California Institute of Technology
Seismological Laboratory Division of Geological and Planetary Sciences


SISIE, South Island, Subduction Inititiation Experiment

SISIESISIE, South Island, Subduction Inititiation Experiment, is an integrated, active-source seismic experiment on the Puysegur-Fiordland boundary south of South Island New Zealand and will test the hypothesis that this type-example of incipient subduction is making a transition from a forced to a self-sustaining state. SISIE was motivated to understand the forces that drive subduction , specifically by models that suggest that during subduction initiation, there is a transition from far-field plate stresses to the negative buoyancy of the slab. Puysegur is a segment of the Macquarie Ridge Complex (MRC; the boundary between the Australian and Pacific Plates), and is the best margin globally to study the early stages of subduction. The team will address, several fundamental questions, including: First, during subduction initiation what controls the transition from a forced to self-sustaining state? And, second, what are the mechanical conditions of the oceanic plate during incipient subduction?

The Team from Caltech, UT Austin and Victoria University of Wellington formulated their hypothesis from both an empirical direction and a theoretical one. The dynamic history of the overriding plate (vertical motions and stress regime) is linked to the amount of convergence across the trench. A key observation that they will exploit in SISIE is the extreme gravity anomaly in the Snares zone -- a region that experienced large amounts of subsidence after a period of uplift. Models show that this situation is expected dynamically before the margin transitions from forced subduction to self-sustaining subduction. The team will test the hypothesize that a strengthening of the slab pull force causes the subsidence along the ridge. SISIE will test this plate tectonic scenario with refined gravity models of the Puysegur region and seismic constraints on the crustal structure and sedimentary basin history. Moreover, initially steep fracture zones and normal faults transition or evolve to shallow thrust planes during subduction initiation, key aspects of the geodynamics. The team is being guided by forward modeling of the seismic source-receiver configuration that shows that the dip and other features of the slab at two positions along strike, each with a different degree of compression will be resolvable with the seismic methods employed.

The center piece of SISIE is comprised of two wide-angle refraction lines running perpendicular to the strike of the trench, one across this Snares zone and the other in an even more juvenile setting. The R/V Marcus Langseth, a ship owned by the National Science Foundation and operated by the Lamont-Doherty Earth Observatory of Columbia University, will carry out the survey and would deploy and recover 23 UTIG OBSs along the northern line and 20 OBS along the southern line. The OBS lines will constrain the crustal thickness, the nature of the plate interface, including its dip, and the seismic wave speeds in the crust and upper mantle across the plate boundary. Roughly 1730 km of additional MCS lines would constrain the upper basement, image upper plate faulting and map the stratigraphy of the Solander Basin, all keys to constraining a transient phase of compression as the force balance transitions from forced to self-sustaining subduction. Underway magnetics and gravity would further constrain regional anomalies and seafloor ages.

SISIE will address the main science questions concerning subduction initiation presented in the GeoPrisms Science and Implementation plans. A key outcome will be refined predictions of volatile release and electrical conductivity to help guide future measurements at this unique type locality for subduction initiation. New Zealand scientists will conduct onshore seismic recordings on the nature of the over-riding continental crust concurrent with the active source experiment. The team will hold a UT & Caltech marine geophysics course during the fieldwork so as to prepare the next generation of students for such research.

Caltech Participants

UT Austin Participants

Victoria University of Wellington Participants