4D stress signals in the upper plate record subduction nucleation and lateral propagation

Subduction zones are fundamental to Earth’s plate tectonic history yet details of how they initiate remain enigmatic. Geodynamic models suggest that early stages of subduction depend on whether underthrusting is driven by horizontal or vertical forces. If horizontal forces dominate, the upper plate experiences compression and uplift followed by extension and subsidence, whereas vertically-forced subduction involves only extension. Geologic evidence from the Izu-Bonin-Mariana forearc supports a ~1 Myr rapid transition, whereas observations from Oman indicate a >8 Myr time lag between initial underthrusting and the onset of upper plate extension. We present seismic images of the incipient Puysegur subduction zone south of New Zealand. Our data show evidence for a stress signal (compression followed by extension) that spread from north to south as the trench initiated and propagated along the plate boundary. Both the magnitude and duration of the compressional phase diminish from ~8 Myrs long in the north to ~5 Myrs in the south. This timing indicates that the transition to self-sustaining subduction is more rapid when an adjacent downgoing slab contributes a driving force that aids subduction initiation. We therefore argue for a new framework in which horizontal forces dominate at sites of subduction nucleation and vertical forces gradually strengthen during later propagation as the developing plate boundary weakens and the slab-pull force intensifies. Our findings corroborate evidence for ancient horizontally-forced subduction initiation events and suggest that the geologic record may be biased, since vertically-forced scenarios of subduction propagation are more likely to be preserved than destructive subduction nucleation events.