Extension at oceanic spreading centers ranges from ultra-slow (dominantly tectonic) to ultra-fast spreading (dominantly magmatic). This variation is reflected in the morphology of the spreading ridge segments and the magmatic productivity observed on the seafloor. These relationships are well understood at Mid-Ocean Ridges (MOR), but less is known about spreading centers above subduction zones. This study is part of a larger initiative to create the first 1:1,000,000 scale geological maps of different subduction zones at the Indo-Australian margin. This is a region of some of the fastest-growing crust on Earth and exhibits prolific magmatic-hydrothermal activity in back-arc basins. Previous work has shown that crustal growth associated with westward subduction of the Pacific Plate is characterized by highly distributed extension in back-arc basins, with numerous and simultaneously active spreading centers. In the NW Lau Basin, two of the spreading centers are punctuated by large-scale magmatic centers that coincide with anomalous mantle input (as documented by large-scale mantle helium anomalies) − features that are not well known in other basins. Detailed mapping at 1:200,000 scale shows that these spreading centers are related to near-field transcurrent faulting that developed in the early stages of the Lau back-arc basin. Translation across two oppositely moving fault zones induced rotation of the intervening crust and two anomalous spreading centers (Rochambeau Rifts and the Northwest Lau Spreading Center) opened obliquely to these structures. Both show inflated axial volcanic ridges that may be a product of an anomalous melt supply relative to the spreading rate. The marked variation in the morphology and magmatic output are thought to be controlled by input of melt from adjacent sources (Samoan plume) or the channeling of melt into a zone of thicker pre-existing crust, or both. These findings have important implications for understanding the origins of large-scale magmatic input in back-arc basins, where many fossil ore deposits have formed, thus providing important guides for resource exploration in ancient volcanic terranes on land.