Eocene and Oligocene basins and ridges of the Coral Sea-New Caledonia region: Tectonic link between Melanesia, Fiji, and Zealandia

Abstract

This paper presents 34 geochemical analyses, 24 Ar-Ar ages, and two U-Pb ages of igneous rocks from the back-arc basins and submarine ridges in the Coral Sea-New Caledonia region. The D'Entrecasteaux Ridge is a composite structural feature. Primitive arc tholeiites of Eocene age (34–56 Ma) are present along a 200 km length of the ridge and arguably were part of the initial line of subduction inception between Fiji and the Marianas; substantial Eocene arc edifices are only evident at the eastern end where Bougainville Guyot andesite breccias are dated at 40 ± 2 Ma. The South Rennell Trough is confidently identified as a 28–29 Ma (early Oligocene) fossil spreading ridge, and hence, the flanking Santa Cruz and D'Entrecasteaux basins belong in the group of SW Pacific Eocene-Early Miocene back-arc basins that include the Solomon Sea, North Loyalty, and South Fiji basins. The rate and duration of spreading in the North Loyalty Basin is revised to 43 mm/yr between 28 and 44 Ma, longer and faster than previously recognized. The direction of its opening was to the southeast, that is, parallel to the continent-ocean boundary and perpendicular to the direction of coeval New Caledonia ophiolite emplacement. Medium- and high-K alkaline lavas of 23–25 Ma (late Oligocene) age on the northern Norfolk Ridge are an additional magmatic response to Pacific trench rollback.