Shimizu et al. Earth, Planets and Space (2020) 72:11 https://doi.org/10.1186/s40623-020-1140-5 EXPRESS LETTER Open Access Near-ridge-axis volcanism afected by hotspot: insights from efective elastic thickness and topography of the Ojin Rise Seamounts, east of Shatsky Rise in the northwest Pacifc Ocean Shoka Shimizu1* , Masao Nakanishi2 and Takashi Sano3 Abstract We used recently collected bathymetric data and published gravity data to examine the efective elastic thickness of the lithosphere and the crustal thickness beneath the Ojin Rise Seamounts, located east of Shatsky Rise in the northwest Pacifc Ocean. An admittance analysis of the bathymetric and gravity data indicates that the efective elastic thickness of the Pacifc plate under the Ojin Rise Seamounts is 2.7 0.1 km, implying that the seamounts were formed on or near the spreading ridge between the Pacifc and Farallon plates.± The mean crustal thickness beneath the seamounts estimated from the mantle Bouguer anomaly is 10.1 1.7 km, which is thicker than the surrounding crust. The thick crust was probably formed by the interaction between± the Pacifc–Farallon ridge and a hotspot form- ing Shatsky Rise. Our results indicate that late-stage volcanism after the formation of the main edifces of Shatsky Rise spread widely beyond the eastern side of the rise, forming the Ojin Rise Seamounts. Keywords: Efective elastic thickness, Ojin Rise Seamounts, Shatsky Rise Introduction the plume tail (e.g., Hawaiian–Emperor volcanic chain; Te formation of oceanic plateaus is an important unre- Clague and Dalrymple 1987). Rejuvenated-stage vol- solved problem in Earth science. Oceanic plateaus are canism may occur as a result of melting of lithospheric thought to be emplaced by rapid, voluminous eruptions mantle beneath the plateau (e.g., Tejada et al. 2015). Late- over a surfacing mantle plume head (e.g., Duncan and stage volcanism produced volcanic cones on the Mani- Richards 1991). However, secondary (late-stage or post- hiki Plateau (Pietsch and Uenzelmann-Neben 2015) and plateau) volcanism also plays an important role in the may have thickened the crust of the Ontong Java Plateau formation of oceanic plateaus (e.g., Ito and Clift 1998; (Ito and Clift 1998). Tus, a comprehensive understand- Pietsch and Uenzelmann-Neben 2015). Late-stage vol- ing of the formation of oceanic plateaus requires ade- canism includes volcanism along the hotspot track and quate knowledge of late-stage volcanism. rejuvenated-stage volcanism. According to the plume Shatsky Rise is proposed to have formed by the emer- model, hotspot-track volcanism represents activity of gence of a mantle plume head at the ridge–ridge–ridge triple junction among the Pacifc, Izanagi, and Faral- lon plates (Nakanishi et al. 1999). It consists of three *Correspondence: [email protected] major volcanic edifces, the Tamu, Ori, and Shirshov 1 Graduate School of Science and Engineering, Chiba University, massifs, and the bathymetric high of Papanin Ridge Chiba 2638522, Japan Full list of author information is available at the end of the article (Fig. 1a). Drilled samples have yielded radiometric dates © The Author(s) 2020. 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Earth, Planets and Space (2020) 72:11 Page 2 of 8 a −5000 −5000 Japan Papanin Ridge Shatsky Rise 40˚N Shirshov Massif −4 0 00 Cooperation Ori Massif −5 Seamount 000 −4000 35˚N Ojin Rise −6000Seamounts Toront Ridge −4000 −6000 −500 −3000 0 Tamu Massif −6000 −6000 30˚N −6000 −6000 −6000 155˚E 160˚E 165˚E 170˚E c M10N M9 M8 M7 M10 M9 b 39˚N 000 −5 37˚30'N 00 −5000 M4 −40 M5 −500 M6 M1 M2 0 M9 −5000 M3 M2 −5 0 00 37˚00'N 37˚N −5000 −500 M5 M3 −5 0 M4 −500 M1 00 M8 0 M10N M10 M8 M11 M7 0 M10N M9 36˚30'N M10 M11 165˚E 166˚E 35˚N 164˚E 166˚E 168˚E −6000 −5000 −4000 −3000 Topography (m) Fig. 1 Bathymetry of the study area. a Bathymetric map around the Ojin Rise Seamounts and Shatsky Rise. Bathymetric data are from Smith and Sandwell (1997); contour interval is 500 m. Shatsky Rise is outlined in red at the 5000 m bathymetric contour. Thin black lines represent magnetic anomaly lineations (Nakanishi et al. 1999). The white-edged black line at lower center is the seismic refection survey line of Ohira et al. (2017). The two dashed rectangles are the areas shown in b and c. The inset shows the locations of Shatsky Rise, Japan, and present plate boundaries (dashed lines). b Bathymetric map based on data collected during cruise KR14-07. c Bathymetric map of the study area of ~ 145 Ma for the Tamu Massif and ~ 134 Ma for the mid-ocean ridges (e.g., Sano et al. 2012). Previous studies Ori Massif (Mahoney et al. 2005; Geldmacher et al. 2014; have also suggested that late-stage volcanism occurred Heaton and Koppers 2014). Te maximum crustal thick- on Shatsky Rise (e.g., Sager et al. 2016; Shimizu et al. ness of the Tamu Massif is estimated to be 30 km on the 2013; Tejada et al. 2016). basis of seismic refraction surveys (Korenaga and Sager Te Ojin Rise Seamounts (ORS) lies east of Shatsky Rise 2012). Several studies have proposed that the thick crust and consists of approximately 80 volcanic edifces. Tors- of the massif was produced by deeper melting, with a vik et al. (2019) devised an absolute Late Jurassic–Creta- higher degree of partial melting, than is typical at normal ceous Pacifc plate model and showed that the ORS is a Shimizu et al. Earth, Planets and Space (2020) 72:11 Page 3 of 8 hotspot track connected to Shatsky Rise. Nakanishi et al. where ∆g(k) and H(k) are the Fourier transforms of the (1999) speculated that part of the ORS (around 37° 00′ N observed FGA and bathymetry, respectively, and the and 165° 30′ E) was formed near a spreading ridge because asterisk denotes the complex conjugate. the elongation of the seamounts and the magnetic anomaly Te Te value can be estimated by comparing the admit- lineations have similar orientations (Fig. 1a). Te crustal tance function of an elastic plate model with observa- structure under the ORS is unknown. Tejada et al. (2016) tional admittances (Watts 2001). Te elastic plate model indicated that the ORS was formed by late-stage volcanism used in this study is one in which the load of a seamount related to the formation of Shatsky Rise and proposed two is compensated by fexure of a plate. Te admittance models of formation based on the geochemical data. One is function of this model is defned by that the ORS was formed by the interaction between vol- −kd −kt Z(k) = 2πG(ρc−ρw)e 1 − Φe(k)e , canism of a spreading ridge and a hotspot. Te other is that (4) the ORS was formed only by deformation-related shallow- where d is the mean water depth, G is the universal gravi- mantle volcanism. Age information for the ORS is indis- tational constant, t is the mean crustal thickness, and ρw pensable for testing these models. and ρc are the densities of seawater and crust, respec- Te efective elastic thickness (T ) value of the litho- e tively. Te fexural response function of the lithosphere sphere under a seamount is an indicator of its tectonic Φe(k) is (Walcott 1976) setting (Watts 1978). Te Te value, the thickness of the lith- −1 osphere behaving as an elastic body, can be estimated from Dk4 Φe(k) = + 1 , (ρm−ρw)g (5) bathymetric and gravity data. Te relationship between Te and the age of the lithosphere at the time of loading was where g is the average gravitational acceleration, ρm is the frst proposed by Watts (1978). Small Te values (0–8 km) suggest that the seamounts were formed on or near a density of the mantle, and D is the fexural rigidity of the lithosphere defned by spreading ridge. For example, Te value of the lithosphere 3 under the Foundation Seamounts near the Pacifc–Ant- D = ETe , arctic Ridge is 0–5 km, suggesting an age diference of 12(1−ν2) (6) 0–10 Myr between the seamounts and the lithosphere where E is Young’s modulus and ν is Poisson’s ratio. Tese under them (Maia and Arkani-Hamed 2002), and Te of the lithosphere under the Hawaiian Islands is 30–40 km, indi- and other parameters used in this study are listed in cating an age diference of 70–80 Myr between the islands Table 1. and the adjacent seafoor (Watts 1978). In this study, we Te most suitable Te value was determined by mini- mizing the root-mean-square (RMS) misft between determined the crustal thickness and the Te value of the lithosphere under the ORS to estimate its formation age, theoretical and observational admittances for various Te tectonic setting, and crustal structure.