Newly-discovered depleted intraplate volcanism recordsPacific-wide plate deformation (62-47 Ma)J. O’Connor1,5, K. Hoernle2, F. Hauff2, J. Phipps Morgan3, D. Sandwell4, J. Wijbrans5, P.Stoffers61GeoZentrum Nordbayern, Erlangen and Alfred Wegener Institute for Polar and Marine Research, Bremerhaven2SFB 574 and Leibniz Institute of Marine Sciences (IFM-GEOMAR), University of Kiel, Germany3 Earth and Atmospheric Sciences, Cornell University, Ithaca, NY4Scripps Institution of Oceanography, La Jolla, California5Department of Petrology, FALW, VU University, Amsterdam6Institute for Geosciences, Christian-Albrechts-University, Kiel

The classic bend in the Hawaiian-Emperor (H-E) seamount chain is themost prominent feature on the seafloor of the Pacific plate. What causedsuch a sharp bend is one of the most elusive questions in Earth Sciencesgiving rise to answers ranging from changes in plate motion over a fixedhotspotMorgan, 1971, slowdown of a moving hotspotTarduno et al., 2003, to a changein plate stress orientationNatland and Winterer, 2006. While distinguishing betweenthese opposing hotspot and plate tectonic mechanisms is essential fortesting hypotheses about global plate motion, mantle convection andmantle plume motionTarduno et al., 2003; Steinberger et al., 2004, Whittaker et al., 2007a,b, novolcanic record has been previously recognized for dating changes inlithospheric stress caused by large-scale tectonic forces acting on thePacific plate. Here we report 40Ar/39Ar ages (62-47 Ma) and geochemistryfor a newly recognized type of volcanism sampled on linear Cretaceousseafloor structures across the Pacific Plate. The depleted trace elementand isotopic composition of this volcanism suggests derivation from ashallow MORB-type reservoir through processes related to platedeformation. We show that the ~15 Myr interval of increased platedeformation is synchronous with both formation of H-E type bends and aseries of changes in circum-Pacific subduction-zone tectonicsWhittaker et al.,2007a,b. We conclude that subduction-driven plate reorganization initiated at~62 Ma caused a combined change in asthenospheric flow (hotspot drift)and plate motion that can explain H-E type bends. A high angle between amajor decrease in (mantle flow)Steinberger et al., 2004 and a major increase inplate Whittaker et al., 2007 motion in the north Pacific resulted in the short sharp≥50-47 Ma H-E bend, while a lower angle between these contrastingprocesses produced the long slow ~62-47 Ma bends in the Louisville andTokelau chains in the south.

While H-E type bends are a key prediction of the mantle plume hypothesis,they can also be explained, together with non-hotspot volcanic lineaments, bychanging midplate stress orientations without any need for hotspots or shifts inplate motionNatland & Winterer, 2005 and references therein. Our study addresses thiscontroversy by reporting age and geochemical data from a newly discoveredPacific-wide type of depleted intraplate volcanism that is related to platedeformation. The age of this volcanism allows us for the first time to date large-scale deformational events of the seafloor and show that H-E type bendsformed synchronously with a 15 Ma interval of Pacific-wide plate deformationbetween 64-47 Ma. Below we summarize our new and published age andgeochemical data for this volcanism.

In the northern central Pacific, the Musicians Seamount Chain (Fig. 1; seedetailed map as Supplementary Information), located to the northwest of theHawaiian Islands, is a NE trending age-progressive Cretaceous hotspot trail,best explained by movement of the plate over a now extinct MusicianshotspotPringle et al, 1993. During the RV Sonne SO142 expedition in 1999, weinvestigated the basement structure of a series of en echelon volcanic elongateridges (VERs) extending a maximum distance of 400 km eastward from theMusicians Seamount ChainKopp et al., 2003. Seismic profiles collected during thisexpedition show that VERs are formed by extrusive volcanism rather than bythe usual intrusive underplating (in crustal layer 3) found in most hotspot-relatedaseismic ridges. The Musicians VERs are therefore attributed to volcanismabove Cretaceous mantle ‘flow-channels’ between the Musicians hotspot andan active Pacific-Farallon spreading centre to the eastKopp et al., 2003, Sleep 2008,consistent with the 83 Ma 40Ar/39Ar feldspar ages for a dredge sample from thenorthern VERs. But unexpectedly all our other 40Ar/39Ar feldspar ages showmulti-stage, ~100 km to 300 km long-lines, of synchronous late-stage volcanismbetween 62 Ma and 47 Ma, >30 Ma younger than the hotspot volcanismforming the Musicians seamounts Pringle et al, 1993 (Supplementary Information)Seismic data and undated ash layers in ODP holes drilled in the ~76-81 MaDetroit Seamount (Fig. 1) also provide evidence for late stage volcanism in theEocene (beginning at ca. 52 Ma), overlapping in age with the late-stagevolcanism on the Musicians Ridges. This volcanism has been attributed toregional changes in plate motionKerr et al., 2005.

During the RV Sonne SO167 expedition in 2006, we dredge sampled both theOsbourn Trough, a fossil spreading ridgeWorthington et al., 2006 located on theopposite end (southwestern margin) of the Pacific plate, and the Louisvillehotspot track located ~200 km to the SW (Fig. 1; see SupplementaryInformation for detailed map). The Osbourn Trough is located on the 3000 kmstretch of seafloor between the Manihiki and Hikurangi oceanic plateaus (largeigneous provinces) that formed during the Cretaceous Magnetic Superchron(~120-84 Ma). But our Ar/Ar age dating unexpectedly shows multi-stage, verylate volcanism between ~62 Ma and ~52 Ma (Supplementary Information) thatis strikingly synchronous with late stage volcanism in the Musicians VERs.While SO167 sample ages for the Louisville hotspot trail confirm the generallyage-progressive nature of the oldest volcanism along the chain, we again findvery-late volcanism between ~61±1 Ma and ~45±1 Ma that is at least 23 Myryounger, and possibly up to 32 Myr younger than predicted by the ageprogressionBeier et al., 2011, O’Connor et al,.submitted(a). A sample from the isolated Tuataraseamount on the seafloor adjacent to the NE margin of the Hikurangi Plateau(Fig. 1) has also been dated at 52 Ma and is thus 65-70 Ma younger than theHikurangi Plateau or the underlying seafloor formed at the now fossil Osbournspreading centerHoernle et al., 2010.

The geochemistry of this late-stage volcanism can help constrain its origin. Thesamples from the Musicians VERs, Osbourn Trough and Osbourn Seamounts(Tuatara and Moa) are tholeiitic, whereas those from Louisville seamounts arealkalic to transitional. All of the aforementioned late-stage volcanism has

depleted incompatible element and isotopic compositions similar to mid-oceanridge basaltsHoernle et al., 2010 (MORB; Fig. 2; Supplementary Information),consistent with derivation from a shallow asthenospheric source rather than adeep plume-type source commonly invoked for explaining the origin of intraplatevolcanism. Late-stage (referred to commonly as rejuventated or post-erosional)volcanism is however prevalent in many hotspot volcanic chains. Whereasrejuvenated volcanism has more depleted isotopic compositions than the mainshield-building stage of volcanism and is often similar to MORB in isotopiccomposition, it is alkalic and has enriched, ocean-island-basalt (OIB) typeincompatible element abundances (ie. enrichment in more incompatible, e.g.Th, Nb, Ta and light rare earth, to less incompatible, e.g. Y and heavy rareearth, element abundances). These geochemical characteristics areinconsistent with the Musician, Osbourn Trough and Osbourn Seamountvolcanism being hotspot-related rejuventated volcanism. The very late-stagevolcanism on the Louisville Seamounts has even more depleted incompatibleelement abundances than earlier, age-progressive (shield stage) lavas from thesame volcanoBeier et al., 2011, O’Connor et al,.submitted(a) (Supplementary Information).Therefore, with the exception of the Osbourn seamounts (see below)Hoernle et al.,

2010, the depleted intraplate volcanism is most likely to be derived from theshallow (probably MORB-source) asthenospheric and/or lithospheric mantle.

Since this depleted intraplate volcanism occurs on a variety of older volcanicstructures (linear ridges formed by plume-ridge interaction, fossil spreadingcentre, hotspot and non-hotspot volcanic edifices), the most reasonableexplanation for its origin is that it is related to reactivation of these diversevolcanic/tectonic structures. In the case of the Musicians VERs, their lineartrend and elongated volcanic edifices on the top of ridge segments are verysimilar to the Pukapuka RidgeKopp et al., 2003, where such features are explainedby subduction-related tensional plate crackingSandwell et al., 1995. The OsbournTrough also has a pull-apart morphology reflecting a late-stage increase inPacific plate tensionWorthington et al., 2006. The Louisville Chain late-stage volcanismis too young to result from volcano loading effects and is generally associatedwith structural features suggesting tectonic reactivation O’Connor et al,.submitted(a).. TheCrossgrain ridges, located in the central PacificWinterer and Sandwell, 1987 and cross-cutting the Line Islands hotspot trailDavies et al., 2002, consists of groups of linear enechelon volcanic ridges Winterer and Sandwell, 1987 very similar to the Musicians VERs(Fig. 1). No direct ages exist for the en echelon Crossgrain VERsWinterer and

Sandwell, 1987; however, stratigraphic evidence shows that they are older than 43Ma and Winterer and Sandwell (1987) noted that the (southern) Crossgrainridges form H-E type bends. Therefore they are likely to have formed between∼62-47 MaWinterer and Sandwell, 1987, Lynch, 1999and may represent a central Pacificanalogue of the Musicians VERs. In addition to forming in roughly similar mid-plate locations equidistant from circum-Pacific subduction zones, both theMusicians and Crossgrain VERs intrersect Cretaceous hotspot trails wherehotspot-spreading ridge flow channels rendered the lithosphere susceptible tolater stress reactivation. Numerous morphologic features in the CrossgrainVERs indicate that they originate also as tension (‘pull-apart’) cracks in thePacific plateWinterer and Sandwell, 1987, Sandwell et al., 1995, Lynch, 1999, Natland & Winterer 2005.

Finally, the Osbourn seamounts (Fig. 1), based on their young age andgeochemistry, are interpreted as having formed through detachment, triggeredby a regional tectonic event, and melting of mafic cumulates from the base ofthe Hikurangi PlateauHoernle et al., 2010, again pointing to a large-scale tectonicevent. In summary, tectonic activity that also caused plate deformation might bethe ultimate source of the volcanism. Extension, as well as regional changes inmantle flow, could cause local upwelling and decompression melting.Alternatively melts may be present in the low-velocity zone of the uppermostasthenosphere, even in the absence of significant decompressione.g. Presnall and

Gudmundur, 2011. In either case, plate deformation (extension and cracking) alongolder Cretaceous structures and seafloor would facilitate the rise ofasthenopsheric melts to the surface.

This newly-discovered depleted intraplate volcanism allows us for the first timeto date the timing of the major plate-wide deformation, associated with theformation of H-E type bends (Fig. 3). Our finding of synchronous late-stagedepleted intraplate volcanism in very different forms of pre-existing structures(fossil spreading ridge, VERs, primary hotspot trail) with pull-apart orreactivation morphologies implies that the entire Pacific plate was subjected todeformation over ~15 Myr long period (≥62 to ≤47 Ma) consistent with a majorplate reorganization. It also provides for the first time a mechanism for datingplate-wide deformation and shows that deformation of the Pacific Plate began atleast 15 Ma before the H-E bend. This finding is supported by recent platereconstructions showing that plate-wide 62-47 Ma volcanism coincides roughlywith a series of subduction-driven circum-Pacific tectonic eventsWhittaker et al.,

2007a,b. The onset of very late-stage volcanism is roughly synchronous with thestart ∼60 Ma of proposed subparallel rapid subduction of the Pacific-Izanagi (P-I) mid-ocean ridge over ~3000 km (Fig. 1) that triggered a chain reaction ofplate reorganisationWhittaker et al., 2007a,b. Our ages suggest that plate deformationrelated to the reorganisation began at ≥62 Ma, implying that P-I subduction alsobegan at ≥62 Ma. Once the P-I ridge had subducted (and the Izanagi plate hadbeen consumed) at ∼55 Ma, the forces acting on the Pacific changed fromridge-push to slab-pull, which could have changed Pacific absolute platemotions from NNW to WWhittaker et al., 2007b. The resulting changes in Pacific (andAustralian) plate motion between 53 and 50 Ma initiated both the Tonga-Kermadec (T-K) and the Izu-Bonin-Marianas (IBM) subduction systems (Fig. 3).New ages of igneous rocks from the forearc basement of the IBM systemindicate that subduction initiation occurred at ≥53 Ma and continued until ~44Ma, with two exceptions (36-37 Ma). The descending slabs of the IBM and T-Ksubduction zones might have increasingly impeded lateral sub-Pacific mantleflow measured by the corresponding slowdown of the Hawaiian hotspot andformation of the H-E BendTarduno et al., 2003, Whittaker et al., 2007a,b.

Figure 3a illustrates the 62-47 Ma synchronicity between H-E type bends,stress-related volcanism, and plate reorganisation events, implying in our view,a causal relationship. The plate reorganization, in particular subduction of the P-I ridge system and initiation of new subduction zones, changed both mantle flowpatterns and thus hotspot motion and caused a change in plate motion, either

direction or speed or both. Interplay of these two distinct processes (mantle flowand plate motion) could affect hotspot track orientation. In the case of theHawaiian hotspot, both mantle flow models and age-progressive paleo-latitude,predict that the Hawaiian hotspot was moving rapidly southward until ~47 Ma,whereas the Louisville hotspot was moving more slowly to the southeastSteinberger, 2004; Tarduno et al., 2003. The resulting high angle between the Hawaiianhotspot and Pacific plate motions therefore predicts the sharp H-E bend (≥50-47Ma), while the lower angle between the motions of the Louisville hotspot andPacific plate predicts the more gradual (62-47 Ma) bends evident in hotspottracks further to the south (Louisville, Tokelau, and possibly Gilbert).Furthermore, minor ‘stress-bends’, gaps and morphologically less-pronounced(lower volume) hotspot trail volcanism, and incomplete bend formation (Fig. 1)show local lithosphere response to stressWinterer & Sandwell, 1987, Lynch, 1999, Natland &

Winterer, 2005, Koppers & Staudigel, 2005, Koppers et al., 2007, reflecting factors such as distancefrom subduction zones and age/strength of the lithosphere (including possiblepre-weakening and existing tectonic structures). If mantle flow is influenced bythe drag of the overlying Pacific plate and possibly also by increasingobstruction by subducting slabsWhattaker et al., 2007a,b, then hotspot upwelling mighthave been inhibited or turbulance/small-scale convection might have increasedunder the entire or large portions of the plate, affecting the continuity andmorphology of the hotspot track.

We speculate further that the virtual disappearance of the H-E trail startingwhen plate stress decreased ~47 Ma until about 25-30 Myr agoDavies, 1992 (Fig. 1)implies that lower amounts of melt reached the surface reflecting a shift from atensile (subduction of the P-I ridge) to a more compressive (Tonga-Kermadecand Izu-Bonin subduction initiation) stress regime and/or that the platereorganization changed the mantle flow regime causing the hotspot to becomemore diffuse and less productive. Thus, the surge in young hotspot and non-hotspot volcanic lineamentsKoppers et al., 2003 in the central South Pacific 25-30 Myrago (e.g., Foundation O’Connor et al., 2002 and Cook-AustralMcNutt et al., 1997 hotspottrails, and the non-hotspot Pukapuka RidgeSandwell et al., 1995 might reflect a returnto a more tensile stress regime (and possible increased local or regional mantledynamics). Moreover, the formation of unusual and subtle patterns of gravityanomalies over younger parts of the of fast moving Pacific plateHaxby and Weissel,

1986 and their enigmatic association with the en echelon volcanic lineamentssuch as PukapukaSandwell et al., 1995 may reflect this return to increased tensileplate stress since 30-35 Ma.. Similarly, we infer an earlier more tensile stressregime from two major episodes of synchronous (86-81 Ma and 73-68 Ma)volcanism, extending for at least 1200 km and >4000 km along the eastern andwestern sides of the Line Islands hotspot tracks, respectivelyDavies et al. 2002.

In conclusion, our finding that the age and morphology of non-hotspot relatedintraplate volcanism record and date deformation caused by changes in platemotion provide a valuable tool to identify and constrain plate reorganization andto resolve plate and mantle/hotspot motions in better detail. Based on the firstsampling of this newly-discovered volcanism, we find that H-E bends were allcreated by the same mechanism _ interplay between plate and mantle (hotspot)

motions _ and that the unique sharpness of the H-E bend reflects their higherrelative angle in the north Pacific. We also find that interplay between platedeformation and local decompression melting is an important mechanismcontrolling local morphology of H-E type bends. Thus, we propose that a fusionbetween opposing plate tectonicNatland and Winterer, 2006 and plate-hotspot(plume)models Morgan, 1971, Tarduno et al., 2003 best explains H-E type bends.

Methods Summary• Max 300 words40Ar/39Ar age dating was carried out at the Laserprobe dating facility at the

VU University Amsterdam. Data acquisition and reduction, corrections for massdiscrimination and age calculation have been described in detailpreviouslyKoppers et al., 2000; Koppers, 2002; O’Connor et al., 2004; Kuiper et al., 2008. Threeplagioclase size fractions, 250-125 or 74-48 µm, were separated using acombination of heavy liquid and paramagnetic methods. The 250-125µmfraction was cleaned with 5-8% HF for 5 minutes, 1 N HNO3 for one hour andfinally washed in distilled H2O ultrasonic bath. However, in order to preventunacceptable sample loss the smaller 74-48 µm separates were cleanedwithout the HF step. Samples were irradiated in the cadmium-shielded CLICITfacility in the TRIGA reactor at Oregon State University and incrementallyheated due to seawater alteration. Ages have been calculated using theFreeware program ArArCalcKoppers, 2002. ArArCalc data files are available asSupplementary Information.

The 40Ar/39Ar ages reported here meet the following acceptability criteria:• Experiments were carried out exclusively on plagioclase phenocryst and

micro phenocryst phases that are far more resistant against hydrothermaland seawater alteration compared to the more commonly used whole rockor bulk groundmass separates.

• 40Ar/39Ar ages have been successfully replicated at least twice for eachsample.

• Plateaus contain at least 79% of released 39Ar.• Isochron (and total fusion) ages all support plateau ages within analytical

uncertainty.ICP-MS geochemistry analyses were carried out at the Institute of Geosciences,University of Kiel using sample preparation and analytical procedures describedin Garbe-Schönberg (1993) and Worthington et al. (2006).

Sr, Nd, Pb isotope analyses were carried out at IFM-GEOMAR by thermalionization mass spectrometry (TIMS), using a Triton and a MAT262 RPQ2+

TIMS, respectively. Sr data are measured on plagioclase mineral separates toavoid problems associated with seawater alteration. Sample preparation andanalysis methods are as described in Hoernle et al. (2008, 2010).

• Contributions should be organized in the sequence: title, text, methods, references, Supplementary Informationline (if any), acknowledgements, author contributions (optional), author information (containing data depositionstatement, interest declaration and corresponding author line), tables, and figure legends.

References• Max 30

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Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

AcknowledgementsThis research was supported by the Bundesministerium für Bildung und Forschung (BMBF) andThe Netherlands Organisation for Scientific Research (NWO). We thank the captain, crew andscientific members of the SO142 and 167 R/V Sonne Expeditions.

Figure legends• 800 max words

Each figure legend should begin with a brief title for the whole figure and continue with a short description of eachpanel and the symbols used. For contributions with methods sections, legends should not contain details of methods, orexceed 100 words (fewer than 500 words for the whole paper). In contributions without methods sections, legendsshould be less than 300 words (less than 800 words in total).

Figure 1. Distribution of newly-discovered (62-47 Ma) depleted intraplatevolcanism. Increased plate deformation at ~62 Ma coincides with a previouslyuncommented sharp bend-like change in morphology in the H-E chain, which isfollowed by a pause (gap) and thereafter by an overall decrease in volcanism62-47 Ma. The Tokelau and Gilberts hotspot trails show roughly similar ~62 Ma‘stress’ bendsKoppers & Staudigel, 2005, Koppers et al., 2007, followed in the Gilberts trail by apause in volcanism. While no obvious 62 Ma stress related bend is evident inthe Louisville there seems to be a gap followed by an overall decrease involcanism starting at about 62 Ma. Thus, increased plate stress (deformation) at~62-47 Ma is reflected in the formation of minor ‘stress-bends’ supporting theproposed formation of the Tokelau and Gilbert bends by ‘jerk-like’ plateextensions that reactivated ‘hotspot-pre-conditioned’ lithosphereKoppers and Staudigel,

2006, Koppers et al., 2007. Whereas the H-E trail began bending at the earliest ~≤53-50Ma forming a short sharp bend, the Tokealu trail began changing orientation asearly as ~62 Ma, resulting in long/slow bending (until the chain disappears at~54 Ma). While no age data are available for the 62-47 Ma interval in theGilberts trail, it shows a similar long, gradual bend like change in morphology,assuming continuation along the Tuvalu seamounts to the southKoppers et al., 2007.

The Louisville trail shows a similar long gradual bend. Measured ages areshown for the H-E bend (Sharp & Clague, 2006, O’Connor et al., submittedmanuscriptb), interpolations between measured ages are shown for theLouisville bend (O’Connor et al., submitted manuscripta). Extrapolated bendlocation ages shown for the Tokelau bend (and for the Gilbert bend) are inferredfrom the volcanic propagation rate of ~5 cm/yr inferred from measured bendages from Matai to Ufiata seamounts. Black arrows are inferred shapes of H-Etype bends (see Figure 3 for more information). Sources for ages are Tuataraand Moa (Osbourn Seamounts)Hoernle et al., 2010, Austral-CookBonneville et al., 2006,Crossgrain ridgesWinterer & Sandwell,1987, Louisville, O’Connor er al., submitted(b), DetroitKerr et al.,

2005, H-ESharp & Clague, 2006, O’Connor et al., submitted manuscript(b), Louisville O’Connor et al., submitted

manuscript(a). Subduction zone tectonics from Whittaker et al., 2007a,b and GlobalMulti-Resolution Topography (GMRT) base map from Ryan et al., 2009

Figure 2. Tectonomagmatic incompatible-element discrimination diagrams andvariation in lead and neodymium isotopic composition show that late-stagevolcanism in the Musicians linear ridges and Osbourn Trough are consistent

with being derived from an upper mantle, MORB-type source rather than ahotspot-type mantle source.(a) Hf/3-Th-Ta tectonomagmatic discrimination diagram for basalts and moredifferentiated rocks (after Woods 1980), showing that Musicians and Osbourn Troughlate-stage volcanic rocks have compositions similar to those from mid-ocean-ridgebasalts (MORB), rather than ocean island basalts (OIB) or subduction-related (arc)basalts. Very late stage volcanism on the Louisville hotspot trail has enriched (E)MORB type compositionsBeier et al., submitted. The more enriched compositions for the late-stage Musicians and the very late-stage Louisville samples may reflect lithosphericinteraction with the earlier OIB-type rocks forming the Cretaceous volcanic structuresor melting of residual plume material at the base of the lithosphere beneath thesevolcanoes. Data for Osbourn Seamount (Moa & Tuatara) from Hoernle et al., 2010,Cook-Austral from Bonneville et al., 2006, and Louisville hotspot trail from Beier et al.,submitted and GEOROC database (htpp://georoc.mpch-mainz.gwdg.de).(b) Isotopic compositions of Musicians (this study) and Osbourn TroughWorthington et al. 2006

lavas are age-corrected using ages reported here, except for sample DR130-1 that iscorrected using the age for DR130-2. Field for Pacific N-MORB is from literaturesources available from the PETDB database (http://www.petdb.org) and is agecorrected to 57 Ma. The islands or island (OIB) groups, age corrected to 57 Ma, are asselected by Hofmann (2003) to represent extreme isotopic compositions reflecting the“type localities” for HIMU (Cook-Austral Islands and St. Helena), EM-1 (Pitcairn-Gambier and Tristan), EM-2 (Society Islands, Marquesas), and PREMA (HawaiianIslands). Only fields for truly intraplate hotspots (OIB) are shown. OIB data wereassembled from the GEOROC database (htpp://georoc.mpch-mainz.gwdg.de). Data forOsbourn Seamount (Moa & Tuatara) from Hoernle et al., 2010, Cook-Austral fromBonneville et al., 2006, and Louisville hotspot trail from Beier et al., submitted andGEOROC database (htpp://georoc.mpch-mainz.gwdg.de). Other literature sourcescited but not referred to in the main text are available as Supplementary information.Other details as for (a).

Figure 3. Synchronicity between depleted intraplate volcanism (62-47 Ma), H-Etype bends and major circum-Pacific tectonic events.Red symbols are for measured and extrapolated/inferred 62-44 Ma hotspot trail agesduring the interval of H-E bend formation (see Figure 1). Blue symbols and dashedlines show isotopically-depleted low-volume, late-stage intraplate volcanism (62-47 Ma)(see Figure 1). Orange circles are from the oldest forearc basement rocks for differentsubduction zonesIshizuka et al., 2011, Clift et al., 1998 and references therein. Orange lines show the bestestimate of when circum-Pacific tectonic events beganWhittaker et al., 2007a,b progressingfrom subduction of the Pacific-Izanagi (P-I) mid-ocean ridge ~62-53 Ma, changes inPacific (and Australian) plate motion between ~50-53 Ma and initiation of both theTonga-Kermadec (TK) and the Izu-Bonin-Marianas (IBM) subduction systems ~53 Maand ~50 Ma, respectively. Blue dots are for total fusion ages for basalt drill samplesfrom Detroit Seamount (H-E)Duncan & Keller, 2004, Keller et al., 1995 that are in agreement withages for ash layers intercalated in overlying marine sedimentsKerr et al., 2005. Gilbert andTolkau ages are from Koppers & Staudigel (2006) and Koppers et al. (2007). Other

details as in Figure 1. Long red arrows show schematically bending of the hotspottracks during the (62->47 Ma) interval of plate deformation defined by depletedintraplate volcanism. Just after arc initiation the short sharp H-E bend startsforming in the north. Short red arrows (47-44 Ma) show the sharp change in theazimuth of the H-E trail and unchanged trend in hotspot trails to the south, see(b).(b) Schematic vector diagrams showing formation of short sharp versus long-slow H-E type bends. Southward and southeastward motions of the Hawaiianand Louisville hotspots until about 47 Ma or earlier, is predicted by mantle-flowmodelsSteinberger et al., 2004, Koppers et al., 2004 and, in the case of the H-E chain, also byage-progressive paleo-latitude Tarduno e al., 2003. Orange arrows show plate motionfrom Whittaker et al. (2007b). Slowdown of a hotspot drifting at a high anglerelative to a tectonic plate produces a short/sharp change in hotspot trail shape.But if the hotspot is moving in roughly the opposite direction to the plate therewill be little change in hotspot trail shape resulting in longer and more gradualbends.

Supplementary Information (Online)

1. Detailed sample location maps

2. Sample information

3. Ar/Ar summary table

4. ArAr Calc data files

5. Geochemistry data

Figure 1. Distribution of late stage volcanism across the Pacific plate. (a) Marinegravity map from radar altimetrySandwell and Smith, 2009 prepared with GeoMapAppRyan et al 2009

illustrating the location of stress-reactivated linear structures. (b) High-resolutionbathymetric maps of the en echelon Musicians linear ridges (adapted from Kopp et al.,2003). Blue circles show locations of SO142 expedition dredge-sample sites; adjacentnumbers are for dredge sample numbers and measured isotopic ages. (c) High-resolution bathymetric map of the western part of Osbourn Trough surveyed andsampled during the SO167 RV Sonne expedition (adapted from Worthington et al.,2006). White lines and arrows show the axial valleys and inferred spreading directions,respectively. Small white dots are for the locations of SO167 dredge stations withlarger numbers for measured isotopic ages and uncertainties.

NOTE: Let’s not include figures 2 and 3 unless the reviewers ask for such info.Will do, but keeping them in for now so that we don’t loose track of them but will delete themfrom the submission version.

References

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