Keenan and Encarnación suggest that the absence of pre-subduction inception basement in the drill core data taken from site U1438 raises ambiguity in our conclusion of spontaneous subduction initiation in the Izu–Bonin–Mariana system¹. However, there is no evidence for uplift in the earliest products of the Izu–Bonin–Mariana system preceding rifting preserved anywhere in the region. Three sub-parallel ridges — from north to south, the Amami plateau, Daito and Oki-Daito ridges (Fig. 1 from ref. 1) — comprise magmatic products of Mesozoic–Tertiary arcs formed prior to the Izu–Bonin–Mariana system. These ridges generally strike east–west orthogonally to the Kyushu–Palau ridge — the earliest stratovolcano chain of the Izu–Bonin–Mariana system. If compression preceded inception, as implied by the forced subduction model², we anticipate uplift of the old arc ridges, diminishing in effect westwards, and sediment shedding from uplifted regions into adjacent basins.

None of these predicted effects are observed. The Amami plateau, Daito and Oki-Daito ridges do not shallow eastwards, rather they shallow westwards³, possibly due to subsidence of the eastern sectors following inception of the Izu–Bonin–Mariana system⁴. Crustal thicknesses change rapidly across-strike of the ridges, showing no east–west compressional thickening^3,4. Furthermore, Lower Eocene sedimentary sequences in the Minami Daito basin, which predate Izu–Bonin–Mariana inception, are clast-free mudstone⁵. Basalt sills intercalated with these sediments are alkaline intraplate types lacking subduction zone input, consistent with a rifting environment⁶. This lack of evidence for compression of pre-Izu–Bonin–Mariana basement, coupled with the rifting and seafloor spreading accompanying the earliest arc products that we document¹ is consistent with a spontaneous initiation model.

Both forced and spontaneous subduction inception models² are oversimplified. The process is likely to be strongly three-dimensional and probably propagates along-strike from an extant subduction system⁷. For the Izu–Bonin–Mariana system, northward propagation from a subduction zone on the southern boundary of the proto-Philippine Sea plate as the latter rotated clockwise is possible¹. Juxtaposition of old, dense Pacific plate lithosphere against the relatively buoyant lithosphere of the Mesozoic to Lower Tertiary arcs could have been critical for the spontaneous nucleation of a new subduction zone⁸.