David Summer , Melissa Anderson, Alan Baxter, Philipp Brandl

Intra-oceanic backarc are characterized by crustal accretion along seafloor spreading axes; however, little is known about the initial rifting of the over-riding plate prior to the establishment of stable seafloor spreading. To address this knowledge gap, we investigate the ~3.5 Ma backarc troughs in the New Hebrides Subduction Zone. Using available bathymetric data, we developed remote-predictive geologic maps at a scale of 1:100,000 over an area of ~234,000 km2. Interpretation of seafloor morphologies, lineament analyses of seafloor fabric, fault kinematics revealed by earthquake moment tensor data, and cross-cutting relationships demonstrate distinct stress regime changes during sequential backarc rifting. South of 10.5°S, three phases of backarc opening are identified: 1) Initial arc rifting accommodating clockwise rotation of the arc, ~3.5–2.7 Ma (preserved in the Duff Horst and Graben Domain); 2) East-west-directed rifting and incipient seafloor spreading, ~2.7–1.1 Ma (preserved in the Jean Charcot Troughs); 3) Northeast-southwest- to NNE-SSW-directed rifting and counter-clockwise rotation of the arc, ~1.1 Ma-present (preserved in the actively rifting Santa Cruz Troughs). Two prominent roughly east-west oriented backarc volcanic corridors stretch from the relict arc and track arc migration along deep crustal-scale structures. North of ~10.5°S, two grabens formed from a single rift event occurring before the crustal scale ruptures that severed the Reef Islands Platform from the New Hebrides Arc. The outcome of this work reveals new insights in the geodynamic processes that bridge periods of stable subduction with the formation of mature backarc basins with crustal accretion occurring along spreading axes.