Abstract Back‐arc basins provide insights into the processes governing the evolution of continental rifting to seafloor spreading. The Bransfield basin hosts a back arc rift that is hypothesized to be in the late stages of this transition. Orca volcano is a submarine volcano that lies on the most evolved portion of the rift. It coincides with a transition in the rifting style from a clearly delineated magmatic rift to the southwest to a wide and deeper faulted graben with thicker sediment to the northeast. We conducted an active‐source tomography experiment to image the three‐dimensional isotropic and anisotropic P‐wave velocity structures of the upper‐crust of Orca Volcano. We developed a method to incorporate secondary arrivals to improve the imaging of the volcano's magma chamber. The magma chamber extends from ∼1 km beneath the seafloor to 3–4 km depth with a maximum melt fraction of 16%–41% and a melt volume of 0.8–2.3 km3. The southwest rift of Orca volcano is characterized by a narrow zone of low velocities that connects to the magma chamber and by strong rift parallel anisotropy at shallow depths. Its structure has similarities to a slow spreading ridge consistent with a rift that is transitioning to seafloor spreading. Extension to the northeast of Orca is more distributed and not clearly linked to the volcano. We infer that the change in extensional style results from a more hydrated and weaker mantle to the northeast that may be caused by a tear in the Phoenix slab and a consequent change in slab depth.
Kidiwela et al. (Sun,) studied this question.