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Active tectonism within the Fiji platform, southwest Pacific

¹ Department of Geology, Indiana University, Bloomington, Indiana 47405
² Mineral Resources Department, Private Mail Bag, G.P.O., Suva, Fiji
³ Department of Geological Sciences and Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853

New observational data on the velocity structure, seismicity, and state of stress of the large submarine platform that includes the Fiji Islands demonstrate that the platform is an active participant in the pervasive regional deformation of the Pacific/Indian-Australian plate boundary. Refraction data collected within the Fiji platform demonstrate that its crustal thickness (15-25 km) and seismic velocity structure are comparable to those of active island arcs.Uppermost mantle velocity beneath the platform is anomalously low (7.55 km/s), in the same range as that observed beneath the neighboring marginal basins and island arcs. Shallow seismicity is observed both within and along the boundaries of the Fiji platform. Moderate to large earthquakes within the platform occur more sporadically than those along its margins; these intraplate events are separated by long periods of quiescence. Earthquake depths within the platform are concentrated in the upper 13-16 km, as in other areas of extensional/strike-slip tectonics. Both the large 1953 Suva earthquake (M_s = 6.75) and microearthquakes within and around the island of Viti Levu occur by strike-slip faulting, under the same stress field that is deforming the North Fiji and Lau basins. The low upper-mantle velocities, presence of widespread seismicity and Neogene volcanism, and state of stress of the Fiji platform contradict the notion of a stable land mass surrounded by zones of deformation in the neighboring back-arc basins.

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