Abstract Geomagnetic vector anomaly measurements were conducted in the southernmost Lau Basin and Havre Trough during the R/V Yokosuka LAUHAVRE cruise in order to understand better their evolutional processes. This arc-backarc system was poorly surveyed previously except for the southernmost segment near New Zealand. The southernmost tip of a propagating rift, the southern extension of the Valu Fa spreading ridge, occurs at 24¡00'S, 177¡10'W in the southernmost Lau Basin, where a prominent graben occupies. Strong negative down-component magnetic anomalies (positive magnetization) probably associated with the oceanic crust of an Brunhes age can be traced down to 23¡50'S. The entire Havre Trough including the junction to the Lau Basin is in a rifting stage at present. No magnetic anomalies indicative of true seafloor-spreading was identified, although lineated magnetic anomalies probably caused by dikes and other intrusions emplaced in arc crust along the rift zone were observed. In the central Havre Trough, the occurrence of NE-SW trending magnetic structure was estimated, which is a little oblique to the trend of the active Kermadec Ridge and the remnant Colville Ridge.