Seismic Stratigraphy and Structure of the Northern Tonga Platform

Abstract

The Tonga Platform crowns one of the classic examples of a simple intra-oceanic subduction system. Its central and southern parts have been moderately well explored. This study examines the structural and stratigraphic evolution of the northern Tonga Platform and makes a preliminary assessment of its hydrocarbon potential, It shows that the northern Tonga Platform has undergone similar evolution to the platform further south except that it has not been affected by the subduction and underplating of the Louisville Ridge, which began in early Pliocene time at the latitude of Vava'u. Much of the data, including multichannel seismic records, single channel seismic records, dredge samples and magnetometry was collected in December 1985 as part of the Natsushima '84 joint Plate Boundary Study and processed in Tokyo. Supplementary Seabeam data from the Lau Basin was collected aboard R.V. Thomas Washington during Leg 5 of its Papatua Expedition in 1986. Existing oil company records were examined in Nuku'alofa. Interpretation of seismic data revealed four reflectors, that defined primary unconformity - bounded sedimentary sequences. These reflectors are inferred to be mid Eocene, early Oligocene, early Miocene and early Pliocene in age. Basement older than mid Eocene is igneous at most places and sedimentary at some places. Most later sediments are inferred to be mixtures of volcaniclastic sediments and pelagic or shallow water limestone. Mound structures in upper Oligocene - Lowermost Miocene sediments are inferred to be reefs that formed at time of slow submergence during the opening of the South Fiji Basin. Palanispastic reconstructions carried out on profiles on the back-are platform revealed the structural effects of several stages in the opening of the Lau Basin. The loading exerted by the Tofua Volcanic Arc, when compared with the geomorphology of the Hawaii Archipelago, reveals a very thin, warm, elastic lithosphere (6km thick) underneath the volcanic arc of the northern Tonga Platform. A Lopatin Reconstruction conducted over the mound structures, just north of Vava'u, indicates that hydrocarbon maturity could have been reached in this region. The present "Oil Window" is calculated to lie at depth between 3000 and 4500m.