NEW OBSERVATIONS ON MID-PLATE VOLCANISM AND THE TECTONIC HISTORY OF THE PACIFIC PLATE, TAHITI TO EASTER MICROPLATE

We describe the geology and tectonics of a continuous swathe of seaflo or between Tahiti and the western edge of the Easter microplate imaged by GLORIA and Sea Beam on two separate cruise transits in 1987 and 19 88. The data reveal that mid-plate volcanism is common in this region, even on deep seafloor hundreds of kilometres from major lines of seam ounts and islands. This supports the idea of a thin weak lithosphere o ver the Pacific Superswell, and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km. The mid -plate volcanism occurs in two distinct forms. Over most of our traver se it appears as fields of relatively young and acoustically strongly backscattering lava flows, often accompanied by groups of numerous sma ll, circular volcanoes. East of 122 degrees W (about chron 5A), howeve r, we observed a distinct form: major, sharp-crested, constructional v olcanic ridges, many tens of kilometres long, individually trending EN E, but lying en-echelon along an E-W regional trend. These ridges appe ar morphologically identical to the 'cross-grain ridges' seen elsewher e in the Pacific. We attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness. Ho wever, although these ridges are parallel to fracture zone trends seen farther west, they are morphologically very different from any known fracture zone. Moreover, individual ridges are somewhat oblique to the tectonic spreading fabric around them, and so do not seem to follow a ctual fracture zone traces. The whole line of en-echelon ridges lies a long part of the predicted trace of Fracture Zone 2 of Okal and Cazena ve [15], and is probably its morphological expression. However, nowher e did we see a convincing 'conventional' fracture zone trace in or fol lowing the predicted position or orientation. We suggest instead that magma from an independent source has used lines of weakness along mino r fracture zones to produce these en-echelon features. The Austral Fra cture Zone is the only major fracture zone crossed in our transit, and here is characterised by four fossil transform strands. Its marked po sition on the AAPG and GEBCO maps is found to be in error. Finally, we found that the expected change from NNW- to NNE-trending spreading fa bric at chron 6C did not occur in a clear-cut way, as predicted by ear lier tectonic histories of the Pacific. Instead, the post-chron 6C fab ric oscillates in a confused way between NNE and NNW, suggesting to us that this area has been characterised by an unstable plate boundary, probably associated with a succession of propagating rifts or micropla tes from chron 6C to the present.