Newly-discovered depleted intraplate volcanism records Pacific-wide plate deformation (62-47 Ma) J. O’Connor1,5, K. Hoernle2, F. Hauff2, J. Phipps Morgan3, D. Sandwell4, J. Wijbrans5, P. Stoffers6 1GeoZentrum Nordbayern, Erlangen and Alfred Wegener Institute for Polar and Marine Research, Bremerhaven 2SFB 574 and Leibniz Institute of Marine Sciences (IFM-GEOMAR), University of Kiel, Germany 3 Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 4Scripps Institution of Oceanography, La Jolla, California 5Department of Petrology, FALW, VU University, Amsterdam 6Institute for Geosciences, Christian-Albrechts-University, Kiel

The classic bend in the Hawaiian-Emperor (H-E) seamount chain is the most prominent feature on the seafloor of the Pacific plate. What caused such a sharp bend is one of the most elusive questions in Earth Sciences giving rise to answers ranging from changes in plate motion over a fixed hotspotMorgan, 1971, slowdown of a moving hotspotTarduno et al., 2003, to a change in plate stress orientationNatland and Winterer, 2006. While distinguishing between these opposing hotspot and plate tectonic mechanisms is essential for testing hypotheses about global plate motion, mantle convection and mantle plume motionTarduno et al., 2003; Steinberger et al., 2004, Whittaker et al., 2007a,b, no volcanic record has been previously recognized for dating changes in lithospheric stress caused by large-scale tectonic forces acting on the Pacific plate. Here we report 40Ar/39Ar ages (62-47 Ma) and geochemistry for a newly recognized type of volcanism sampled on linear Cretaceous seafloor structures across the Pacific Plate. The depleted trace element and isotopic composition of this volcanism suggests derivation from a shallow MORB-type reservoir through processes related to plate deformation. We show that the ~15 Myr interval of increased plate deformation is synchronous with both formation of H-E type bends and a series 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 a major decrease in (mantle flow)Steinberger et al., 2004 and a major increase in plate 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 contrasting processes produced the long slow ~62-47 Ma bends in the Louisville and Tokelau 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, by changing midplate stress orientations without any need for hotspots or shifts in plate motionNatland & Winterer, 2005 and references therein. Our study addresses this controversy by reporting age and geochemical data from a newly discovered Pacific-wide type of depleted intraplate volcanism that is related to plate deformation. 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 bends formed synchronously with a 15 Ma interval of Pacific-wide plate deformation between 64-47 Ma. Below we summarize our new and published age and geochemical data for this volcanism. In the northern central Pacific, the Musicians Seamount Chain (Fig. 1; see detailed map as Supplementary Information), located to the northwest of the Hawaiian Islands, is a NE trending age-progressive Cretaceous hotspot trail, best explained by movement of the plate over a now extinct Musicians hotspotPringle et al, 1993. During the RV Sonne SO142 expedition in 1999, we investigated the basement structure of a series of en echelon volcanic elongate ridges (VERs) extending a maximum distance of 400 km eastward from the Musicians Seamount ChainKopp et al., 2003. Seismic profiles collected during this expedition show that VERs are formed by extrusive volcanism rather than by the usual intrusive underplating (in crustal layer 3) found in most hotspot-related aseismic ridges. The Musicians VERs are therefore attributed to volcanism above Cretaceous mantle ‘flow-channels’ between the Musicians hotspot and an 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 the northern VERs. But unexpectedly all our other 40Ar/39Ar feldspar ages show multi-stage, ~100 km to 300 km long-lines, of synchronous late-stage volcanism between 62 Ma and 47 Ma, >30 Ma younger than the hotspot volcanism forming the Musicians seamounts Pringle et al, 1993 (Supplementary Information) Seismic data and undated ash layers in ODP holes drilled in the ~76-81 Ma Detroit Seamount (Fig. 1) also provide evidence for late stage volcanism in the Eocene (beginning at ca. 52 Ma), overlapping in age with the late-stage volcanism on the Musicians Ridges. This volcanism has been attributed to regional changes in plate motionKerr et al., 2005.

During the RV Sonne SO167 expedition in 2006, we dredge sampled both the Osbourn Trough, a fossil spreading ridgeWorthington et al., 2006 located on the opposite end (southwestern margin) of the Pacific plate, and the Louisville hotspot track located ~200 km to the SW (Fig. 1; see Supplementary Information for detailed map). The Osbourn Trough is located on the 3000 km stretch of seafloor between the Manihiki and Hikurangi oceanic plateaus (large igneous provinces) that formed during the Cretaceous Magnetic Superchron (~120-84 Ma). But our Ar/Ar age dating unexpectedly shows multi-stage, very late volcanism between ~62 Ma and ~52 Ma (Supplementary Information) that is strikingly synchronous with late stage volcanism in the Musicians VERs. While SO167 sample ages for the Louisville hotspot trail confirm the generally age-progressive nature of the oldest volcanism along the chain, we again find very-late volcanism between ~61±1 Ma and ~45±1 Ma that is at least 23 Myr younger, and possibly up to 32 Myr younger than predicted by the age progressionBeier et al., 2011, O’Connor et al,.submitted(a). A sample from the isolated Tuatara seamount 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 the Hikurangi Plateau or the underlying seafloor formed at the now fossil Osbourn spreading centerHoernle et al., 2010.

The geochemistry of this late-stage volcanism can help constrain its origin. The samples from the Musicians VERs, Osbourn Trough and Osbourn Seamounts (Tuatara and Moa) are tholeiitic, whereas those from Louisville seamounts are alkalic to transitional. All of the aforementioned late-stage volcanism has depleted incompatible element and isotopic compositions similar to mid-ocean ridge basaltsHoernle et al., 2010 (MORB; Fig. 2; Supplementary Information), consistent with derivation from a shallow asthenospheric source rather than a deep plume-type source commonly invoked for explaining the origin of intraplate volcanism. Late-stage (referred to commonly as rejuventated or post-erosional) volcanism is however prevalent in many hotspot volcanic chains. Whereas rejuvenated volcanism has more depleted isotopic compositions than the main shield-building stage of volcanism and is often similar to MORB in isotopic composition, it is alkalic and has enriched, ocean-island-basalt (OIB) type incompatible element abundances (ie. enrichment in more incompatible, e.g. Th, Nb, Ta and light rare earth, to less incompatible, e.g. Y and heavy rare earth, element abundances). These geochemical characteristics are inconsistent with the Musician, Osbourn Trough and Osbourn Seamount volcanism being hotspot-related rejuventated volcanism. The very late-stage volcanism on the Louisville Seamounts has even more depleted incompatible element abundances than earlier, age-progressive (shield stage) lavas from the same 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 the shallow (probably MORB-source) asthenospheric and/or lithospheric mantle.

Since this depleted intraplate volcanism occurs on a variety of older volcanic structures (linear ridges formed by plume-ridge interaction, fossil spreading centre, hotspot and non-hotspot volcanic edifices), the most reasonable explanation for its origin is that it is related to reactivation of these diverse volcanic/tectonic structures. In the case of the Musicians VERs, their linear trend and elongated volcanic edifices on the top of ridge segments are very similar to the Pukapuka RidgeKopp et al., 2003, where such features are explained by subduction-related tensional plate crackingSandwell et al., 1995. The Osbourn Trough also has a pull-apart morphology reflecting a late-stage increase in Pacific plate tensionWorthington et al., 2006. The Louisville Chain late-stage volcanism is too young to result from volcano loading effects and is generally associated with structural features suggesting tectonic reactivation O’Connor et al,.submitted(a).. The Crossgrain 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 en echelon 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 43 Ma and Winterer and Sandwell (1987) noted that the (southern) Crossgrain ridges 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 Pacific analogue of the Musicians VERs. In addition to forming in roughly similar mid- plate locations equidistant from circum-Pacific subduction zones, both the Musicians and Crossgrain VERs intrersect Cretaceous hotspot trails where hotspot-spreading ridge flow channels rendered the lithosphere susceptible to later stress reactivation. Numerous morphologic features in the Crossgrain VERs indicate that they originate also as tension (‘pull-apart’) cracks in the Pacific plateWinterer and Sandwell, 1987, Sandwell et al., 1995, Lynch, 1999, Natland & Winterer 2005. Finally, the Osbourn seamounts (Fig. 1), based on their young age and geochemistry, are interpreted as having formed through detachment, triggered by a regional tectonic event, and melting of mafic cumulates from the base of the Hikurangi PlateauHoernle et al., 2010, again pointing to a large-scale tectonic event. In summary, tectonic activity that also caused plate deformation might be the ultimate source of the volcanism. Extension, as well as regional changes in mantle flow, could cause local upwelling and decompression melting. Alternatively melts may be present in the low-velocity zone of the uppermost asthenosphere, even in the absence of significant decompressione.g. Presnall and Gudmundur, 2011. In either case, plate deformation (extension and cracking) along older Cretaceous structures and seafloor would facilitate the rise of asthenopsheric melts to the surface. This newly-discovered depleted intraplate volcanism allows us for the first time to date the timing of the major plate-wide deformation, associated with the formation of H-E type bends (Fig. 3). Our finding of synchronous late-stage depleted intraplate volcanism in very different forms of pre-existing structures (fossil spreading ridge, VERs, primary hotspot trail) with pull-apart or reactivation morphologies implies that the entire Pacific plate was subjected to deformation over ~15 Myr long period (≥62 to ≤47 Ma) consistent with a major plate reorganization. It also provides for the first time a mechanism for dating plate-wide deformation and shows that deformation of the Pacific Plate began at least 15 Ma before the H-E bend. This finding is supported by recent plate reconstructions showing that plate-wide 62-47 Ma volcanism coincides roughly with a series of subduction-driven circum-Pacific tectonic eventsWhittaker et al., 2007a,b. The onset of very late-stage volcanism is roughly synchronous with the start ∼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 of plate reorganisationWhittaker et al., 2007a,b. Our ages suggest that plate deformation related to the reorganisation began at ≥62 Ma, implying that P-I subduction also began at ≥62 Ma. Once the P-I ridge had subducted (and the Izanagi plate had been consumed) at ∼55 Ma, the forces acting on the Pacific changed from ridge-push to slab-pull, which could have changed Pacific absolute plate motions from NNW to WWhittaker et al., 2007b. The resulting changes in Pacific (and Australian) 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 system indicate that subduction initiation occurred at ≥53 Ma and continued until ~44 Ma, with two exceptions (36-37 Ma). The descending slabs of the IBM and T-K subduction zones might have increasingly impeded lateral sub-Pacific mantle flow measured by the corresponding slowdown of the Hawaiian hotspot and formation 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 flow patterns and thus hotspot motion and caused a change in plate motion, either direction or speed or both. Interplay of these two distinct processes (mantle flow and plate motion) could affect hotspot track orientation. In the case of the Hawaiian 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 southeast Steinberger, 2004; Tarduno et al., 2003. The resulting high angle between the Hawaiian hotspot and Pacific plate motions therefore predicts the sharp H-E bend (≥50-47 Ma), while the lower angle between the motions of the Louisville hotspot and Pacific plate predicts the more gradual (62-47 Ma) bends evident in hotspot tracks 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 distance from subduction zones and age/strength of the lithosphere (including possible pre-weakening and existing tectonic structures). If mantle flow is influenced by the drag of the overlying Pacific plate and possibly also by increasing obstruction by subducting slabsWhattaker et al., 2007a,b, then hotspot upwelling might have been inhibited or turbulance/small-scale convection might have increased under the entire or large portions of the plate, affecting the continuity and morphology of the hotspot track.

We speculate further that the virtual disappearance of the H-E trail starting when 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 a tensile (subduction of the P-I ridge) to a more compressive (Tonga-Kermadec and Izu-Bonin subduction initiation) stress regime and/or that the plate reorganization changed the mantle flow regime causing the hotspot to become more 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 Myr ago (e.g., Foundation O’Connor et al., 2002 and Cook-AustralMcNutt et al., 1997 hotspot trails, and the non-hotspot Pukapuka RidgeSandwell et al., 1995 might reflect a return to a more tensile stress regime (and possible increased local or regional mantle dynamics). Moreover, the formation of unusual and subtle patterns of gravity anomalies over younger parts of the of fast moving Pacific plateHaxby and Weissel, 1986 and their enigmatic association with the en echelon volcanic lineaments such as PukapukaSandwell et al., 1995 may reflect this return to increased tensile plate stress since 30-35 Ma.. Similarly, we infer an earlier more tensile stress regime 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 and western sides of the Line Islands hotspot tracks, respectivelyDavies et al. 2002.

In conclusion, our finding that the age and morphology of non-hotspot related intraplate volcanism record and date deformation caused by changes in plate motion provide a valuable tool to identify and constrain plate reorganization and to resolve plate and mantle/hotspot motions in better detail. Based on the first sampling of this newly-discovered volcanism, we find that H-E bends were all created by the same mechanism _ interplay between plate and mantle (hotspot) motions _ and that the unique sharpness of the H-E bend reflects their higher relative angle in the north Pacific. We also find that interplay between plate deformation and local decompression melting is an important mechanism controlling local morphology of H-E type bends. Thus, we propose that a fusion between 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 words 40Ar/39Ar age dating was carried out at the Laserprobe dating facility at the VU University Amsterdam. Data acquisition and reduction, corrections for mass discrimination and age calculation have been described in detail previouslyKoppers et al., 2000; Koppers, 2002; O’Connor et al., 2004; Kuiper et al., 2008. Three plagioclase size fractions, 250-125 or 74-48 µm, were separated using a combination of heavy liquid and paramagnetic methods. The 250-125µm fraction was cleaned with 5-8% HF for 5 minutes, 1 N HNO3 for one hour and finally washed in distilled H2O ultrasonic bath. However, in order to prevent unacceptable sample loss the smaller 74-48 µm separates were cleaned without the HF step. Samples were irradiated in the cadmium-shielded CLICIT facility in the TRIGA reactor at Oregon State University and incrementally heated due to seawater alteration. Ages have been calculated using the Freeware program ArArCalcKoppers, 2002. ArArCalc data files are available as Supplementary 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 hydrothermal and seawater alteration compared to the more commonly used whole rock or bulk groundmass separates. • 40Ar/39Ar ages have been successfully replicated at least twice for each sample. • 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 described in Garbe-Schönberg (1993) and Worthington et al. (2006).

Sr, Nd, Pb isotope analyses were carried out at IFM-GEOMAR by thermal ionization mass spectrometry (TIMS), using a Triton and a MAT262 RPQ2+ TIMS, respectively. Sr data are measured on plagioclase mineral separates to avoid problems associated with seawater alteration. Sample preparation and analysis methods are as described in Hoernle et al. (2008, 2010).

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

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

Acknowledgements This research was supported by the Bundesministerium für Bildung und Forschung (BMBF) and The Netherlands Organisation for Scientific Research (NWO). We thank the captain, crew and scientific 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 each panel and the symbols used. For contributions with methods sections, legends should not contain details of methods, or exceed 100 words (fewer than 500 words for the whole paper). In contributions without methods sections, legends should be less than 300 words (less than 800 words in total). Figure 1. Distribution of newly-discovered (62-47 Ma) depleted intraplate volcanism. Increased plate deformation at ~62 Ma coincides with a previously uncommented sharp bend-like change in morphology in the H-E chain, which is followed by a pause (gap) and thereafter by an overall decrease in volcanism 62-47 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 a pause in volcanism. While no obvious 62 Ma stress related bend is evident in the Louisville there seems to be a gap followed by an overall decrease in volcanism starting at about 62 Ma. Thus, increased plate stress (deformation) at ~62-47 Ma is reflected in the formation of minor ‘stress-bends’ supporting the proposed formation of the Tokelau and Gilbert bends by ‘jerk-like’ plate extensions that reactivated ‘hotspot-pre-conditioned’ lithosphereKoppers and Staudigel, 2006, Koppers et al., 2007. Whereas the H-E trail began bending at the earliest ~≤53-50 Ma forming a short sharp bend, the Tokealu trail began changing orientation as early 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 the Gilberts 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 are shown for the H-E bend (Sharp & Clague, 2006, O’Connor et al., submitted manuscriptb), interpolations between measured ages are shown for the Louisville bend (O’Connor et al., submitted manuscripta). Extrapolated bend location ages shown for the Tokelau bend (and for the Gilbert bend) are inferred from the volcanic propagation rate of ~5 cm/yr inferred from measured bend ages from Matai to Ufiata seamounts. Black arrows are inferred shapes of H-E type bends (see Figure 3 for more information). Sources for ages are Tuatara and 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 Global Multi-Resolution Topography (GMRT) base map from Ryan et al., 2009

Figure 2. Tectonomagmatic incompatible-element discrimination diagrams and variation in lead and neodymium isotopic composition show that late-stage volcanism in the Musicians linear ridges and Osbourn Trough are consistent with being derived from an upper mantle, MORB-type source rather than a hotspot-type mantle source. (a) Hf/3-Th-Ta tectonomagmatic discrimination diagram for basalts and more differentiated rocks (after Woods 1980), showing that Musicians and Osbourn Trough late-stage volcanic rocks have compositions similar to those from mid-ocean-ridge basalts (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 lithospheric interaction with the earlier OIB-type rocks forming the Cretaceous volcanic structures or melting of residual plume material at the base of the lithosphere beneath these volcanoes. 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 is corrected using the age for DR130-2. Field for Pacific N-MORB is from literature sources available from the PETDB database (http://www.petdb.org) and is age corrected to 57 Ma. The islands or island (OIB) groups, age corrected to 57 Ma, are as selected 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 (Hawaiian Islands). Only fields for truly intraplate hotspots (OIB) are shown. OIB data were assembled from the GEOROC database (htpp://georoc.mpch-mainz.gwdg.de). 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). Other literature sources cited 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-E type bends and major circum-Pacific tectonic events. Red symbols are for measured and extrapolated/inferred 62-44 Ma hotspot trail ages during the interval of H-E bend formation (see Figure 1). Blue symbols and dashed lines 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 different subduction zonesIshizuka et al., 2011, Clift et al., 1998 and references therein. Orange lines show the best estimate of when circum-Pacific tectonic events beganWhittaker et al., 2007a,b progressing from subduction of the Pacific-Izanagi (P-I) mid-ocean ridge ~62-53 Ma, changes in Pacific (and Australian) plate motion between ~50-53 Ma and initiation of both the Tonga-Kermadec (TK) and the Izu-Bonin-Marianas (IBM) subduction systems ~53 Ma and ~50 Ma, respectively. Blue dots are for total fusion ages for basalt drill samples from Detroit Seamount (H-E)Duncan & Keller, 2004, Keller et al., 1995 that are in agreement with ages for ash layers intercalated in overlying marine sedimentsKerr et al., 2005. Gilbert and Tolkau 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 hotspot tracks during the (62->47 Ma) interval of plate deformation defined by depleted intraplate volcanism. Just after arc initiation the short sharp H-E bend starts forming in the north. Short red arrows (47-44 Ma) show the sharp change in the azimuth 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 Hawaiian and Louisville hotspots until about 47 Ma or earlier, is predicted by mantle-flow modelsSteinberger et al., 2004, Koppers et al., 2004 and, in the case of the H-E chain, also by age-progressive paleo-latitude Tarduno e al., 2003. Orange arrows show plate motion from Whittaker et al. (2007b). Slowdown of a hotspot drifting at a high angle relative 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 there will be little change in hotspot trail shape resulting in longer and more gradual bends.

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) Marine gravity map from radar altimetrySandwell and Smith, 2009 prepared with GeoMapAppRyan et al 2009 illustrating the location of stress-reactivated linear structures. (b) High-resolution bathymetric maps of the en echelon Musicians linear ridges (adapted from Kopp et al., 2003). Blue circles show locations of SO142 expedition dredge-sample sites; adjacent numbers are for dredge sample numbers and measured isotopic ages. (c) High- resolution bathymetric map of the western part of Osbourn Trough surveyed and sampled 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 with larger 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 them from the submission version.

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