Shallow fault segmentation of the Alpine fault zone,New Zealand revealed from 2- and 3-D GPR surveying |
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| Authors: | Alastair F McClymont Alan G Green Anna Kaiser Heinrich Horstmeyer Robert Langridge |
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| Institution: | 1. Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand;2. GNS Science, Lower Hutt, New Zealand;3. School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand;1. A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;2. A.P. Karpinsky Russian Geological Research Institute (VSEGEI), Srednii pr. 74, St. Petersburg, 199106, Russia;3. Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia;1. Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand;2. Institute of Hazard, Risk and Resilience, Durham University, South Road, Lower Mountjoy, Durham DH1 3LE, UK;3. Centre for Sustainability, University of Otago, PO Box 56, Dunedin, New Zealand |
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| Abstract: | Where they are preserved, landforms that have been truncated and offset by past fault movements provide potentially valuable quantitative data that can be used to estimate slip rates. At such locations, it is important to investigate the fault zone in sufficient detail to understand how displacements are accommodated on individual fault strands. At a site along a northern section of the Alpine fault zone on the South Island of New Zealand, surface mapping of a series of faulted river terraces and channels has revealed a complicated and poorly understood paleoearthquake history. We have acquired high-resolution 2- and 3-D ground-penetrating radar (GPR) data over a large area (~ 500 × 500 m) of the terraces to map along-strike changes in shallow (<20 m) fault zone morphology. By identifying distinct reflection patterns within the topographically migrated 3-D GPR volumes and extrapolating them to the longer and more widely spaced GPR profiles, we determined the subsurface extent of two main structural/depositional facies that were juxtaposed by three left-stepping en-echelon fault strands. Two regions of warped strata are interpreted to result from transpressive folding between the overlapping strands, where displacement is transferred from one fault to the next. We suggest that diffuse deformation between the overlapping fault tips results in anomalously low estimates for horizontal and vertical fault displacements of some geomorphic features. |
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