HR: 09:10h
AN: V31A-05    [PDF]
TI: Was Shatsky Rise Formed by a Plume Head or Plate Boundary Processes?
AU: * Sager, W W
EM: wsager@ocean.tamu.edu
AF: Department of Oceanography, Texas A\&M University, College Station, TX 77843 United States
AB: Shatsky Rise is a large oceanic plateau located about 1600 km east of Japan, in the NW Pacific Ocean. It is a basaltic mountain range with an area nearly equal to Japan or California, qualifying it as one of the globe's larger LIPs. Existing geologic and geophysical data are consistent with formation by a starting plume head, yet coincidences with spreading ridge tectonics suggest otherwise. Most knowledge of Shatsky Rise tectonics comes from magnetic lineations mapped around and within the rise. They show that the rise sits along the track of the Pacific-Farallon-Izanagi (PFI) triple junction from M21 to M3 time (147-125 Ma) and consists of three large volcanoes, separated by minimally-disturbed lithosphere, and a linear volcanic ridge. All volcanic edifices appear to be the same age as the lithosphere upon which they sit, implying progressive volcanism becoming younger to the northeast. The largest volcanic edifice, with a volume of 2.7 x 10$^{6}$ km$^{3}$, erupted rapidly, as shown by its consistent reversed magnetic polarity and the near coincidence of dates from basalts cored on its upper flanks with the age of the lithosphere. Shallow water fossils from the upper reaches of this volcano imply its summit was above sea level. Magnetic lineations indicate that the triple junction jumped at least nine times, with larger jumps taking it to positions at the edges of the volcanic edifices and implying a connection between volcanism and ridge jumps. Furthermore, the triple junction migrated in a direction and with a velocity inconsistent with a single, stable configuration, as if its behavior were perturbed by factors other than plate velocities. These observations fit reasonably well with predictions of the plume or plume head hypothesis: (1) rapid and voluminous initial eruptions, (2) the volcano grew above sea level (dynamic uplift), (3) massive plume head eruptions transitioned to lesser-flux plume tail, and (4) extra heat and uplift from the plume caused the triple junction to jump repeatedly to the plume site. In addition, the trends of Shatsky and Hess rises are similar to that of the Mid-Pacific Mountains, implying all were formed by motion of the Pacific plate over mantle hotspots. Other observations fit the plume hypothesis less well. Geochemical and isotopic data mostly show MORB-like signatures, arguing against a plume source unless it was diluted or swamped by sub-ridge melting. More troublesome, however, is the affinity of Pacific plate plateaus for triple junctions. Shatsky Rise formed at a triple junction, beginning at the time of a plate reorganization that formed one or two microplates. Hess Rise and another microplate probably formed at the location of a later PFI triple junction jump. Furthermore, the Mid-Pacific Mountains, Magellan Rise, Manihiki Rise, and two additional microplates seem to have formed along the track of the Pacific-Farallon-Phoenix triple junction and the tracks of the two triple junctions have different trends (i.e., cannot be explained by motion over fixed hotspots). This coincidence of ridges and triple junctions only makes sense if plate boundary processes give rise to the plateau volcanism. In summary, Shatsky Rise has characteristics that fit formation either by plume or plate boundary processes, but new data are required to decide which. What's more, the connection between plumes, microplates, and triple junctions is unclear and seems fertile ground for further study.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8157 Plate motions--past (3040)
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting