New Zealand Journal of Geology and Geophysics abstracts
Upper crustal structure beneath the eastern Southern Alps and the Mackenzie
Basin, New Zealand, derived from seismic reflection data
D. T. Long*
S. C. Cox
S. Bannister
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
M. C. Gerstenberger
Swiss Seismological Service
ETH-Hoenggerberg/HPPP
CH 8093, Zurich, Switzerland
D. Okaya
Institute of Geophyics
University of Southern California
Los Angeles, CA 90089-0740, USA
*Present address: Petroleum Geo-Services, Level 4, IBM Centre, 1060 Hay St,
West Perth, WA6005, Australia.
Abstract A 65 km seismic reflection transect was shot
in 1998 across Mackenzie Basin to Mount Cook Village, South Island, New Zealand,
to provide a detailed image of the crustal structure in the central Southern
Alps. The first 5 s (two-way time (TWT)) of data were processed separately,
with a maximum offset of 14 km for each shot, to image the upper 12-15 km
of the crust. Data were processed as one continuous section, although the
line was physically segmented due to an area of relatively steep topography
with no vehicular access. No major, continuous regional-scale features >10
km are present in the data, but numerous 2-3 km scale reflections and discontinuities
occur which are consistent with the known geology of monotonous greywacke
sequences overlying schist. Strong, well-defined reflections mark the active
Irishman Creek Fault and confirm it to be a southeasterly dipping reverse
fault with c. 1300-1700 m of Late Cretaceous-Pleistocene sediments preserved
in the footwall and an uplifted greywacke basement “high” in the hanging
wall. Some evidence exists for active faults beneath latest Quaternary gravels
at the Jollie valley and Tekapo River. Oppositely dipping reflections and
discontinuities define a large, c. 15 km wavelength antiform beneath Tasman
valley and Mount Cook that is imaged to 10 ± 2 km depths (3.5 s TWT).
Two “end-member” interpretations are consistent with the seismic data observations,
velocity models, and constraining features of exposed geology, and extend
existing geological cross-sections to 10-15 km depth. One interpretation
assumes imaged structures are primarily backthrusts developed in response
to distributed Cenozoic deformation southeast of the Alpine Fault plate boundary,
incorporating features observed in contemporary geodetic strain and numerical
plate boundary models. The second interpretation assumes structures are mostly
Mesozoic, either reactivated or preserved by late Cenozoic deformation. The
main difference between the interpretative cross-sections is the degree to
which active structures link into basal detachment and high-strain zones
at depth.
Keywords Southern Alps; seismic reflection; SIGHT;
Mackenzie Basin; faults; tectonics; deformation
G01035 Received 29 October 2001; accepted 24 September 2002; published
21 March 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 21-39
0028-8306/03/4601-0021 $7.00/0 © The Royal Society of New Zealand
2003
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