Back-arc basins, characterized by intense extension behind island arcs or along continental margins adjacent to trenches, are commonly attributed to subduction processes involving multiple thermal and mechanical factors. However, subduction alone is not sufficient to trigger the back-arc extension as the latter is missing in some ongoing subduction zones. The Bransfield Basin is a young (~4 Ma) back-arc basin related to the remnant subduction of the Phoenix Plate that once existed along the entire Pacific margin of the Antarctic Peninsula. As a young back-arc basin formed in a long-term subduction setting, the Bransfield Basin is an ideal site to learn how the back-arc extension initiates and what is the driving mechanism behind its evolution. Within the BRAnsfield VOlcanoes SEISmology (BRAVOSEIS) project, a dense, amphibious broadband seismic network was deployed in the Central Bransfield Basin (CBB) and adjacent areas between 2018 to 2020. Here, based on these new seismic observations, we perform ambient noise tomography to image S-wave velocity (Vs) structures in this area. Our images reveal that the CBB suffers strongest extension in the northeast, featured by coupled crust and mantle deformations with mantle exhumation, which weakens to the southwest with decoupled deformations, indicating that the CBB experiences different along-strike extensional phases of continental rift-seafloor spreading transition. We suggest that the along-strike variation of extension was a consequence of the slab window formation and the forearc rotation, which are associated with the Phoenix Plate detachment during the ridge–trench collision at the southwest of the Hero Fracture Zone. The role of the Phoenix Plate in this process implies that the ridge–trench collision and subsequent slab detachment are the main initiation mechanism of back-arc extension in the CBB.