https://doi.org/10.1130/G48436.1

Manuscript received 15 May 2020 Revised manuscript received 12 November 2020 Manuscript accepted 15 November 2020

© 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 30 December 2020

Hikurangi Plateau a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific) K. Hoernle1,2, J. Gill3, C. Timm1,4, F. Hauff1, R. Werner1, D. Garbe-Schönberg2 and M. Gutjahr1 1GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany 2Institute of Geosciences, Kiel University, 24118 Kiel, Germany 3Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA 4GNS Science, PO Box 30-368, Lower Hutt 5040,

ABSTRACT et al., 2011). It formed at ca. 125 Ma as part Splitting of the Vitiaz arc formed the -Kermadec and Lau-Colville Ridges (south- of the Ontong Java––Hikurangi super- western Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the plateau, which broke apart shortly after forma- south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec tion (e.g., Davy et al., 2008; Hoernle et al., and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) 2010). The basement of the plateau fragments compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, consists of two distinct geochemical types: (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of (1) low-Ti (Kroenke and Kwaimbaita Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) groups on Ontong Java) have isotopically inter- elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville mediate compositions similar to that of primi- Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely tive mantle, and (2) high-Ti basalts (Singallo overlapping isotope ratios, consistent with an origin through lower degrees of melting of more group on Ontong Java) have enriched mantle enriched in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc 1 (EM1)–type basement with unradiogenic (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 206Pb/204Pb but radiogenic Sr isotope ratios 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern (e.g., Tejada et al., 2004; Hoernle et al., 2010; Hemisphere regression line] and 87Sr/86Sr) compared to older and younger arc lavas. Timm et al., 2011; Golowin et al., 2018). Where High-Ti basalts from the , once joined to the , could serve stratigraphic information is available, the high- as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen Ti basalts overlie the low-Ti basalts. Between only in the isotope ratios of mobile elements, was transported by hydrous fluids from the ca. 117 and 79 Ma, spreading along the Osbourn western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into Trough paleo–spreading center, now located at the overlying mantle wedge. Our results are consistent with plateau subduction triggering ∼25.5°S latitude, created ∼3000 km of seafloor arc splitting and backarc opening. between the Hikurangi and Manihiki Plateaus (e.g., Mortimer et al., 2019). The northern tip INTRODUCTION can et al., 1985; Wright et al., 1996; Timm et al., of the subducting Hikurangi Plateau is presently Volcanic arcs play a key role in the plate 2019; Caratori Tontini et al., 2019). Mechanisms located at ∼36°S. tectonic paradigm, being the surface expres- for triggering arc splitting, however, are con- Here we present new trace element and Sr- sion of plate convergence. Nevertheless, little troversial (Sdrolias and Müller, 2006; Wallace Nd-Hf-Pb isotopic data from 40 locations on is known about the long-term tectonic and geo- et al., 2009). the Kermadec (KR) and Colville (CR) Ridges chemical evolution of submarine remnant arc Subduction of young igneous oceanic pla- between ∼28°S and 36°S (Fig. S1 in the Supple- systems formed by arc splitting and backarc teaus is unlikely due to their buoyancy, as dem- mental Material1), recovered primarily on the basin opening, largely due to their inaccessi- onstrated by the Hikurangi Plateau when it col- R/V Sonne cruise SO255. We show that geo- bility. Contemporaneous Neogene volcanism on lided with and accreted to the chemical variations along both ridges are nearly the Tonga-Kermadec and Lau-Colville Ridges at ca. 105 Ma, becoming part of the identical, confirming that they once formed a (southwestern Pacific Ocean;Fig. 1 ) supports microcontinent. The present-day Hikurangi Pla- single arc, and that differences between the the idea that the subparallel ridges were once teau represents a rare example of an oceanic ridges reflect the KR having been the frontal part of a single volcanic arc (termed the Vitiaz plateau being subducted into Earth’s mantle arc and the CR the rear arc of the Neogene Vitiaz arc), which split at ca. 5.5–3 Ma to form the Lau beneath the North Island of New Zealand and arc. Some of the late Neogene (8–3 Ma) Vitiaz Basin and Havre Trough (e.g., Gill, 1976; Dun- the southern Kermadec arc (Fig. 1) (Reyners arc had an enriched composition distinct from

1Supplemental Material. Supplemental information about the samples and analytical methods, including Fig. S1), Table S1 (geochemical data), and Table S2 (replicates and reference materials). Please visit https://doi​.org/10.1130/GEOL.S.13377182 to access the supplemental material, and contact editing@geosociety. org with any questions.

CITATION: Hoernle, K., et al., 2021, Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific): Geology, v. 49, p. 536–540, https://doi.org/10.1130/G48436.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/536/5273370/536.pdf by guest on 01 October 2021 ca. 8–3 Ma and depleted volcanism perhaps continuously since the early Miocene. Plots of isotope ratios versus latitude (with 1° latitude added to CR samples to compen- sate for northwest-southeast opening of the Havre Trough) show that isotopic variations are nearly identical along the KR and CR (Fig. 3), confirming that they once formed a single vol- canic arc (Gill, 1976; Timm et al., 2019; Cara- tori Tontini et al., 2019). The shift to higher Figure 1. (A) Map showing more-incompatible to less-incompatible ele- location of the Tonga-Ker- ment ratios in the CR than the KR lavas at simi- madec arc-backarc system and Hikurangi Plateau. lar isotopic composition suggests derivation Red box shows location of of CR lavas through lower degrees of melting the map in Figure S1 (see beneath the rear arc. KCR lavas with enriched footnote 1). Base map is compositions were found between 29°S and from GEBCO_2014 Grid 37°S (corrected CR; Fig. 3) with the strongest (version 20150318; http:// www.gebco.net). enriched signal being located at ∼33°S, charac- terized by the lowest 206Pb/204Pb and 207Pb/204Pb and highest 87Sr/86Sr ratios. Therefore, the enriched arc signature appears to have been limited both temporally and spatially, although more geochronology is necessary to define its exact duration. We now review possible origins of the enriched end member, beginning with the mantle wedge. Assuming corner flow, enriched intraplate mantle as found in South Basin and intraplate CR samples could have flowed from the backarc beneath the that of the Quaternary Kermadec volcanic arc, (≤18.37) and 207Pb/204Pb (≤15.53) and higher older arc. The South Fiji and intra- consistent with subduction of the Hikurangi Pla- 87Sr/86Sr (≥0.7047) at similar 208Pb/204Pb, Nd, plate CR source, however, has higher 206Pb/204Pb teau or a plateau fragment. The enriched lavas and Hf isotope ratios, indicating an enriched and lower 87Sr/86Sr isotope ratios than the KCR occur along a segment of the arc where part of mantle (EM1)–type component in the source of lavas and therefore cannot explain the enriched the forearc is missing, consistent with removal the KCR lavas (between ∼29.5°S and 36.5°S), (EM1-type) signature (Fig. 2). There is also no by plateau subduction. Plateau collision and sub- not yet found in the Quaternary lavas. evidence of a plume beneath the arc, because duction is a possible mechanism for causing arc Alkalic seamounts behind and late-stage the enriched lavas show typical subduction zone splitting and backarc basin opening. cones on the CR (designated “intraplate CR”) incompatible element abundances, e.g., low have higher Nb/Th (4.3, 9.5–15.5), Ce/Pb (3–32), Ce/Pb (2.0–11.2, n = 74) and Nb/U (0.7–7.6, RESULTS Nb/U (9–50, LOI <4 wt%) and 206Pb/204Pb, and n = 74) ratios, well below typical mantle values We report analytical methods in the Supple- lower 87Sr/86Sr, suggesting a similar intraplate of 25 ± 5 and 47 ± 10, respectively, in global mental Material, trace element and isotope data source as for seamounts in the South Fiji Basin intraplate lavas (Hofmann et al., 1986). There in Table S1, and replicate materials and replicates (Mortimer et al., 2007; Todd et al., 2011). is also no geophysical (Bassett et al., 2016) or in Table S2, for lavas of the KR and CR (together geochemical evidence that continental litho- designated “KCR lavas”). Compared to the pri- DISCUSSION AND CONCLUSIONS sphere underlies the KR and CR. marily tholeiitic KR samples, the primarily calc- Origin of Enriched End Member The enrichment of fluid-mobile elements alkaline CR samples (no adakites or boninites Published age data can be used to con- relative to fluid-immobile elements is con- were found) have higher more-incompatible to strain the duration of the enriched volca- sistent with the enriched component coming less-incompatible immobile element ratios (Ta/ nism (Fig. S1). Six 40Ar-39Ar ages published from the subduction input. A decrease in Ce/ Yb, Nb/Yb, Th/Yb, La/Yb, and La/Sm). KCR from enriched lavas range from 7.5 ± 2.0 Pb from 25 (average for global dry oceanic lavas also have low Nb/Th (<3.4), low Ce/Pb to 2.63 ± 0.23 Ma (three from the KR, 4.4– mantle, after Hofmann et al. [1986]) to 6 (aver- (≤12, with two exceptions), and low Nb/U (<10 4.8 Ma; three from the CR, 7.5–2.6 Ma; Timm age in enriched KCR lavas) represents a four- when loss on ignition [LOI] ≤4 wt%). Although et al., 2019). Depleted lavas yielded 40Ar-39Ar fold addition of fluid-mobile Pb compared to the KR and CR lavas largely overlap in isotopic ages of 3.40 ± 0.24 Ma from the KR (Timm fluid-immobile Ce, implying enrichment of composition (Fig. 2), each can be divided into et al., 2019) and 16.7 ± 0.1 Ma from the CR the mantle wedge primarily by hydrous fluids isotopically depleted and enriched groups. The (Mortimer et al., 2010). Samples classified as rather than melts. Enrichment by fluid addi- depleted KCR lavas between ∼28°S and 37°S depleted based on K content (0.17–0.29 wt%) tion can also explain why the isotope ratios of largely overlap with the Quaternary Kermadec from the KR have K-Ar ages of 1.25–2.04 Ma fluid-mobile elements (Pb and Sr) have been arc and Havre Trough backarc lavas isotopi- (n = 8, ∼32.3°S) and 7.84 ± 0.69 Ma (31.6°S) affected more than those of fluid-immobile cally (Fig. 2). Compared to the depleted KCR (Balance et al., 1999). In summary, volcanism Nd and Hf (Turner and Hawkesworth, 1998). lavas, the enriched KCR lavas extend to higher on the KR (1.2–7.8 Ma) and CR (2.6–16.7 Ma) There is, however, no evidence of an EM1-type

K2O and incompatible element contents for a extends into the early Miocene, with enriched track or seamounts being subducted given MgO content, and to lower 206Pb/204Pb volcanism being restricted conservatively to along 900 km of this part of the arc that could

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C D

Figure 2. 206Pb/204Pb versus 208Pb/204Pb (A), 207Pb/204Pb (B), 87Sr/86Sr (C), and 143Nd/144Nd (D) isotope diagrams for depleted and enriched (EM1- type; ca. 8–3 Ma) Kermadec Ridge and Colville Ridge lavas, Quaternary Kermadec volcanic arc (QKVA) lavas, Havre Trough backarc (HTBA) lavas, sediments data, and Manihiki North Plateau samples (Golowin et al., 2018; Timm et al., 2019, and references therein; Hauff et al., 2021; Gill et al., 2021). In C, the arrow labelled “Subducted sediments” points to the subducted sediment field, which plots above the range in the diagram. Pacific and Indian mid-oceanic ridge basalt (MORB) is from the PetDB http://www.earthchem.org/petdb( ).

explain the enriched composition (Castillo have extended as far north as 29°S. Alterna- propose that between ca. 8 and 3 Ma, high-Ti et al., 2009; Hoernle et al., 2010). tively, EM1 plateau material may have also plateau basalts with an EM1-type composition, The Hikurangi Plateau currently subducts been incorporated in the ocean lithosphere similar to Manihiki North Plateau lavas, sub- south of 36°S, but isotopic evidence suggests ( and/or mantle) formed directly after ducted beneath the southern Vitiaz arc possibly that it may underlie the present arc as far north plateau breakup at ca. 117 Ma (Fig. 4). We as far north as ∼29°S. as ∼32°S (Timm et al., 2014). The enriched KCR isotopic signature, however, cannot be explained by the composition of the low-Ti Hikurangi basement, sampled along ∼50 km of the Rapuhia scarp (Hoernle et al., 2010). Detailed sampling and geochemical studies have also been conducted on the Manihiki Figure 3. Plot of Plateau, which was once joined to the north- 206Pb/204Pb versus latitude, ern part of the Hikurangi Plateau. The high-Ti showing a peak in enrich- lavas from the Manihiki North Plateau have ment (lowest 206Pb/204Pb) appropriate Pb and Sr isotopic compositions at 33°S. One degree of latitude has been added to (Timm et al., 2011; Golowin et al., 2018) to Colville Ridge samples to serve as the EM1 component in the enriched correct for the opening of KCR lavas (Fig. 2). Although estimates of the the Havre Trough. Refer- maximum size of the Hikurangi Plateau based ences are as for Figure 2. on super-plateau reconstructions extend the plateau to ∼32°S (Timm et al., 2014), it is possible that a relatively thin finger of plateau material or plateau fragment, separated from the western edge of the Manihiki Plateau, may

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/536/5273370/536.pdf by guest on 01 October 2021 subduction, and (2) Vitiaz arc splitting and A C Havre Trough opening. Because there has been only minor clockwise rotation of the Kermadec forearc from 10 Ma to the present (Sdrolias and Müller, 2006), another possible mechanism is removal of forearc litho- sphere (Wolf and Huismans, 2019). Subduc- tion of a positive bathymetric anomaly on the downgoing plate, such as an aseismic ridge or plateau fragment, would enhance basal litho- spheric erosion of the overriding plate, result- ing in subsidence and extension of the margin B D (Clift et al., 2003). Between 29°S and 34°S, the lithosphere thickness beneath the forearc thins to ∼10–11 km but reaches thicknesses of Enriched oceanic 16–17 km to the north (25°S–26°S) (Stratford lithosphere ? et al., 2015; Bassett et al., 2016), which may reflect basal forearc removal by plateau sub- duction. In order to explain the absence of the ∼110-km-wide Tonga Ridge (located ∼160 km from the trench at ∼25°S–26°S) in the forearc south of ∼30.5°S, Collot and Davy (1998) pro- posed frontal forearc removal. Removal of a E large block of the forearc could have resulted in extension as the overriding plate moved trench- ward to compensate for forearc removal, and could explain migration of the arc westward away from the trench (Keppie et al., 2009) beginning at ∼30°S and away from the KR into the eastern Havre Trough south of ∼32°S (Bas- sett et al., 2016). Thus, lithospheric removal by plateau collision and subduction could also be an important mechanism contributing to arc splitting and backarc basin opening. Finally, the enriched plateau signal disap- Figure 4. Model to explain the presence of enriched signal in Kermadec Ridge and Colville pears in the KR and CR volcanism at ca. 3 Ma, Ridge lavas between ca. 8 and 3 Ma. (A) At ca. 120 Ma, Ontong Java rifts away from the Mani- and normal oceanic crust is presently subduct- hiki + Hikurangi plateau fragment. (B) At ca. 117–97 Ma, spreading along the Osbourn spreading ing in the region where the enriched signal was center creates ∼3000 km of seafloor between Manihiki and Hikurangi (Mortimer et al., 2019). Some oceanic lithosphere formed near rifted margins of plateau fragments may also have observed (29°S–36°S). Once the plateau frag- enriched plateau-like composition. At ca. 105 Ma, Hikurangi collides with the sub- ment fully subducted and thinner “normal” Cre- duction margin, which later becomes the Chatham Rise of Zealandia. (C) At ca. 10 Ma, the taceous oceanic crust started subducting again, western margin of Hikurangi is just outboard of the Kermadec–North Island (New Zealand) some slab rollback would have been likely. This trench. (D) From ca. 8 to 3 Ma, the western subducts beneath the Vitiaz arc, could also have been an important mechanism which splits into Colville Ridge (CR) and Kermadec Ridge (KR), forming the Havre Trough. (E) Present configuration. (Modified from Davy et al., 2008; Timm et al., 2014.) causing or contributing to backarc rifting and/ or spreading. We need better constraints on the timing of Havre Trough opening and plateau Causes of Arc Splitting and Backarc Basin the incoming plate (e.g., seamount province, subduction in order to constrain the exact mech- Opening hotspot track, or ) with the anism further. The causes of splitting of the Vitiaz arc subduction margin. The collision “pins” the As is clear from Ontong Java, plateau col- into the KR and CR and the formation of the subduction zone, resulting in trenchward rota- lision with a subduction zone will not always Havre Trough backarc basin are enigmatic. tion of the forearc about the pinned pivot point, result in arc splitting and backarc rifting. Due Extension in the overriding plate can trig- causing extension, arc splitting, and backarc to its larger size, greater thickness, and younger ger backarc rifting and/or spreading (Karig, rifting and/or spreading (McCabe, 1984). An age at collision than the Hikurangi plateau mar- 1971). Rollback of the subducting slab causes important criterion for causing backarc opening gin or plateau fragment when it collided with trench retreat, resulting in extension in the by a collision is that the “indentor enter[s] the New Zealand, the was too overriding plate (Jurdy, 1979), but rollback subduction margin just prior to the initiation buoyant and thick to subduct and clogged the is unlikely to be a major process during sub- of the back-arc rifting event” (Wallace et al., subduction zone, resulting in subduction polar- duction of thickened plateau crust (e.g., as 2009, p. 9). Based on the presently available ity reversal, so that normal ocean crust could much as 35 km beneath the Hikurangi Plateau; age and geochemical data, the first evidence again subduct. In summary, there appears to Reyners et al., 2011). for plateau subduction was at ca. 8 Ma, pre- be a continuum from collision and subduc- Another mechanism for generating exten- ceding the initial Havre Trough opening at ca. tion of seamount clusters and hotspot tracks sion in the overriding plate is forearc rotation 5.5–3 Ma (Caratori Tontini et al., 2019), con- resulting in forearc rotation and backarc rift- caused by collision of a buoyant indentor on sistent with a connection between (1) plateau ing (Wallace et al., 2009), to collision of older

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/536/5273370/536.pdf by guest on 01 October 2021 plateau fragments­ causing lithospheric removal Gill, J., Hoernle, K., Todd, E., Hauff, F., Werner, R., Reyners, M., Eberhart-Phillips, D., and Bannister, (­accompanied by arc splitting and backarc open- Timm, C., Garbe-Schönberg, D., Gutjahr, M., S., 2011, Tracking repeated subduction of the ing), to collision of large and thick plateaus over 2021, Basalt geochemistry and mantle flow dur- Hikurangi Plateau beneath New Zealand: Earth ing early backarc basin evolution: Havre Trough and Planetary Science Letters, v. 311, p. 165– a long stretch of an arc that shuts down subduc- and Kermadec Arc, southwest Pacific: Geo- 171, https://doi​.org/10.1016/​j.epsl.2011.09.011. tion, causing polarity reversal in oceanic sub- chemistry Geophysics Geosystems, https://doi. 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