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EIE-07 High-resolution geophysical imaging of geological structures and processes in environmental studies
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Shallow imaging of the northern Alpine Fault zone from high resolution 3D seismic reflection data
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Anna Kaiser, ETH Zürich (Switzerland)
Fiona Campbell, ETH Zürich (Switzerland)
Wanda Stratford, University of Copenhagen (Denmark)
Heinrich Horstmeyer, ETH Zürich (Switzerland)
Robert Langridge, GNS Science (New Zealand)
Michael Finnemore, University of Canterbury (New Zealand)
Jacques Ernst, ETH Zürich (Switzerland)
David Nobes, University of Canterbury (New Zealand)
Alan Green, ETH Zürich (Switzerland)
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The Alpine Fault marks the boundary between the Australian and Pacific plates through the South Island of New Zealand. It accommodates much of the relative transpressional plate motion. Offset basement terranes suggest ∼470 km of dextral strike-slip movement has occurred along its length, and a smaller compressional component of movement is apparent from the uplift of the Southern Alps east of the fault. Our study site is well known for a concrete wall erected across the fault trace in the 1960s. It shows no evidence of fault creep over the past ∼40 years. Nevertheless, displaced Quaternary river terraces at the site are evidence for relatively recent movements along the fault, with the last event estimated to have occurred ∼1530-1700 AD. Ground-penetrating radar data and trenching delineate a steeply dipping fault zone through Quaternary gravels to depths of 15m. Our high-resolution 3D seismic reflection survey aims to image fault zone structure beyond the sedimentary cover into basement rock to ∼300 m depth. We have used a pseudo-3D shooting configuration to survey an area of ∼190 x 500m, yielding up to 50-fold data with a CMP spacing of 4 x 2 m. In addition, a 360 m-long ultra-high resolution seismic reflection line provides ∼60-fold data at 0.25 m intervals. This line targets the detailed shallow structure, yielding a direct tie to the existing ground-penetrating radar data. Relatively standard seismic reflection processing techniques have yielded high-quality stacked sections that reveal sedimentary layering in shallow river gravels and a strong reflection from the sediment-basement contact.
The dipping basement reflection is offset by ∼40 m across the main trace of the Alpine fault and outlines a hanging wall drag-fold structure. We speculate that basement faulting has offset an erosional surface that formed during the most recent major period of glaciation. Such an interpretation results in a Quaternary dip-slip rate of ∼2mm/yr, in accord with other estimates along the fault in this region. A second notable basement feature is imaged approximately 300 m from the main fault in the corner of our survey area. It may be evidence for erosional basement relief or a subsidiary fault strand. Further processing and tracing of structures in 3D will aid our interpretation of this feature. We also intend to apply more sophisticated processing techniques in an attempt to image relatively steeply dipping structures within the fault zone, with the aim of accurately tracing the location of the fault plane itself.
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