International Geologiical Congress - Oslo 2008

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EIE-07 High-resolution geophysical imaging of geological structures and processes in environmental studies

 

Investigating the structure of the ostler fault zone, south island, New Zealand, using high-resolution seismic reflection

 

Fiona Campbell, ETH Zürich (Switzerland)
Anna Kaiser, ETH Zürich (Switzerland)
Wanda Stratford, University of Copenhagen (Denmark)
Heinrich Horstmeyer, ETH Zürich (Switzerland)
Michael Finnemore, University of Canterbury (New Zealand)
Laurent Marescot, ETH Zürich (Switzerland)
David Nobes, University of Canterbury (New Zealand)
Alan Green, ETH Zürich (Switzerland)
 

 

The Ostler Fault Zone is one of several structures to the east of the Alpine Fault on the South Island of New Zealand that accommodate some of the compressional component of the ∼45 mm/yr oblique convergence between the Pacific and Australian plates. This fault zone consists of a series of predominantly west-dipping, highly segmented, surface rupturing pure thrust faults that transect Quaternary glacial outwash terraces in the Mackenzie Basin and have a total slip rate of 1 - 2 mm/year. It has been the subject of extensive surface mapping and geomorphological studies, and ground-penetrating radar data studies have imaged steeply dipping faults at depths less than 5 m, but although several models have been suggested, little is known of the faul structure at greater depths.

This study focuses on the Benmore section of the fault zone, where folding and a series of small faults accommodate displacement in a transfer zone between two non-overlapping fault segments. We carried out a high resolution seismic reflection survey across this section in 2007 to (i) determine the structure of the fault, and associated deformation in the hanging wall and (ii) track lateral variations in these structures across the transfer zone. Twelve 1.2km-long seismic lines were recorded perpendicular to the fault strike and two additional tie lines were recorded parallel to strike, covering ∼1.6km of fault length. We used a 240-channel acquisition system with 3m and 6m receiver and shot spacings, respectively. Application of basic seismic reflection processing techniques to this high quality data set reveals complex dipping sedimentary layering in both the hangingwall and the footwall down to 650ms traveltime, where a strong sub-horizontal reflection occurs. Additional, weaker, sub-horizontal reflections are recorded down to 1300ms. More sophisticated processing techniques are being applied, both to clarify the complex dipping structures and to image steeper dips. Structures identified so far indicate that compression in the hanging wall of the fault is accommodated by both folding and subsidiary faulting, with strong lateral variations across the transfer zone.

 

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