3. QUATERNARY PLATE CONVERGENCE RATES AT THE NEW HEBRIDES ISLAND ARC FROM THE CHRONOSTRATIGRAPHY OF BOUGAINVILLE GUYOT (SITE 831)1

We have determined convergence rates of the Australia plate with the New Hebrides Island Arc using the chronostratigraphy of Bougainville Guyot, drilled at Site 831, Ocean Drilling Program Leg 134. The convergence rate at the New Hebrides Island Arc is the vectorial sum of convergence rates between the Australia and Pacific plates (8.8 cm/yr at Espiritu Santo Island) and the opening rate of the North Fiji Basin. We assume that the relative motion of the Australia and Pacific plates is unchanging on the 1.5 m.y. time scale and that any changes of rate occurred in the North Fiji Basin. Convergence rates can be calculated because we know the distances at which carbonate sedimentation would cease and resume as the Bougainville Guyot emerged and submerged during its crossing of the outer rise flexure west of the New Hebrides Island Arc. From 1.42 to 0.393 Ma, Bougainville Guyot was subaerially exposed as it moved approximately 177 km across the outer rise and no sediment was deposited. The mean convergence rate during this time interval was 17.2 ± 7 cm/yr, as determined from strontium-isotope and uranium-series ages of the last carbonates before emergence and the first carbonates deposited after submergence. The Australia plate has converged approximately 52 km with the New Hebrides Island Arc at a mean rate of 13.2 ± 1 cm/yr since 0.393 ± .011 Ma when Bougainville Guyot re-submerged and carbonate sedimentation resumed. This age is based on a precise mass-spectrometric Th age measurement and is reliable because the uranium isotopic composition of the sample indicates no diagenetic alteration. The change in convergence rates from 17.2 to 13.2 cm/yr indicates a significant change in the opening rate of the North Fiji Basin. However, this conclusion depends on the age of initial opening of the North Fiji Basin. If the North Fiji Basin began to open at 10 Ma, then the average opening rate at Espiritu Santo Island has been 6 cm/yr. If opening began at 12 Ma, then the average rate had to be 5 cm/yr. Because the relative motion between the Australia and Pacific plates is 8.8 cm/yr, the net convergence rate at the central New Hebrides Island Arc must have averaged 13.8 to 14.8 cm/yr. Younger dates of initial opening would require higher average convergence rates. If the convergence rate of 13.2 cm/yr at Espiritu Santo Island had prevailed for the entire opening of the North Fiji Basin, then the basin would have taken 13-14 m.y. to open at a mean rate of 4.4 cm/yr. This is contrary to hypotheses for the time of origin of the North Fiji Basin.

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