Many of our ideas of where mantle plumes originate, how they interact with the convecting mantle, and how plates have moved in the past rest on interpretations of the Hawaiian-Emperor hotspot track. One reason this volcanic lineament has attained this conceptual stature lies in its prominent bend at 43 Ma. The bend, which separates the westward-trending Hawaiian islands from the northward-trending Emperor Seamounts (Fig. F1) has no equal among the Earth's hotspot tracks; it is the clearest physical manifestation of a change in plate motion in a fixed hotspot reference frame. Because the bend is so distinct, it can be used to estimate plume diameters and to place bounds on the convecting mantle wind that may deflect plumes (Duncan and Richards, 1991). However, shortly after hotspots were used as a frame of reference (Morgan, 1971), apparent discrepancies involving the Hawaiian-Emperor track arose (Molnar and Atwater, 1973). Attempts to model past plate motions failed to predict the bend; instead, a more westerly track was derived (Solomon et al., 1977). Tests of the fixed hotspot hypothesis based on global plate circuits suggested large relative motions between Hawaii and hotspots in the Atlantic and Indian Ocean basins (Molnar and Atwater, 1973; Molnar and Stock, 1987), but uncertainties in the relative plate motions employed in these tests limited their resolving power (Acton and Gordon, 1994).
Several works have readdressed these questions. Norton (1995) suggested that the Hawaiian-Emperor bend records the time when the moving hotspot became fixed in the mantle. Prior to 43 Ma, Norton argues that the hotspot moved southward, creating the Emperor Seamount chain. This work is difficult to assess because of the lack of formal error analyses, but the interpretation reiterates findings of updated plate circuit studies that consider rotation pole errors (Cande et al., 1995). In addition, no obvious change occurs in the spreading rate at 43 Ma for the well-studied marine magnetic anomaly record of the North Pacific (Atwater, 1989). Many feel the lack of such a response by overlying plates to a change of absolute plate motion as large as that indicated by the Hawaiian-Emperor bend is reason enough to question hotspot fixity. New modeling efforts, utilizing a viscosity structure based on geoid constraints, mantle flow fields consistent with tomographic data, and plate motion estimates, also predict motion of hotspot groups (Steinberger and O'Connell, 1997). For the Emperor trend, the predicted motion is 1015 mm/yr (Steinberger, 1996) (Fig. F2).
Whereas these recent studies have revitalized discussions regarding hotspot fixity (see also Christensen, 1998; Wessel and Kroenke, 1998), they face some fundamental data limitations. However, the hypothesis of hotspot motion can be tested independently using paleomagnetism (e.g., Duncan et al., 1972; McElhinny, 1973; Hargraves and Duncan, 1973). The most direct approach is to sample volcanoes that construct a given hotspot track. In the example of the Hawaiian hotspot, the paleolatitudes of extinct volcanic edifices of the Emperor chain should match the present-day latitude of Hawaii if the hotspot has remained fixed with respect to the Earth's spin axis. But this type of test is difficult, in practice, to apply. Paleolatitude values derived from the paleomagnetic analysis of deep-sea sediment overlying seamounts must be interpreted carefully because compaction can induce a flattening of inclinations (Celaya and Clement, 1988; Arason and Levi, 1990; Tarduno, 1990). Such problems can be avoided through the study of drill cores from well-dated lava flows. But until recently, only a few sites had sufficient depth penetration to conduct direct paleomagnetic tests of hotspot fixity. This situation improved after Pacific drilling during Ocean Drilling Program (ODP) Legs 143145. Data from Legs 143 and 144 indicate significant motions between hotspot groups in the Atlantic and Pacific Oceans during the mid-Cretaceous (12895 Ma) (Tarduno and Gee, 1995). The motion is rapid, at speeds within the range of lithospheric plate velocities (30 mm/yr).
These findings indicate an older episode of hotspot motion and, coupled with the inferences based on relative plate motions, suggest that Hawaiian hotspot motion is a viable hypothesis that should be tested further; this test became the primary objective of ODP Leg 197. Data obtained from the analysis of cores obtained during ODP Leg 145 (Tarduno and Cottrell, 1997) and Deep Sea Drilling Project (DSDP) Leg 55 (Kono, 1980) on the Emperor chain (Fig. F1), summarized below, allowed a preliminary test (Cottrell and Tarduno, in press) that guided the drilling plan of Leg 197. The sites chosen to address the question of hotspot fixity also were designed to obtain geochemical data needed for understanding the compositional variability of volcanic products from the Hawaiian hotspot, another important goal of Leg 197.
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