Tectonic deformation in the North Fiji Basin

Abstract

The North Fiji Basin is a structurally complex marginal basin set between the obliquely converging Australia and Pacific Plates. This study uses recently acquired SeaMARC II imagery and bathymetry and GLORIA imagery as well as other geophysical data to examine the present structure and recent tectonic history of the basin. Three areas are studied in detail: The first area is the ridge-ridge- ridge triple junction at 17°S, 174°E which shows evidence of a recent realignment in spreading directions. This triple junction formed by a jump eastward from its former location to a site of extension within the proto-Fiji Transform Fault. The current morphological character is that the northern and eastern limbs are deep grabens and the southern limb has typical spreading ridge morphology. The second area is the Fiji Transform Fault north and west of Fiji which shows evidence of transtensional stress. Between 174°E and 176°E, the fault has a wide principal displacement zone and shows evidence of extension in a NW-SE direction. This is compatible with a broad zone of left lateral shear or an overstep In the fault trace but its activity is uncertain. Adjacent to the Fiji Platform, the fault trace proceeds through a series of left-stepping extensional relay zones (ERZ's). One of these ERZs appears to be propagating into the Fiji Platform breaking off a fragment of the island arc crust. The Balmoral Reef and Braemar Ridge are probably older fragments of the Fiji Platform which presumably formed similarly. The third area is the Pandora Ridge at the northern end of the basin which is undergoing extension and shearing. This area shows evidence of recent faulting and volcanism and probably marks the southern boundary of the Pacific Plate. The current structural complexity of the North Fiji Basin is the product of frequent changes in the location and nature of its tectonic elements. These changes reflect a history of a rapidly changing stress regime which is probably caused by the ongoing rotations of Fiji and the New Hebrides Arc. Furthermore, while the tectonic elements have undergone rapid evolution, this evolution does not suggest basin-wide non-rigid deformation. Indeed, on a large scale, there does not appear to be any non-rigid behavior. Deformation occurs within, or adjacent to, fairly narrow spreading rifts and faults.