From Rifting to Subduction: Evidence for the Role of Past Tectonics Influencing Subduction Initiation at the Puysegur Trench, New Zealand

The critical ingredients and conditions to initiate a new subduction zone are widely debated. However, there is a general agreement that subduction initiation likely takes advantage of previously weakened lithosphere. To evaluate how pre-existing lithospheric structures may facilitate underthrusting and lead to a mature subduction zone, we present an analysis of the Puysegur margin, a young deep-sea trench with a diverse tectonic history.

The Puysegur margin exhibits several characteristics of a subduction zone, including ongoing plate convergence, an active Benioff zone, and adakitic volcanism. Plate reconstructions show that the margin has experienced a complicated tectonic transformation from rifting to seafloor spreading, to strike-slip motion, and most recently to incipient subduction, all in the last 45 million years.

Here we present new seismic images from the South Island Subduction Initiation Experiment (SISIE) which surveyed the Puysegur region in February-March, 2018. SISIE acquired 1252 km of deep-penetrating multichannel seismic (MCS) data on 8 lines, including 2 critical dip lines which extend from the incoming Australian plate to across the forearc Solander Basin.

Our preliminary MCS profiles reveal several important findings within the Solander Basin: (1) The crust is extended and tilted on large normal faults. We infer that the basement here is thinned continental Campbell Plateau crust formed during Eocene rifting. (2) In the south, we see thinner continental crust stretched over a wider distance and evidence of numerous volcanic sills, which suggests that there is a north-south transition in crustal structure, possibly related to a temporal evolution in rifting processes. (3) The basin contains up to 3 km of sediment including distinguishable packages of pre-, syn-, and post-rift sedimentary sequences. (4) We find fault zones deforming of all these sequences, suggesting some of the normal faults inverted upon compression likely related to subduction initiation.

These early, yet convincing images suggest subduction at the Puysegur Trench was assisted by inherited buoyancy and reactivation of structural weaknesses that were imprinted into the lithosphere during an earlier phase of continental rifting which aided plate failure and subduction nucleation 30 million years later.