HR: 08:35h
AN: T31G-03    [Abstracts]
TI: Seismic Potential of a Compressional Inversion Province, NW South Island, New Zealand
AU: * Ghisetti, F C
EM: francesca.ghisetti@otago.ac.nz
AF: Department of Geology, University of Otago, Leith Street, Dunedin, 9001 New Zealand
AU: Sibson, R H
EM: rick.sibson@otago.ac.nz
AF: Department of Geology, University of Otago, Leith Street, Dunedin, 9001 New Zealand
AU: Berryman, K
EM: k.berryman@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, 69 Gracefield Road, Lower Hutt, 6009 New Zealand
AB: In the NW South Island of New Zealand, large historical earthquake ruptures (1929 Murchison M7.7 and 1968 Inangahua M7.2) have dominant components of reverse slip. At the surface, active reverse faulting occurs along a segmented system of closely-spaced (10-20 km apart), N-S to NNE-SSW trending faults continuous over $>$ 150 km, and dipping 50-$80\deg$ both east and west. The faults truncate subparallel folds that deform the Tertiary sequence overlying a composite Paleozoic and Mesozoic crystalline basement. Some of the high-angle faults are inherited from Cretaceous extension, and have been subsequently inverted, consequent on right-lateral shearing and transpression on the Alpine Fault since the early Miocene. However, the deep geometry of these faults, their penetration into the basement and their relationship to the Alpine Fault are poorly understood. These problems have been addressed by reconstructing the structural contours at the base of the calcareous Oligocene sequence for the whole northwestern margin of the South Island. This represents a convenient strain marker, because: (i) it was deposited close to sea level; (ii) it is extensively preserved on land; (iii) it stands out as a high energy reflector in seismic lines; and, (iv) it has been penetrated by a number of oil exploration boreholes. Regional deformation of this Oligocene marker results from shortening episodes since the early Miocene with the development of sets of elongated and narrow N-S folds (wavelength $<$ 10 km). Crests of anticlines range in elevation from 1000 to 3000 m asl, whereas troughs of synclines are as low as -2000 m bsl. These large vertical displacements of the Oligocene marker are partly accommodated by reverse faulting (throws $<$ 3000 m), and partly by bedding-parallel slip along the steep flanks linking anticline-syncline pairs. Shortening calculated from section restoration is highly variable, ranging from $<$ 10%\ up to 30%\. These estimates represent a minimum because large-scale detachments of the cover sequence are likely to have been accommodated by blind thrusts that ramp across the crystalline basement and follow layer-parallel trajectories at the basement-cover interface, or within low competence horizons. The deformation of the whole region is indicative of large-scale bulk shortening of the sedimentary sequence, between two basement buttresses. The eastern basement block is bounded by the sub-vertical to steeply E-dipping Alpine Fault. The western margin is a complex basement high that can be followed all along the coast line, bounded by a segmented system of interconnected high-angle inverted normal faults and low-angle thrusts dipping both east and west. The overall style of deformation is suggestive of strong components of shortening to the west and in the footwall of the Alpine Fault, accommodated by inversion of high-angle normal faults in the basement, folding and detachment of the sedimentary covers, and propagation of new, low-angle thrust faults, that cut across the early structures and the detached folds. Nucleation of seismic ruptures in the basement at depths of $\sim$ 10-15 km may therefore lead to distributed deformation in the near-surface. Conversely, large seismogenic faults that have not ruptured in the past 200 years may be concealed beneath thick detached portions of the cover sequence.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting