REVIEW ity implies that velocity gradients are small and that inertial forces pale in comparison to viscous ones; thus, the mantle transmits stresses nearly The Bent Hawaiian-Emperor Hotspot instantaneously. Given both premises, any plate motion change must go along with changes in the internal mantle flow. Hager and O’Connell (6) Track: Inheriting the Mantle Wind used this insight to infer deep mantle motion from the surface plate tectonic pattern in the decade fol- John Tarduno,1,2* Hans-Peter Bunge,3 Norm Sleep,4 Ulrich Hansen5 lowing the first use of a hotspot reference frame. Subsequent advances in seismic tomography Bends in volcanic hotspot lineaments, best represented by the large elbow in the Hawaiian- and past plate motion modeling allowed model- Emperor chain, were thought to directly record changes in plate motion. Several lines of ing of the temporal evolution of the mantle back geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by in time. Steinberger and O’Connell (7) produced mantle dynamics causes bends in hotspot tracks. Inverse modeling suggests that although deep a synoptic view of past mantle circulation that flow near the core-mantle boundary may have played a role in the Hawaiian-Emperor bend, suggested southward motion of the Hawaiian capture of a plume by a ridge, followed by changes in sub-Pacific mantle flow, can better explain hotspot, consistent with paleomagnetic data (8). the observations. Thus, hotspot tracks can reveal patterns of past mantle circulation. Geophysical cruise results and paleomagnetic studies resolved potential gaps in plate circuits he Hawaiian-Emperor track (Fig. 1) is the Two premises embedded in the dynamic [e.g., (9, 10)], and global paleomagnetic tests archetype used to illustrate plate motion equations of how the mantle moves preclude excluded polar wander (11–13). These develop- Tover a hotspot. In the classic view, volcan- stationary hotspots. The first is the principle of ments have set the stage for a reassessment of ism on the surface recorded regular plate motion mass conservation: Any plate movement at the mantle processes that could be recorded by large over a plume deep within Earth’smantlethat surface must be balanced by motion deeper in the bends in hotspot tracks. produced a fixed region of melting near the sur- mantle. The second premise is that the mantle can face (hotspot). The great bend separating the be taken as a highly viscous fluid—a nominal vis- An Alternative Hawaiian-Emperor Track on April 16, 2009 northwestern-trending Hawaiian chain from the cosity of 1021 Pa·s is suggested from studies of To visualize the implications of hotspot motion, north-south–oriented Emperor Seamounts was postglacial rebound (5). The large mantle viscos- we consider what the Hawaiian-Emperor track thought to trace a ~60°change in absolute plate motion. However, a paleomagnetic test based on 60° ocean drilling in 2001 revealed that the islands and seamounts had formed at different latitudes (1). The Emperor trend seamounts thus record motion of the upwelling in the mantle. Why didn’t the community accept hotspot www.sciencemag.org mobility earlier? The elegant and captivating ge- Emperor Seamounts ometry of the fixed hotspot idea (2) compelled a 50° search for alternatives to emerging contradictory Plate motion findings. It had been recognized in reconstructing 81 - 47 Ma plate motions that a fixed Hawaiian hotspot was inconsistent with fixed hotspots in the Indo- Atlantic realm (3). However, early reconstructions 40° relied on a critical plate circuit link through the Downloaded from then poorly understood Antarctic and southern- most Pacific region. The possibility of undocu- mented plate motions provided an option that Hawaiian Chain avoided hotspot drift. Paleomagnetic data from a 30° single Pacific site also indicated hotspot motion, but they could be interpreted as a record of polar Plate motion wander (4), the rotation of the entire solid Earth in response to shifts in its mass heterogeneities. In 20° 0 - 47 Ma addition, although plumes could easily have been envisioned as swaying in the mantle, the geo- dynamic details were wanting. 10° 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA. 2Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, 0° USA. 3Department of Earth and Environmental Sciences, Ludwig-Maximilians Universität, München, 80333 München, 4 Germany. Department of Geophysics, Stanford University, 150° 160° 170° 180° 190° 200° 210° 220° 230° Stanford, CA 94305, USA. 5Institut für Geophysik, Universität Münster, 48149 Münster, Germany. Fig. 1. Bathymetry of the northwest Pacific Ocean basin. The traditional interpretation is highlighted: The *To whom correspondence should be addressed. E-mail: Hawaiian volcanic chain records absolute Pacific plate motion to the northwest, whereas the Emperor [email protected] Seamounts reflect a more northerly plate direction. 50 3 APRIL 2009 VOL 324 SCIENCE www.sciencemag.org REVIEW might have looked like had the A C Hawaii hotspot actually been fixed 150° 170° 190° 210° 150° 170° 190° 210° relative to the deep mantle. To do this, we use plate circuits and assume that volcanic chains in the Indo-Atlantic realm reflect fixed hotspots. These Detroit 75-81 Ma predictions are thus still not free of 50° 50° the fixed hotspot hypothesis, but Suiko 61 Ma paleomagnetic studies suggest that Rapid Nintoku 56 Ma iii ii the hotspots in the Indo-Atlantic have hotspot i motion not moved much since the Late- Koko 49 Ma Cretaceous-Paleogene, when the Diakakuji 47 Ma Emperor Seamounts were formed 30° 30° Midway 28 Ma (14). The plate circuit that traverses the Necker 10 Ma Indo-Atlantic and Pacific realms via Hawaii Antarctica (9, 14) yields a hypotheti- cal trace that is similar to the Hawai- 10° 10° 160 180 ian chain for the last ~30 million years ° ° ° (Fig.2,AtoC).Foroldertimes,dif- B 200 D 150° 170° 190° 210° ferences build. The Hawaiian-Emperor 140 Lord bend of the extant track is replaced by Howe ° Pacific plate a gentle ~30° of curvature in the Rise -30 Australia hypothetical trace (Fig. 2D). This ° 50° moderate curvature is similar to that ° ° 220 seen in the Louisville chain, the other Campbellplateau -30 on April 16, 2009 hotspot track from the Pacific Ocean 120 basin with a long, clear age progres- ° sion (15) that is not confused by ° 240 lithospheric structure. Moreover, the 30° hypothetical trace resembles an oce- 100° AntarcticaWest anic fracture zone, a feature of Earth’s East 260° surface known to record plate motion Antarctica -60 340 320 300 16 ° ° ° over tens of millions of years ( ). As 60 ° ° ° 40 0 ° ° isthecasewithfracturezones,wecan ° 20 -60 10° www.sciencemag.org fit the trace with several small circle segments, the junction of each repre- 0.0 11.0 19.7 33.3 39.5 47.2 56.7 67.8 84.0 124.6 129.8 135.3 142.1 Seafloor Age (Ma) senting a change in plate motion. The bend in the actual track might repre- Fig. 2. Alternative Hawaiian-Emperor traces. (A) Present-day track with ages and episode of rapid hotspot motion sent one of these junction times in the highlighted. (B) Differences in how plate circuits connect the Indo-Atlantic realm to the Pacific for Late Cretaceous to hypothetical trace. middle Eocene times [from (14)]. In the standard reconstruction [e.g., (9)], the connection is through East-West Antarctica An alternative plate circuit in- (black arrows); in a more recent circuit, the connection is through Australia and the Lord Howe Rise (white arrows) (17). 14 19 volves transfer from Antarctica to (C) Predicted tracks from plate circuits [see ( )and( ) for ages and uncertainty analysis]: (i) East-West Antarctica circuit Downloaded from Australia, and then through Lord (red); (ii) Australia-Lord Howe Rise circuit (green); (iii) Australia-Lord Howe Rise circuit modified with a revised Antarctic- Howe Rise (Fig. 2, B and C). In the Australia spreading history [from (17)] (blue). Note that path (iii) converges to path (i). (D) Trace that would have been predictions arising from use of this produced had the Hawaiian hotspot been fixed in the deep mantle (16). Predictions based on East-West Antarctica circuit. circuit, a somewhat sharper bend remains, which could imply a larger plate motion change (17). Both plate circuits paleomagnetic determinations from ocean large volume flux (~350 m3 s−1)(21), spatial pass paleomagnetic consistency tests (14); we drilling (1, 8). pattern, and longevity are best explained by the cannot distinguish between them with the mantle plume hypothesis. Some studies of al- resolution of the available data. However, while Candidate Processes ternative mechanisms (e.g., a propagating crack) avoiding the Antarctic-Pacific connection, the Two models have been proposed to explain hot- show that they are inadequate (21), whereas other second circuit relies on a contentious assump- spots. One is a “top-down” plate-driven process in modeling has shown that deep mantle plumes tion: Lord Howe Rise is locked to the Campbell which rifting of the lithosphere stimulates shallow contribute substantially to the mantle energy bud- Plateau between 84 and 47 million years ago mantle melting (20). The second is a “bottom-up” get (22). Below, we identify five physical pro- (Ma). Moreover, when a revised history of model in which an upwelling or plume (2)risesin cesses that could have caused hotspot motion and Antarctic-Australian spreading history (18)is the mantle from a thermal boundary layer (the the Hawaiian-Emperor bend. We use 47 Ma as incorporated into the circuit through Lord Howe transition zone at ~700 km or as deep as the core- the best age for the bend; an older age of 50 Rise, the predictions for the locations of the mantle boundary).