The Pacific/Australian plate boundary through South Island (the Alpine Fault plate boundary) is recognised as one of the best localities in the world to examine processes associated with continent-continent transpression and associated mountain building (the Southern Alps orogen). The boundary has some of the fastest rates of convergence and exhumation in the world. In its central portion the plate boundary structure is essentially 2D.
A concentrated effort to study this continental transform in New Zealand began in the mid- 1990s as an international collaboration between New Zealand and USA geoscientists. The effort was primarily focused in two coincident seismic experiments, and active source experiment - SIGHT, and a passive seismology experiment - SAPSE, although there were several ancillary research programmes. Altogether, approximately fifty professors, researchers, and students from New Zealand and USA participated in these projects and more than 40 peer-reviewed articles have been published to date in international journals (see Selected Publications ). At the core of this research was a major multi-disciplinary project funded by the NSF Continental Dynamics program and by the New Zealand Foundation for Research, Science and Technology (FRST).
The aim of the investigations was to understand the crustal-upper mantle deformation that occurs at a continental obliquely compressive plate boundary, including :
Pukaki seismic experiment (1994)
A ‘pilot’ multi-channel seismic survey was carried out along the edge of Lake Pukaki in 1994, as a pilot experiment for the subsequent and larger SIGHT96 and SIGHT98 experiments. The data from the Pukaki seismic experiment revealed seismic reflections dipping to the south-east. These seismic reflections were subsequently interpreted as reflections associated with the Alpine fault zone (Davey et al., 1995).
SIGHT96 (1996)
Multi-channel seismic lines were shot in 1996 offshore both the east and west coast of the South Island, using the RV Maurice Ewing. This seismic data acquisition was termed ‘SIGHT96’ (South Island GeopHysical Transect). The primary goal of SIGHT96 was to obtain detailed seismic velocity models along two profiles across the central South Island, and for the offshore region. The location of these lines are shown in the figure (brown lines). Seismic arrivals from airguns on the R/V Maurice Ewing were recorded onshore by several hundred receivers placed in two parallel transects which crossed the South Island, and offshore by 19 - 20 ocean bottom seismographs (OBS) on either side of the island. Land explosions were also recorded by recorders on these transects.
The seismic data collected in SIGHT96 have subsequently provided a strong basis for multiple lines of research, primarily targeting the crustal structure beneath the Southern Alps but also for investigating crustal and sedimentary structure offshore of the west and east coast. See Selected Publications : for a subset of the published research.
Location of SIGHT96 ship-tracks and offshore transects marked in brown
SAPSE (1994-1997)
A passive-seismic experiment, SAPSE (Southern Alps Passive Seismic Experiment) was carried out from 1994 to 1997 across all of South Island. The timing of SAPSE was coordinated so that earthquake recorders used in SAPSE also detected the SIGHT explosions and airgun sound waves, providing additional data for combined tomographic studies. The SAPSE deployment involved deploying 26 broadband sensors across South Island, mostly between 1995-1996 although some preliminary work was undertaken in 1994. These broadband sensors were supplemented by 14 short period sensors. Data from an additional 4 stations were utilised from outside South Island, from stations at Chatham Island, Macquarie Island, Whitehall and Riverview (Australia).
Figure above : Map views of 3-D Vp model at (a) 6-km depth (b) 14-km depth (c) 30-km depth (d) y=40 cross-section to 65 km depth.
The primary objectives for SAPSE included obtaining :
All of these objectives were met, and are described in a range of publications (See Selected Publications )
SIGHT98 (January-February 1998)
A high resolution crustal-scale seismic reflection profile, SIGHT98, was carried out in central South Island in 1998, to image the crustal root of the Southern Alps, and to attempt to image the Alpine fault. Conventional low-fold processing of the CDP data has lead to the seismic image shown above. The profile reveals that the central South Island crustal root lies beneath southern Lake Pukaki, and is laterally offset from the Southern Alps. The depth of the crustal root is approximately 40-45 km, thinning towards the east (Burkes Pass) to a depth of approximately 30-35 km. Although reflections are visible in raw shot data they are not well located in the above image, due to their steep dip. Further advanced migration processing is being carried out to sharpen the steeply dipping seismic reflections close to the Alpine Fault.
You can contact
Dr Stephen Bannister
Dr Donna Eberhart-Phillips
Street Address : 1 Fairway Drive, Avalon, 5010.
Postal Address : PO Box 30368, Lower Hutt 5040,
New Zealand
Tel:+64 4 570 1444
Fax:+64 4 570 4600
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November 14, 2006