F Martinez, B Taylor, A M Goodliffe (SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, (808) 956-6882
The Woodlark basin is opening at intermediate rates in a NNW-SSE direction, separating the continental Woodlark andPocklington rises and forming a westward-pointing v-shapedoceanic basin. Seafloor spreading initiated earlier in theeast and is propagating to the west into the thickened crustof Papua New Guinea. The geometry of the margins, transformazimuths, and magnetic isochrons indicate the pole of opening is located to the west of the basin and that spreading rates decrease westward (67-36 mm/yr). The Moresbytransform near 154.2E divides the basin into western andeastern parts having contrasting geophysical characteristicsopposite to what would be predicted from the spreading ratevariation. West of the transform, seafloor is about 200 mshallower, Bouguer gravity anomalies are about 20 mGalslower, and the rift axis has the morphology of a fasterspreading center, forming a relative high, whereas to theeast the spreading center forms an axial valley. Thegeophysical variations across Moresby transform indicatethat the western basin is thermally more robust than theeastern part despite its slower opening rate. A possibleexplanation for this variation is induced convection causedby horizontal temperature gradients generated as a result of rifting (Buck, 1986). The rifting causes the coolerlithospheric roots of the margins to be juxtaposed againstthe intervening rising hot asthenosphere in the basin. Thetemperature contrast enhances the convective system,causing downwelling near the edges and bringing up more hotasthenosphere near the center of the basin than would beproduced by passive plate separation alone. The decrease inthis effect eastward across the Moresby transform mayresult from the expected dissipation of the horizontalthermal gradients in the older wider basin, but a moreimportant factor may be that the margins are much thinnerand narrower east of the transform, and therefore would nothave induced the effect to the same degree in the firstplace.