ABSTRACTThe bend in the Hawaiian-Emperor chain is the best example of a change in plate motion recorded in a fixed-hotspot frame of reference. Alternatively, the bend might primarily record differences in motion of the Hawaiian hotspot relative to the Pacific lithosphere. Four lines of inquiry support the latter view: (1) global plate motions predicted from relative plate motion data, (2) spreading rate data from the North Pacific basin, (3) mantle flow modeling utilizing geoid and seismic tomography constraints, and (4) new paleomagnetic data from the Emperor chain. Although the rate of motion is difficult to constrain because previous drilling has been limited, the best available paleomagnetic data suggest Pacific hotspots may have moved at rates comparable to those of lithospheric plates in Late Cretaceous to early Tertiary times (81-43 Ma). If correct, this requires a major change in how we view mantle dynamics and the history of plate motions. This leg seeks to test the hypothesis of southward motion of the Hawaiian hotspot by drilling five to six basement sites in the Emperor seamount trend. The principal drilling objective is to achieve moderate basement penetration (150-250 m) at these sites to obtain cores from lava flows suitable for paleomagnetic paleolatitude and radiometric age determinations. A comparison of these dated paleolatitude values with fixed and moving hotspot predictions form the basis of the proposed test. Our sampling strategy will also allow us to address important geomagnetic questions that require paleomagnetic data from the Pacific plate, including the history of the time-average field and its paleointensity. The data obtained will place fundamental constraints on the Late Cretaceous to early Tertiary motion of the Pacific plate. An improved picture of this motion history is needed if proxy climatic data from previous and future drill sites are to be used to define past latitudinal gradients.
Another important science objective is to determine the geochemical variation of the volcanic products of the Hawaiian hotspot through time. Petrologic and compositional data from cored lava flows will be used to document changes in source and melting conditions (temperature, depth, and extent) over the duration of Emperor seamount formation. The effect of proposed changes in plate setting (near spreading ridge to mid-plate with decreasing age) on magma composition will be evaluated. The well-known stages of Hawaiian island formation (tholeiitic shield, alkalic capping, and post-erosional flows) will be used to assess lava flow compositions in the context of volcano development. Alteration studies will provide estimates of elemental fluxes in submarine or subaerial weathering conditions. Finally, cored lava flows will be examined with regard to the physical volcanology of these Emperor seamounts. Estimates of size and frequency of eruptions and distance from source will be based on the characteristics of Hawaiian island flows (e.g., morphology, vesicularity, and crystallinity).
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