Reply to ‘Unclear causes for subduction’

<p>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 system^{<a href="#ref1">1</a>}. 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. <a href="#ref1">1</a>) — 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 model^{<a href="#ref2">2</a>}, we anticipate uplift of the old arc ridges, diminishing in effect westwards, and sediment shedding from uplifted regions into adjacent basins.</p><p>None of these predicted effects are observed. The Amami plateau, Daito and Oki-Daito ridges do not shallow eastwards, rather they shallow westwards^{<a href="#ref3">3</a>}, possibly due to subsidence of the eastern sectors following inception of the Izu–Bonin–Mariana system^{<a href="#ref4">4</a>}. 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 mudstone^{<a href="#ref5">5</a>}. Basalt sills intercalated with these sediments are alkaline intraplate types lacking subduction zone input, consistent with a rifting environment^{<a href="#ref6">6</a>}. 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 document^{<a href="#ref1">1</a>} is consistent with a spontaneous initiation model.</p><p>Both forced and spontaneous subduction inception models^{<a href="#ref2">2</a>} are oversimplified. The process is likely to be strongly three-dimensional and probably propagates along-strike from an extant subduction system^{<a href="#ref7">7</a>}. 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 possible^{<a href="#ref1">1</a>}. 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 zone^{<a href="#ref8">8</a>}.</p>