T001-0001
When strike-slip comes first: Strain partitioning insights from the Puysegur subduction zone

Monday, 7 December 2020
Poster
Sean P S Gulick1, Brandon Shuck1, Harm J Van Avendonk2, Michael Gurnis3, Erin Hightower4, Joann M Stock5 and Rupert Sutherland6, (1)University of Texas at Austin, Institute for Geophysics & Department of Geological Sciences, Austin, TX, United States, (2)University of Texas at Austin, Institute for Geophysics, Austin, TX, United States, (3)California Institute of Technology, Seismological Laboratory and Division of Geological and Planetary Sciences, Pasadena, CA, United States, (4)California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States, (5)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (6)Victoria University of Wellington, Wellington, New Zealand
Abstract:
Pre-stack depth migrated seismic data from the SISIE project south of New Zealand image deformation and timing relationships within the Puysegur subduction zone that may be instructive for formative mechanisms of strain partitioning. Strain-partitioning was first described in the Sumatra-Andaman subduction zone, wherein oblique convergence is accommodated by discrete zones of thrust faults, such as within an accretionary prism, and by strike-slip faults. Strain-partitioning has been described in numerous oblique subduction zones where major strike-slip faults are proximal to volcanic arcs as well as within forearcs.

The Puysegur subduction zone formed in the Miocene and its volcanic arc consists of a single, still forming edifice. Subduction initiated along a pre-existing strike-slip fault that is the northward extension of the Macquarie Ridge system, which juxtaposed oceanic crust against the stretched continental crust of the Solander Basin. A <35 km wide, 7 km thick deformed sedimentary prism has formed in the northern portion of this recent subduction zone where the still active strike-slip Puysegur Ridge and Snares Zone serve as the backstop. The outer 18 km of the prism is dominated by imbricate thrusting with significant backthrusting. The inner 17 km consists of out-of-sequence thrusts (OOST) that have strike-slip faults in the hanging wall of individual structures. We observe strike-slip faults cutting off OOSTs and vice versa. Inward of this prism, the Puysegur Ridge strike-slip faults offset both sediments and slivers of oceanic crust, such that a forearc sliver exists.

Coincidence of OOSTs and strike-slip was also observed on the outer edge of the Kumano forearc Basin, Nankai Trough, where it was suggested to be mechanically favorable due to strength contrast between the inner and outer wedge. Increasing prevalence of strike-slip landward is similar to that observed in the Aceh forearc Basin, Sumatra, where strike-slip faults overprint older accretionary backthrusts. The Puysegur margin demonstrates that strike-slip faults can not only enable subduction initiation, under the right forcings, but can also serve to partition strain at the outset. Some strain partitioning strike-slip fault systems on other margins may similarly pre-date the subduction zone maintaining a forearc sliver geometry.

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