Research Articles
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RESEARCH ARTICLES Emperor chain should match the present- day latitude of Hawaii (ϳ19°N) if the hot- The Emperor Seamounts: Southward spot has remained fixed with respect to Earth’s spin axis. The most reliable indica- Motion of the Hawaiian Hotspot tors of paleolatitude are basaltic rocks, but their reliability depends on each section Plume in Earth’s Mantle spanning enough time to sample geomag- netic secular variation. Recovery of such John A. Tarduno,1* Robert A. Duncan,2 David W. Scholl,3 samples requires ocean-drilling technology, Rory D. Cottrell,1 Bernhard Steinberger,4 and only a few seamounts have been sam- Thorvaldur Thordarson,5 Bryan C. Kerr,3 Clive R. Neal,6 pled to date. Paleomagnetic analyses of 81-million- Fred A. Frey,7 Masayuki Torii,8 Claire Carvallo9 year-old basalt recovered from Detroit Sea- mount (Site 884) yielded a paleolatitude of The Hawaiian-Emperor hotspot track has a prominent bend, which has ϳ36°N (10), which is discordant with served as the basis for the theory that the Hawaiian hotspot, fixed in the Hawaii. Data from ϳ61-million-year-old deep mantle, traced a change in plate motion. However, paleomagnetic and basalt (9) from Suiko Seamount define a radiometric age data from samples recovered by ocean drilling define an paleolatitude of 27°N (11). These data sug- age-progressive paleolatitude history, indicating that the Emperor Sea- gest that the Emperor Seamounts record mount trend was principally formed by the rapid motion (over 40 milli- southward motion of the hotspot plume in meters per year) of the Hawaiian hotspot plume during Late Cretaceous to the mantle (10). early-Tertiary times (81 to 47 million years ago). Evidence for motion of the A paleomagnetic test. The Ocean Dril- Hawaiian plume affects models of mantle convection and plate tectonics, ling Program (ODP) Leg 197 (12) sought to test changing our understanding of terrestrial dynamics. the hypothesis of southward motion of the Ha- waiian hotspot by drilling additional basement The concept of an age-progressive set of define the directional change at 43 million sites in the Emperor chain (Fig. 1). We collect- volcanic islands, atolls, and seamounts pro- years ago (Ma) (7) that would be expected ed detailed stepwise alternating field (AF) de- duced by a hotspot plume fixed in the deep if such a large change in plate motion had magnetization data aboard the drilling ship mantle was first developed to explain the occurred. There was also a general lack of JOIDES Resolution. Although these shipboard Hawaiian Islands (1). The bend separating circum-Pacific tectonic events (8) docu- data are of high resolution, they alone are in- the westward-trending Hawaiian island mented for this time. Recent age data sug- sufficient to define paleolatitudes. Magnetic chain from the northward-trending Emper- gest a slightly older age for the bend [ϳ47 minerals with intermediate to high coercivities, or Seamounts has most often been inter- Ma (9)], but this revised timing still does carrying magnetizations resistant to AF demag- preted as an example of a change in plate not correspond to an episode of profound netization, are commonly formed during sub- motion recorded in a fixed-hotspot frame of plate motion change recorded within the aerial or seafloor weathering. The magnetiza- reference (2). Pacific basin or on its margins. tions of these mineral phases are easily resolv- However, global plate circuits suggest One approach to examine hotspot fixity able in thermal demagnetization data, which we large relative motions between Hawaii and is to determine the age and paleolatitude of also discuss here (13). hotspots in the Atlantic and Indian Oceans volcanoes that form a given hotspot track. The geomagnetic field at a radius r, co- (3–6). Improved mapping of marine mag- For the Hawaiian hotspot, the paleolati- latitude , and longitude can be described netic anomalies in the Pacific has failed to tudes of extinct volcanic edifices of the by the gradient of the scalar potential (⌽): 1Department of Earth and Environmental Sciences, Fig. 1. Hawaiian-Emperor chain shown University of Rochester, Rochester, NY 14627, USA. with ODP Leg 197 sites (12) and marine 2College of Oceanic and Atmosphere Science, Oregon magnetic-anomaly identifications (40). State University, Corvallis, OR 97331–5503, USA. 3Geophysics Department, Stanford University, Stan- ford, CA 94305, USA. 4Institute for Frontier Research on Earth Evolution, Japan Marine Science and Tech- nology Center, Yokosuka 237–0061, Japan. 5Depart- ment of Geology and Geophysics–School of Ocean and Earth Science and Technology, University of Ha- waii, Honolulu, HI 96822, USA. 6Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA. 7Depart- ment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 8Department of Biosphere- Geosphere System Science, Okayama University of Science, Okayama 700–0005, Japan. 9Department of Physics, Geophysics Division, University of Toronto, Mississauga, ON L5L1C6 Canada. *To whom correspondence should be addressed.E- mail: [email protected] 1064 22 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org R ESEARCH A RTICLES ϱ Ϯ l lϩ1 induced inclination flattening (17). Sediment yield a mean age of 49.15 0.21 Ma (2 re ⌽(r,,) ϭ r ͩ ͪ ϫ magnetizations, however, can record long uncertainty quoted; table S1). e r lϭ1 mϭ0 time intervals. Similarly, chemical remanent Reflected-light microscopy revealed rela- magnetizations (CRMs), carried by minerals tively fresh titanomagnetite grains in the la- formed during weathering, can preserve sta- vas (fig. S2A). AF demagnetization of basalt m͑ ͓͒ m ϩ m Pl cos gl cos m hl sin m ] ble magnetizations that provide insight into samples (n ϭ 74) showed the removal of a m where Pl are partially normalized Schmidt the time-averaged field. small low-coercivity component, followed by functions, re is the radius of Earth, and the Koko Seamount (Site 1206). Site 1206 the definition of a characteristic remanent m m Gauss coefficients gl and hl describe the (Fig. 1) was positioned on the southeastern magnetization (12). Two normal-polarity in- size of spatially varying fields. At least sev- side of the lower summit terrace on Koko tervals were defined, separated by a thin eral millennia must be sampled such that the Seamount with the use of crossing underway reversed-polarity zone; the radiometric age 1 axial dipole term (g 0) becomes dominant, seismic profiles (fig. S1). The base of the thin data are most compatible with the chron 21n- allowing an estimate of the paleolatitude. sediment cover contains nannofossils of 21r-22n sequence. This assignment is con- We used the angular dispersion of incli- Zones NP14 and NP15, which provide an firmed by thermal demagnetization (n ϭ nation averages (14) from independent lava early- to middle-Eocene minimum age for the 113), which revealed univectorial decay after flows (inclination groups) and compared this volcanic basement [43.5 to 49.7 Ma (18)]. the removal of a small overprint (fig. S2, B with global lava data of the same age (15)to Fifteen volcanic formations (including pa- and C). examine whether secular variation has been hoehoe flows, flow foot breccias, and subaerial Seventeen inclination groups were iden- adequately sampled (13, 16). Observations of a,a units) were recovered in 278 m of basement tified in the thermal demagnetization data the physical aspects of the lava flows, as well penetration (12). Thin intercalations of lime- (table S2), with a mean thermal inclination ϭ ϩ6.9° as petrologic and geochemical data, were stone, volcaniclastic sandstone, and a deeply (IT 38.3° Ϫ9.3°; hereafter, all uncertainty used to group cooling units into lava units weathered flow top were also recovered, pro- regions are the 95% confidence interval (12, 13). viding geological evidence of time between unless otherwise noted) that was nearly Marine sediments can also provide useful lava flow units. The lavas are mainly of tholei- identical to that isolated by AF treatment ϭ ϩ8.4° paleolatitude information, but they generally itic composition, although two alkalic flows (IAF 38.5°Ϫ10.9°, based on the 14 incli- provide only minimum values of paleolati- were noted. Plateaus in 40Ar/39Ar incremental nation groups sampled; (fig. S2D). The an- tude because of potential compaction- heating spectra from six whole-rock samples gular dispersion (13) of the thermal data ϭ ϩ4.3° (SF 15.3° Ϫ2.7°) is within the error of that predicted by global lava flows of 45 to 80 Ma (15). A comparison of the inclination units based on thermal demagnetization with a synthetic Fisher distribution (19) (Fig. 2) suggests that the basalt sequence well represents the time-averaged geomag- netic field. Furthermore, a stable magneti- zation carried by hematite [likely a CRM Ͼ with unblocking temperatures (TUB) 580°C] in samples of the deeply weathered ϭ basalt yields a mean inclination (IT 38.2° ϩ5.1° ϭ Ϫ5.6°, n 10) that is indistinguishable from that of the lava flows. Nintoku Seamount (Site 1205). Site 1205 on Nintoku Seamount (Fig. 1) was se- lected with the use of two crossing seismic profiles that defined a ϳ43-m-thick sedimen- tary sequence above a flat igneous basement on the northwestern edge of the summit re- gion (fig. S1). Nannofossils of Zone NP10 in the sediment immediately overlying the base- ment provide a minimum age [53.6 to 54.7 Ma (18)] similar to that obtained by radio- metric dating of basalt from the nearby Deep Sea Drilling Project (DSDP) Site 432 [56.2 Ϯ 0.6 Ma; 1 error (20)]. A sequence of 25 subaerially erupted a’a and pahoehoe lavas and interbedded sedi- ment and soil horizons were recovered in 283 m of basement penetration (12).