|
Western Pacific Region |
Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 0BP, U.K.
Brimfield Hall, Brimfield, Ludlow, Shropshire, U.K.
From the Eocene to the Middle Miocene, Fiji was part of a N-facing Outer Melanesia arc system, stretching from Papua New Guinea to Tonga, and was dominated by tholeiitic arc volcanism. Oligocene back-arc spreading to the S of Fiji led to the formation of the Minerva Plain (South Fiji Basin). Reorganization of the plate boundaries in Outer Melanesia during the Middle Miocene fractured the simple arc system and caused polarity reversal in arc segments W of Fiji. Fiji, the major yield point in the break-up, experienced a compressive event followed by progressive isolation from a subduction regime as arc segments were rotated away from the region. This led to asthenospheric melting with a decreasing subduction component, and a consequent change in Fiji volcanism from arc andesites and tholeiites to alkalic ocean island basalts. During the Upper Miocene to Lower Pliocene, rotation of the Vanuatu arc segment caused opening of the Fiji Plateau marginal basin. This was accompanied by widespread, chemically diverse volcanism in Fiji, in which contamination of rising magma by pre-existing crust may have been an important process. The most recent phase of arc rotation resulted in opening of the Lau Basin between Fiji and Tonga, and effected the final divorce of Fiji from a subduction influence with commencement of ocean island basalt volcanism in the Middle Pliocene.
