The active southern Havre Trough (35¿20'-37¿S) backarc basin is interpreted to form from the evolution and interaction of migrating cross-arc magmatism, and the progressive development of longitudinal rift grabens. The proposed migration of the proto-Kermadec arc front from the Colville arc at ~5 Ma to the active Kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening, and contemporaneously rifting, backarc complex. Migration trails are identified for at least four arc volcano sources. The Rumble V arc migration trail is the most prominent, forming a continuous, high-standing, magmatic arc ridge. These postulated arc trails segment the backarc region and initially limit rift development to the intervening proto-arc crust blocks. Early rifting between the arc volcanoes forms fully developed rift grabens which, with progressive basin widening, propagate longitudinally across the trails of migrating constructional arc magmatism. A model of the balance between rates of constructional arc magma production MR and destructive back-arc (rifting) extension VE is proposed. When VE is high, MR is insufficient to keep abreast of destructional rifting, resulting in small, isolated arc massifs quickly dismembered by rifting. Conversely, when VE is low, MR is sufficiently greater than rifting to produce a continuous, high-standing cross-arc ridge which segments longitudinal rift development. Migrating arc magmatism will be best observed, and preserved, in rifting backarc basins when MR is >600 km3 m.y.-1 100 km-1 of plate boundary and VE is <25 mm a-1. ¿ American Geophysical Union 1996