Moreover, we observed wide-angle refection waves at large ofsets in ocean-bottom seismometer records. The travel time analysis results showed that these waves were refected from mantle refectors at depths of 30–85 km below the seafoor, which are considered to represent hetero- geneities consisting of frozen melts created during the cooling of the plate. Keywords: Hawaiian Arch, Ocean-bottom seismometer, Multi-channel seismic refection data Background a hole completely through the crust beneath the ocean Te Hawaiian Islands, which are situated in the center of (Hess 1959). However, the crustal structure around the the Pacifc Plate, constitute one of the best-known vol- islands is potentially afected by hotspot volcanism, mak- canic island chains formed by a mantle plume (Wilson ing it difcult to target “typical” oceanic crust for drilling. 1963; Morgan 1971). Te volcanic centers are character- Around the Hawaiian Islands, several seismic refrac- ized by an age-progressive island chain on old seafoor tion experiments, including the original refraction survey caused by intraplate volcanism away from a mid-ocean for Project Mohole in 1962 across the northern Hawai- ridge. Te seafoor on the Hawaiian Arch (Fig. 1) sur- ian Arch (Shor and Pollard 1964), have been carried rounding the island chain is anomalously shallow, rela- out with explosives (Fig. 1). Tese active-source studies tive to normal seafoor of the same age (e.g., Crough revealed the basic seismic velocity structures of the crust 1978; Ribe 2004), because of plate fexure caused by the and uppermost mantle beneath the Hawaiian Arch and island load. Terefore, this oceanic lithosphere is older around the island of Hawaii (Shor and Pollard 1964; Hill and colder than would be expected for this bathymetric 1969; Zucca and Hill 1980; Zucca et al. 1982). Shor and depth alone. For this reason, the northern Hawaiian Arch Pollard (1964) reported that beneath the Hawaiian Arch, is one of the feasible candidates for Project Mohole drill- the mean depth to Moho is 10.4 km below sea level and ing (Ildefonse et al. 2010), a plan proposed in 1957 to drill it dips to 13 km beneath the Hawaiian Deep and the northern shelf of Maui. Tey showed that this variation in depth to the mantle between the arch and the deep is *Correspondence: [email protected] due to variations in the thicknesses of the sediment and 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Showa‑machi 3173‑25, Kanazawa‑ku, Yokohama 236‑0001, Japan volcanic layers which have a P-wave velocity (Vp) near Full list of author information is available at the end of the article 4.2 km/s. Tey also identifed the diferent mantle Vp © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ohira et al. Earth, Planets and Space (2018) 70:121 Page 2 of 16 Fig. 1 Bathymetric map of the Hawaiian Islands and the surrounding Pacifc Plate. The seismic lines of this study are shown by solid black lines, and yellow circles show the positions of ocean-bottom seismometers (OBSs). Colored lines show the locations of previous active-source seismic surveys around the island chain. Hawaiian Plume-Lithosphere Undersea Melt Experiment (PLUME) station locations are shown by black dots (e.g., Wolfe et al. 2009; Leahy et al. 2010; Laske et al. 2011). Thin white lines indicate seafoor age (in millions of years; from Muller et al. 2008). The black dashed line indicates the axis of the Hawaiian Arch (e.g., Holcomb and Robinson 2004; Ballmer et al. 2011). The North Arch volcanic feld, characterized by strong acoustic refectivity (Normark et al. 1989; Clague et al. 1990, 2002), is indicated by gray shading values of 8.1 and 8.7 km/s along two survey lines crossing bending stress caused by the products of the upwelling. the arch at right angles. Zucca and Hill (1980) and Zucca Watts et al. (1985) suggested that the fexed oceanic crust et al. (1982), who conducted seismic refraction experi- beneath the island chain is underlain by a 4-km-thick ments adjacent to Kilauea and Mauna Loa volcanoes, deep crustal body with Vp of 7.4–7.8 km/s, which they showed that the oceanic crust bends downward beneath interpreted as a deep crustal sill complex. the island of Hawaii; as a result, the depth to Moho Tese seismic experiments conducted around the increases from about 10 km beneath the open ocean to island chain indicate that the crust thickens toward the roughly 13 and 18 km beneath the high, subaerial fanks islands. However, the “normal” crustal and upper mantle of Kilauea and Mauna Loa, respectively (Hill and Zucca structure of the ocean basin (i.e., without any efects of a 1987). topographic swell), which must be known to determine To investigate fexural deformation of the oceanic crust the efect of the hotspot volcanism, have not yet been due to the volcanic load of the Hawaiian ridge, coin- constrained by recent data acquisition techniques such cident multi-channel seismic refection and refraction as large-volume tuned airgun arrays, ocean-bottom seis- data were obtained by a two-ship seismic survey near the mometers (OBSs), and multi-channel streamer cables. islands of Oahu and Molokai (Watts et al. 1985; Brocher On the other hand, teleseismic receiver function analy- and ten Brink 1987; ten Brink and Brocher 1987, 1988; ses derived from seismic events recorded by the Hawai- Watts and ten Brink 1989). Te 1-D velocity structures ian Plume-Lithosphere Undersea Melt Experiment and lateral extents of sedimentary, crustal, and subcrustal (PLUME) seismic network, which consisted of nearly 70 refections revealed by these data suggest an interaction seafoor sites and 10 land stations (e.g., Wolfe et al. 2009; between upwelling magma from a hotspot source and Leahy et al. 2010; Laske et al. 2011), have constructed a Ohira et al. Earth, Planets and Space (2018) 70:121 Page 3 of 16 much broader model of crustal structure than the mod- Frey et al. 2000; Yang et al. 2003) in response to the load- els constructed by the active-source seismic experiments ing of the Hawaiian Islands (Moore 1970). (e.g., Leahy et al. 2010). Te analysis results show the crustal thickness of the Hawaiian swell and the region Data acquisition and processing where magmatic underplating occurs. According to these For the acquisition of seismic data, fve OBSs were results, the thickness of the total crustal column, which deployed by R/V Marcus G. Langseth and recovered by consists of preexisting crust and the underplated layer, R/V Kilo Moana. A tuned airgun array (volume 7800 ranges from approximately 7 ± 1 km in non-underplated cubic inches) mounted on R/V Kairei was fred at a areas to 17 ± 2 km under the islands. Tey inferred that depth of 10 m at intervals of 50 m along EW, NS2, and magmatic underplating occurs beneath the northern NS5 lines, and at intervals of 50 and 200 m along the Hawaiian Arch and that the presence of underplating is NS1 line. MCS data were also collected with a 444-chan- correlated with the shallow bathymetry of the Hawai- nel, 6000-m-long streamer cable with a 12.5-m interval ian swell, although the crustal structure on the northern between hydrophones, towed at a depth of 12 m. Multi- Hawaiian Arch is not well constrained because of inad- beam bathymetric data were collected with a SeaBeam equate station coverage there (Leahy et al. 2010). 3012 echo sounder on R/V Kairei and EM 122 echo With regard to the deep lithospheric structure, the sounders on R/V Marcus G. Langseth and R/V Kilo lithosphere–asthenosphere boundary has been imaged Moana. at ~ 75–93 km depth beneath Hawaii by using S-to- In this study, we deployed OBSs sparsely, at intervals of P receiver functions (Rychert et al. 2013). On the other 150 km, along only the EW line, because the main pur- hand, long-ofset profling based on active-source seismic pose of our refraction survey was to constrain the crus- data in the Pacifc basin has indicated the existence of tal structure as described above. We also attempted to seismic discontinuities shallower than the lithosphere– observe the long-ofset wide-angle refection phases from asthenosphere boundary (Ohira et al.
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