Stress transition from horizontal to vertical forces during subduction initiation
Subduction zones are fundamental to plate tectonics, yet how they initiate remains enigmatic. Geodynamic models suggest that 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 of past subduction initiation events has been interpreted in terms of these alternatives; however, it is unclear whether they are mutually exclusive or represent different stages of early subduction. Here, we present seismic images of the Puysegur plate boundary south of New Zealand that reveal space–time relations of stress during subduction initiation. Our data show evidence for a stress transition (compression followed by extension) that spread from north to south as the trench nucleated and propagated along the plate boundary. Both the magnitude and duration of compression diminish from north (8 Myr) to south (5 Myr). This indicates that transition to self-sustaining subduction accelerates after nucleation of a downgoing slab increases driving forces and decreases fault strength near the propagating tipline of the nascent trench. Instead of horizontally forced versus vertically forced initiation, we propose a four-dimensional evolution, where horizontal forces initially dominate at the site of nucleation, but with time, vertical forces accelerate, propagate along strike and facilitate the development of self-sustaining subduction.
© 2022 Nature Publishing Group. Received 18 May 2021; Accepted 07 December 2021; Published 10 February 2022. We thank the captain, crew and science party of the RV Marcus Langseth for their efforts during the South Island Subduction Initiation Experiment. Thank you to the University of Texas Institute for Geophysics (UTIG) Marine Geology and Geophysics group for fruitful discussion and feedback, which enhanced this work. Research in this manuscript was supported by the National Science Foundation through awards OCE-1654689 (UT Austin), OCE-1654766 and OCE-2049086 (Caltech). B.S. is grateful for support from a UTIG Ewing and Worzel Graduate Fellowship. This is UTIG contribution no. 3864. Data availability: Uninterpreted and interpreted seismic images shown in this study can be found in the Extended Data (Extended Data Figs. 3–7). Seismic data from the SISIE expedition are available through the Marine Geoscience Data System (https://www.marine-geo.org/tools/entry/MGL1803). Underway geophysical data from MGL1803 are available from the Rolling Deck Repository (https://doi.org/10.7284/907966). Earthquake moment tensors can be accessed online from the gCMT (https://www.globalcmt.org) and GeoNet (https://github.com/GeoNet/data/tree/master/moment-tensor) catalogues. Regional bathymetry data are available through NIWA (https://niwa.co.nz/our-science/oceans/bathymetry) and gravity grids from GNS Science (https://www.gns.cri.nz/Home/Products/Databases/New-Zealand-Region-Gravity-Grids). Code availability: Maps were generated using the Generic Mapping Tools (GMT) software (https://www.generic-mapping-tools.org). Author Contributions: H.J.A.V.A., S.P.S.G., M.G., J.S. and R.S. led acquisition of the SISIE seismic dataset. B.S. conceived the study, reprocessed the EW9601-P1 seismic data and wrote the bulk of the manuscript. B.S. and S.P.S.G. conducted the detailed interpretation of seismic reflection data. S.P.S.G. and H.J.A.V.A. oversaw the project and the analyses. M.G. and R.S. were instrumental in developing the conceptual model of subduction initiation. All authors contributed to writing the manuscript through edits and the concepts outlined in this paper through feedback and discussions. The authors declare no competing interests. Peer review information: Nature Geoscience thanks Fabio Crameri, Susan Ellis and Marc-Andre Gutscher for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely, in collaboration with the Nature Geoscience team.
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