Magnetic and Tectonic Fabric of the North Fiji Basin and Lau Basin
Detailed airborne and shipboard magnetic studies conducted over the eastern marginal basins of the southwest Pacific between New Zealand and the Solomon Islands suggest that seafloor spreading in the North Fiji Basin, which began in the latest Miocene, continues today. The North Fiji Basin is marked by at least two triple junctions, located in the central (16°50’S, 173°45’E) and northeastern (14°S, 179°30’E) parts of the basin. Both were apparently formed during the past five million years in response to continuing adjustments in the strike of the active spreading centers in the North Fiji Basin. Two additional contemporary spreading centers appear to have formed within the last one million years to the north and immediately to the west of Viti Levu. Although magnetic anomalies from 1 to 3 (0–5 Ma) are seen to form a clearly defined lineation pattern over the North Fiji Basin, earthquake foci suggest that only portions of the present North Fiji/Lau Basin spreading center system are currently active.
Analysis of the magnetic anomalies suggests that spreading began in the North Fiji Basin about 8 Ma (shortly before anomaly 4). At about 6 Ma (shortly before anomaly 3), the New Hebrides/Fiji island arc began to fragment through the development of an active spreading center (calculated half-spreading rate of 3.5 cm/yr) located between the New Hebrides and Fiji portions of the island arc. Subsequently, portions of the northeastern sector of the South Fiji Basin Plain were subducted beneath the North Fiji Basin. Between 4.5 Ma and 3.5 Ma (anomalies 3 and 2’), spreading in the North Fiji Basin extended into the Lau Basin and Havre Trough, thus separating the Tonga and Lau ridges. As a result, a distinct series of active spreading centers developed southward along the axis of the Havre Trough/Lau Basin.
The continuum of spreading centers from the North Fiji Basin into the Lau Basin and Havre Trough has been accomplished through the formation of triple junctions located in the North Fiji and northern Lau basins and propagating southward rifts in the southern Lau Basin.
KeywordsMagnetic Anomaly Spreading Center Seafloor Spreading Ridge Segment Tectonic Fabric
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