Quaternary Plate Convergence Rates at the New Hebrides Island Arc from the Chronostratigraphy of Bougainville Guyot (Site 831)

Abstract

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.

Geodetic measurements of convergence across the New Hebrides Subduction Zone

“…The As far as the site on Santo is concerned, the convergence rate derived from the GPS results is much lower than the rates given by L&P and NUVEL-1 and the even larger value given by Taylor et al [1994]. Taylor et al [1987 and…”

First estimate of volcanic SO2 budget for Vanuatu island arc

“…Further work is needed to understand the causes of such elevated degassing. It is worthy of note that the central arc segment is subjected to both a westward inflow of enriched, possibly sulphur-rich Indian-type MORB mantle ( [Monzier et al, 1997] , [Peate et al, 1997] and [Turner et al, 1999] ) and to the collision with the d'Entrecasteaux ridge since 2-3 Ma ( [Collot et al, 1985] , [Collot et al, 1992] , [Burne et al, 1988] , [Greene and Collot, 1994] , [Taylor et al, 1994] and [Laporte et al, 1998] ) that has led to back-arc shortening ( [Collot et al, 1985] and [Louat and Pelletier, 1989] ) and to the development of tranverse fracture zones which may facilitate magma ascent beneath Ambae and Ambrym ( [Pontoise et al, 1994] , [Baker and Condliffe, 1996] and [Monzier et al, 1997] ).…”

Rapid forearc uplift and subsidence caused by impinging bathymetric features: Examples from the New Hebrides and Solomon arcs

“…Global plate motion models and regional GPS analyses indicate that net Australian‐Pacific plate motion at the CNH is ∼84–87 mm/yr in an ENE direction [ DeMets et al , 1990, 1994; Beavan et al , 2002]. However, convergence rates of 110 to 130 mm/yr were estimated based on the motion of New Hebrides outer rise islands [ Dubois et al , 1977; F. W. Taylor et al , 1994]. These paleoconvergence rates compare well with rates of 100 to 120 mm/yr measured by GPS at several southern New Hebrides islands [ F. W. Taylor et al , 1995; Calmant et al , 1995, 2003] (Figure 3).…”

“…The Bougainville Guyot is farther along the trajectory with its cap of reefs that drowned as it descended from sea level to ∼1000 m BSL. Bougainville Guyot now indents the edge of the arc by about 10 km [ Collot et al , 1985; Fisher et al , 1991a; Greene and Collot , 1994; Taylor et al , 1994]. The upper part of the southeast flank of the Bougainville Guyot slopes on the order of 25°, typical of an atoll forereef slope.…”