Subduction initiation at the New Hebrides STEP fault induced by absolute plate motion and mantle flow

Within the last two-million years, subduction has initiated at the southern end of the New Hebrides trench along the ~E-W trending Matthew-Hunter section of the trench (Patriat et al., 2015; 2019). This part of the subduction system originated as a subduction-transform edge propagator (STEP) fault, a transcurrent plate boundary that terminates Australian plate subduction at the southern end of the New Hebrides trench at a slab tear and allows its rapid southwestward rollback (Govers and Wortel, 2005). The down warped torn lithospheric edge of the STEP fault dips northward in the same direction as the absolute plate motion of the Australian plate in a hotspot reference frame. This creates a strong southward mantle flow (~55 km/Myr) against the already failed and weak northward dipping STEP fault, promoting further down bending and subduction. Through this mechanism, subduction and southward rollback of the STEP fault edge has begun, initiating a subduction zone in an extensional stress regime without requiring initial convergence between the Australian plate and the North Fiji Basin. In fact, the North Fiji Basin is in extension, forming rifts and spreading centers and volcanically accreting crust unusually close to the Matthew-Hunter trench. Subduction initiation at the Matthew-Hunter trench has effectively terminated the STEP fault and slab tear, so that subduction now takes place continuously around the corner from the New Hebrides to the Matthew-Hunter section of the trench. This model proposes that STEP faults are favorable tectonic boundaries for subduction initiation, provided that mantle flow induced by absolute plate motion is oriented correctly, as shown by the opposing example of the Tonga step fault, which displays no evidence of initiating subduction despite a much larger lithospheric age contrast (Martinez, 2024).

Govers, R., and M. J. R. Wortel (2005), Lithosphere tearing at STEP faults: Response to edges of subduction zones, Earth and Planetary Science Letters, 236, 505-523.

Martinez, F. (2024), Subduction initiation (or not) due to absolute plate motion at STEP faults: The New Hebrides vs. the Tonga examples, in EGU General Assembly 2024, Vienna, Austria, https://doi.org/10.5194/egusphere-egu24-4189

Patriat, M., et al. (2015), Propagation of back-arc extension into the arc lithosphere in the southern New Hebrides volcanic arc, G-Cubed, 16(9), 3142-3159.

Patriat, M., et al. (2019), Subduction initiation terranes exposed at the front of a 2 Ma volcanically-active subduction zone, Earth and Planetary Science Letters, 508, 30-40.