MORPHOSTRUCTURE AND MAGNETIC FABRIC OF THE NORTHWESTERN NORTH FIJI BASIN

Four successive spreading phases are distinguished in the northwestern part of the North Fiji Basin. After an initial NE-SW opening, a N-S spreading phase took place, up to the northwesternmost tip of the basin, along the South Pandora, Tikopia and 9-degrees-30 Ridges. The N-S spreading phase in the northern North Fiji Basin was followed by an E-W opening phase along the central North Fiji Basin axis. A triple junction was probably active during an intermediate stage between the two phases. E-W spreading underwent a reorganisation that induced the functioning of the 16-degrees-40'S triple junction and the development of the E-W trending Hazel Holme Extensional Zone from the active central spreading axis to the southern tip of the New Hebrides Back-Arc Troughs. Active extension also occurs along the E-W Santa Cruz Trough which crosscuts the arc platform at the northern end of the N-S trending Back-Arc Troughs. The existence of the Back-Arc Troughs is mainly due to the construction of the 400 km-long volcanic Duff Ridge which trapped a piece of the old North Fiji Basin oceanic crust.