PDFArticle Abstract
The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins.