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  • Newly-discovered depleted intraplate volcanism recordsPacific-wide plate deformation (62-47 Ma)J. OConnor1,5, 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 sharp50-47 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 ~611 Ma and ~451 Ma that is at least 23 Myryounger, and possibly up to 32 Myr younger than predicted by the ageprogressionBeier et al., 2011, OConnor 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, OConnor 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 OConnor 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 andSandwell, 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 between62-47 MaWinterer and Sandwell, 1987, Lynch, 1999and may represent a central Pacificanalogue of the Musicians VERs. In addition to forming in ro

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