The main objective of Leg 197 was to test the hypothesis that the Hawaiian hotspot migrated with respect to the Earth's spin axis in Late Cretaceous to early Tertiary times. Paleomagnetic paleolatitude and radiometric age analyses of basalt recovered at sites drilled on the Emperor Seamounts were the principal analytical tools proposed to conduct the test.
The choice of the Hawaiian-Emperor chain for an investigation of hotspot motion returns us to the origins of the hotspot concept itself, which was based on the age-progressive distribution of volcanoes comprising the Hawaiian Islands (Wilson, 1963). This concept was later extended to the entire Hawaiian-Emperor chain of atolls, seamounts, and guyots (Christofferson, 1968). Many subsequent ideas about geodynamic processes were based on the proposition that hotspots like Hawaii were maintained by plumes fixed in the deep mantle that could be used as a stationary frame of reference for plate motions (Morgan, 1971). Of special importance was the bend in the chain at ~43 Ma, separating the Hawaiian and Emperor Seamounts. This bend is commonly considered to be the best example of a hotspot recording of a change in plate motion.
However, a growing body of data has suggested that hotspots may move at rates as high as several centimeters per year, driven by large-scale mantle advection of plumes (e.g., Tarduno and Gee, 1995; Steinberger and O'Connell, 1998). Therefore, hotspot tracks may record drift of the plume in the mantle as well as plate motion. Leg 197 was designed as a paleolatitude test of the hotspot drift hypothesis. If the Hawaiian hotspot has remained fixed, paleolatitudes determined from paleomagnetic analyses of drilled basalt cores from volcanoes in the chain should yield values equal to the present-day latitude of Hawaii (~19°N), allowing us to reject the hypothesis of substantial hotspot motion.
The Emperor Seamounts are especially well suited for a paleomagnetic test because of their wide latitudinal distribution and age range. A sequence of time-independent lava flows sufficient to average geomagnetic secular variation was sought at each drilling site to obtain a high-resolution paleolatitude estimate. At the same time, minimally altered volcanic material was required for radiometric analyses to constrain the crystallization age of the flows. Sites were distributed along the Emperor trend to help evaluate processes responsible for the potential hotspot motion. The basalt and volcanic glass available at these sites were also sought for geomagnetic investigations, including studies of the geometry and intensity of the Late Cretaceous to early Tertiary magnetic field. Currently, the Pacific hemisphere is underrepresented in global geomagnetic data compilations.
Sites proposed on the oldest Emperor edifice, Meiji Guyot, were unavailable for sampling during Leg 197 because clearance for drilling was denied by the Russian government (in May 2001). Detroit Seamount was chosen to sample the oldest (Late Cretaceous) part of the Emperor Seamount trend located in international waters (Fig. F1). Previous paleomagnetic and radiometric age data from basalt recovered at Ocean Drilling Program (ODP) Site 884 on Detroit Seamount indicate a paleolatitude of 36.2° (95% confidence limits = +6.9°/-7.2°) (Tarduno and Cottrell, 1997) at 81 Ma (Keller et al., 1995), suggesting considerable motion of the hotspot. However, the 87 m of basement penetration at the site provided a limited number of independent readings of the Late Cretaceous magnetic field. This resulted in a relatively high uncertainty in the paleolatitude estimate, limiting the use of these data for constraining a plume migration rate. Accordingly, Site 1203 on Detroit Seamount was selected to obtain a longer magnetic record with a more tightly constrained paleolatitude range.
Another important objective of Leg 197 at Site 1203 was to obtain a record useful for tracing the geochemical evolution of the Hawaiian plume through time. In particular, different models have been proposed to explain geochemical signatures derived from analyses of basalt recovered from Detroit Seamount during ODP Leg 145. These models invoke interaction of the hotspot with a spreading ridge axis (Keller et al., 2000) or temporal changes in the composition of the plume (M. Regelous et al., unpubl. data). Basalt from a longer section drilled at Detroit Seamount could help distinguish between these models as well as contribute to our understanding of the geochemical variation of hotspot magmatism on a time scale as long as 81 m.y.
Rotary coring was planned at Site 1203 with one bit change and reentry using a free-fall funnel (FFF). Cores obtained by rotary coring are azimuthally unoriented, and, hence, only paleolatitude and polarity can be determined from standard paleomagnetic analyses. The dominant north-south distribution of the Emperor Seamounts and the proposed sense of hotspot motion (Tarduno and Cottrell, 1997) make a test using only paleolatitude viable. However, a longitudinal component of hotspot drift can also be inferred from studies examining the consistency of the Hawaiian-Emperor track relative to hotspot tracks on other plates (e.g., Cande et al., 1995). Oriented basalt cores could be used to obtain paleodeclination values and paleomagnetic pole positions, which could be used to test potential longitudinal motion of the Hawaiian hotspot.
One method to reorient cores obtained by rotary coring is to use overprints from the Brunhes-age Earth's magnetic field that are sometimes preserved in the total remanent magnetization of basaltic rocks (see "Paleomagnetism and Rock Magnetism" in the "Explanatory Notes" chapter). Alternatively, logging data can be used. At Site 1203 a comprehensive logging plan was proposed to collect data needed to reorient basement cores. Specifically, we planned to use the Formation MicroScanner (FMS) to obtain an oriented image of borehole fractures. Borehole fractures imaged in this way can sometimes be matched to features identified in the recovered cores, allowing for core reorientation (see "Physical Properties" and "Downhole Measurements," both in the "Explanatory Notes" chapter). In addition, we planned to log with the three-axis fluxgate sensor Goettingen Borehole Magnetometer (GBM) and the SUSLOG 403-D magnetic susceptibility tool at Site 1203 to test how well these instruments could characterize the magnetic environment of the borehole and the magnetization of the drilled basement section. Beyond these tools, standard logging with the triple combination and natural gamma ray tools was scheduled to aid in characterizing the stratigraphy of the basement sequence drilled. We also planned to use the dipole sonic imager (DSI) to obtain compressional and shear wave velocities useful for correlation of log and seismic data.
ODP Site 1203 was chosen along a seismic profile (Lonsdale et al., 1993) revealing an area of thin (400-500 m thick) sediment cover on the summit of Detroit Seamount. To better characterize the structural and stratigraphic setting of the site, a seismic survey using the JOIDES Resolution was conducted immediately prior to drilling (see "Underway Geophysics"). This survey confirmed the simple and seemingly flat basement structure at Site 1203. Coring was planned to commence in the sedimentary section to characterize the environment of the basement-sediment transition, determine the age of the transition using micropaleontological analysis, and potentially recover a North Pacific record of the Late Paleocene Thermal Maximum.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
MS 197IR-103