AGU Fall Meeting 2014, San Francisco, USA
1University of Texas; 2Caltech; 3Los Alamos National Laboratory; 4Victoria University of Wellington 5GNS Science, NZ
On Sunday December 14, 2014, from 1:30 to 5 pm, a diverse group of researchers met in the Grand Hyatt San Francisco before the AGU Fall Meeting to discuss coordination of work within the South Island, New Zealand GeoPRISMS primary site. The South Island of New Zealand offers extraordinary opportunities to address subduction cycles and dynamics science questions. Members of the community are gearing up for work in New Zealand and so the time was ripe to foster collaboration between US scientists and others internationally.
Following an introduction from the organizers, Sean Gulick (UT Austin) recapped the science priorities defined for Puysegur and Fiordland in the GeoPRISMS Implementation Plan. Sean described how the South Island of New Zealand offers a wealth of prospects for subduction zone research. The Puysegur Trench region – a juvenile subduction zone “caught in the act” of initiation – provides unique opportunities to investigate the geodynamics of this fundamental plate tectonic process. In Fiordland, tectonic motions have led to deep exhumation of a pristine Cretaceous arc section and offers a prime locale to investigate the root zones of an ancient arc at outcrop scale. Addressing questions on subduction initiation, exhumed terranes, and subduction thrust slip behavior in one region is an exciting opportunity and will require large geophysical field deployments, targeted geological fieldwork, sampling, geochemical analysis and geodynamic models.
The overview was followed by shorter talks describing specific targets or nascent efforts for larger activities. Joshua Schwartz (CS Northridge) described how an exhumed arc root exposed at Fiordland provides opportunities to address how volatiles, fluids, and melts are stored, transferred, and released through the subduction system. Sarah Penniston-Dorland (U Maryland) then described how Fiordland presented an outstanding locale for an ExTerra Field Institute in which a group of experienced scientists and students would spend several weeks in the field familiarizing newcomers to the area, collecting rock samples and making other detailed field observations. Jamie Howarth (GNS Science) discussed surface processes and the history of earthquakes from the sedimentary record. Jamie described his own work using sequences of turbidites to understand landslides and erosion in the Southern Alps and how the large magnitude earthquakes within Fiordland can be better understood through the study of turbidites.
Harm Van Avendonk (UT Austin) gave a talk on measuring crustal and fault structure across Puysegur with active source seismology. Harm described how the fundamental geophysical unknowns in Puysegur limit our understanding of subduction initiation. Through detailed models of seismic wave propagation through Puysegur, Harm showed how crustal structure, crustal thickness and dip of the nascent plate boundary could be determined with east-west active source seismic lines. Recent seismic work elsewhere showed that the necessary data could be acquired with an active source experiment. The field geophysical theme continued with a talk by Michal Kordy and Phil Wannamaker (U of Utah) on constraining mantle volatiles with an MT (magnetotellurics) experiment. They showed how major changes in electrical resistivity are likely associated with volatiles in the mantle and how a combined onshore and offshore MT experiment across Fiordland and Puysegur could constrain the volatile release during subduction initiation. Joann Stock (Caltech) made the case for magnetic measurements along Puysegur – the only subduction zone in which the kinematics of both over-riding and under thrusting plates are well known during the initiation phase.
Brian Jicha (U of Wisconsin) and Gene Yogodzinski (U of South Carolina) gave a talk on adakitic volcanism and subduction initiation at Solander Island. Solander is the only sampled volcanism along Puysegur and the andesites there are adakitic. Brian reviewed the other locations in which adakites are found and that melting of MORB eclogite in the subducting oceanic crust is one aspect of their formation. Most studies of subduction initiation have been made on western Pacific arcs and Puysegur provides an opportunity to study a nascent arc which has a different petrological expression. The case was made that there is a large area of submarine volcanism around Solander that has yet to be sampled and that the time is now ripe to do so.
Several talks explored work currently underway on the South Island that complements those planned for GeoPRISMS. Simon Lamb and Tim Stern (Victoria U of Wellington) gave a talk exploring the putative hyperextended margin of the conjugate to Campbell Plateau that might be the crust now below the central part of the Southern Alps. Martha Savage (Victoria U) gave an overview of several other South Island projects including seismic anisotropy over the extent of the island and drilling within the Alpine Fault.
The talks were followed by open discussion on both the science and logistics of the various plans presented. In terms of science returns, the participants discussed how the seismic experiments link the plate kinematics to the structure and evolving force balance. The MT experiment would map the first appearance of volatile release heralding the transformation of basalt to eclogite that could have provided a major jump in the force driving subduction initiation. Discussed at length was the question of optimizing the logistics of the passive MT and active seismic experiments while providing opportunities to sample volcanic rocks around Solander Island. The two geophysics experiments have different footprints: the seismic lines are more tightly aligned on the Puysegur margin while the MT experiment extends farther afield. The vessel that deploys or recovers the MT instruments might also be able to dredge for samples around Solander. The broader group discussed logistical aspects of holding an ExTerra Field Institute in the remote Fiordland location highlighting the advantages of coordination with any geophysical deployment. The group identified numerous opportunities and ways to coordinate activities through both NSF programs and international collaboration.Reference information