During Leg 145, 87 m of lava flows was penetrated on Detroit Seamount Site 884 (Fig. F1) (Rea et al., 1995). 40Ar-39Ar radiometric analyses yield an age (81.2 that predicted (75 Ma) from hotspot-based best-fit linear plate motion models (Duncan and Clague, 1985). Characteristic magnetizations derived from basalt samples have mainly negative inclinations, indicating reversed polarity. This polarity assignment is consistent with the radiometric age data, suggesting eruption of the basalt during Chron 33R (Tarduno and Cottrell, 1997).
A potential problem in obtaining reliable paleomagnetic data from any basalt drill hole is the uncertain timescale between eruptions. If most flows reflect rapid eruptions, one could easily obtain a biased paleolatitude estimate by giving equal weight to each flow unit. To address this concern, the inclination-only averages derived from each flow unit (McFadden and Reid, 1982) must be checked for serial correlation (Cox, 1970; Kono, 1980; Tarduno and Sager, 1995). These analyses lead to inclination group models (Fig. F3). The directional angular dispersion, estimated from the inclination model data and transformed into pole space (Cox, 1970; Tarduno and Sager, 1995), is indistinguishable from the predicted virtual geomagnetic pole scatter from global data sets (McFadden et al., 1991) (Fig. F3). As discussed below, only one other paleomagnetic data set exists for the Emperor trend that satisfies these geomagnetic sampling requirements.
The preferred inclination group model, where groups are distinct at >95% confidence (N = 10) (Tarduno and Cottrell, 1997), produces a paleolatitude of 36.2° (+6.9°/7.2°), clearly discordant from the present-day latitude of Hawaii (19°) (Fig. F3). This discrepancy is too large to be explained by tectonic tilt. Tilts of 1°3° have been previously reported for some of the northern Emperor Seamounts (Lonsdale et al., 1993). Because these tilts are small and the angle between the remanent magnetization vector and downdip azimuth of tilt is large (>60°), the effect on the paleolatitude is negligible. Measurements made at unit contacts also fail to indicate significant dips.
The new paleomagnetic result directly questions the validity of the Late Cretaceous Pacific apparent polar wander path (APWP) (Fig. F3). But how could these prior results be so errant? Previous Late Cretaceous poles are dominantly or solely based on the inversion of magnetic surveys over seamounts (Gordon, 1983; Sager and Pringle, 1988). Reviews of the methods used to fit these poles suggest they are far more uncertain than commonly supposed (Parker, 1991). Viscous and induced magnetizations can also bias the resulting pole positions (Gee et al., 1989; Cottrell and Tarduno, 2000b). Interestingly, high-latitude poles similar to the new colatitude result (Fig. F3) have been reported from preliminary analyses of the skewness of marine magnetic anomaly data of comparable age (Vasas et al., 1994).
The other paleolatitude value from the Emperor trend that adequately averages secular variation was derived from Suiko Seamount (65 Ma) (Kono, 1980) (Fig. F1). The 8° discrepancy between the Suiko Seamount paleolatitude and the present-day latitude of Hawaii has been previously attributed to early Cenozoic true polar wander (Gordon and Cape, 1981; Sager and Bleil, 1987), which is defined as a rotation of the entire solid Earth in response to mass redistribution (e.g., changes in density heterogeneities in the mantle and growth and disappearance of glacial ice) (Goldreich and Toomre, 1969). True polar wander predictions based on global paleomagnetic data from the continents (Besse and Courtillot, 1991), however, do not agree with the new Detroit Seamount data (Tarduno and Gee, 1995; Tarduno and Cottrell, 1997). Furthermore, renewed tests of Cretaceous true polar wander models show that the solid Earth rotations proposed are not seen in paleomagnetic data from regions where large changes in latitude should be observed (Cottrell and Tarduno, 2000b; Tarduno and Smirnov, 2001). Therefore, the proposed true polar wander rotations appear to be artifacts related to the fixed hotspot reference frame employed.
Because Late Cretaceous true polar wander predictions are inconsistent with Pacific observations, we must now consider hotspot motion as an explanation for the new paleomagnetic data. Although limited in number, paleomagnetic data from the Hawaiian chain younger than the age of the Hawaiian-Emperor bend do not suggest large southward latitudinal displacement relative to the fixed hotspot model (Gromme and Vine, 1972), nor do results from relative plate motion models (e.g., Cande et al., 1995). Thus, the possibility of large latitudinal motion of the Hawaiian hotspot is best examined by focusing our examination on the time interval during which the Emperor Seamounts were formed. We can isolate the latitudinal history of the Emperor Seamounts from that of the Hawaiian chain by subtracting the difference between the present-day latitudes of the 43-Ma bend and Hawaii from the present-day latitudes of each of the Emperor Seamounts. In effect, we slide the Emperor trend down the Hawaiian chain to the present-day latitude of Hawaii (Fig. F4). In so doing, we produce a plot predicting the paleolatitude of the Emperor Seamounts if they were formed by a hotspot moving at constant velocity beneath a stationary plate. Site 884 Detroit Seamount results together with the Suiko Seamount data (Kono, 1980) parallel this predicted trend and provide support for the hotspot motion hypothesis. Differences between the data and predicted values also allow for some northward plate motion. It is difficult to place error bounds on the rate of motion because only two estimates of paleolatitude are available. Nevertheless, the data suggest that the Hawaiian hotspot could have moved southward from 81 to 43 Ma (Norton, 1995) at a constant rate of 3050 mm/yr while the Pacific plate moved slowly northward, in a paleomagnetic (spin axis) frame of reference (Fig. F4).
Interpretations of the Hawaiian-Emperor bend have had a tremendous impact on our understanding of the history and dynamics of plate motions. But the data sets described above suggest that these interpretations may be wrong or, at best, largely incomplete. Our primary motivation during Leg 197, as outlined in "Scientific Objectives" below, was to test the hypothesis of Hawaiian hotspot motion with further drilling in the Emperor Seamounts. This objective provided additional opportunities to learn more about the geometry and paleointensity of the Late Cretaceous to Tertiary geomagnetic field and to study the source and melting history of the Hawaiian hotspot.
Next Section | Table of Contents