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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. 

Subduction initiation remains one of the least understood global processes of plate tectonics. Prominent models have been cast in terms of two broad classes: spontaneous cases due to lithospheric gravitational instabilities and induced cases due to forced plate convergence. Yet gravitationally unstable lithosphere is old, strong, and difficult to begin to bend into a subduction zone and convergent forces necessary to begin subduction are often too large given the plates involved. These models also consider the asthenospheric mantle as passive, even though relative motion between slabs and the asthenosphere has long been regarded as a strong control on subduction dynamics. Here I propose that subduction-transform edge propagator (STEP) faults can initiate subduction depending on the absolute motion of the STEP fault with respect to the asthenosphere. STEP faults form where subduction zones end and the subducting plate tears forming a down flexed transcurrent plate boundary at the surface shearing against the adjacent rear arc lithospheric plate. However, STEP faults are not simple transcurrent boundaries. Absolute motion of the down flexed STEP fault edge with respect to the surrounding asthenosphere can produce a strong sea anchor force that either continues to bend the edge downward, initiating subduction, or opposes slab bending, inhibiting subduction. In the south Pacific, the southern end of the New Hebrides Trench and the northern end of the Tonga Trench are type-example STEP faults with opposite senses of dip but both moving northward with respect to the asthenosphere. The northward dipping New Hebrides STEP fault moves northward in a mantle reference frame creating a strong asthenospheric flow against the STEP fault edge, inducing active subduction at the Matthew-Hunter trench. In contrast, the Tonga STEP fault dips southward but also has a northward component of motion with respect to the mantle. Asthenosphere thus flows southward beneath the down flexed Tonga STEP fault edge opposing further bending.  Subduction does not initiate at the Tonga STEP fault despite a ~100 Myr age contrast between the Pacific and north Fiji and Lau basin lithospheres. Since absolute plate motions reflect the sum of all forces acting on the entire lithospheric plate, a strong sea anchor mantle force may be generated at a STEP fault edge, initiating subduction (or inhibiting it), even where lithosphere is old, strong, and resists bending and without requiring large convergent forces between plates, overcoming these objections to previous models.