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1 Institute of Geological and Nuclear Sciences Ltd., P.O. Box 1320, Wellington, New Zealand
2 Department of Geology, University of Canterbury, Christchurch, New Zealand
Hanmer basin (10 x 20 km), located in northern South Island, New Zealand, is evolving where two major segments of the dextral strike-slip Hope fault are projected to converge across a 6- to 7-km-wide releasing step-over. The structural geometry and development of Hanmer basin does not conform to traditional pull-apart basin models.
The respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault. The Hope River segment terminates in an array of oblique normal faults along the northwestern basin range front, and east-west-striking normal faults on the west Hanmer Plain. Faulted Holocene alluvial-fan surfaces indicate west Hanmer basin is actively subsiding evolving under north-south extension. The Conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces, small pop-up ridges, and graben depressions. Early basin-fill sediments of Pleistocene age are being folded, elevated, and dissected as the eastern part of Hanmer basin is progressively inverted and destroyed by north-south contraction.
The north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step-over. These normal faults we interpret to reflect large-scale upper crustal collapse of the hanging-wall side of the Hope fault.
New seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development. Four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward, with insequence lateral onlaps occurring to the north and east, and also onto basement near the fault-controlled basin margins. The basin depocenter currently contains >1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth. In the western part of the basin, the sediment fill is thinner (<500 m) and is intensely faulted across the entire basin width.
Today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector, with the oldest basin fill now being recycled. We propose a hybrid model for Hanmer strike-slip basin, one in which geometric elements of a fault-wedge basin (downward and upward tipped, spindle-shaped ends) are combined with those of a pull-apart basin (step-over region between the major fault segments). We also conclude that changes in fault geometry (releasing and restraining bends and step-overs) at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three-dimensional basin asymmetry. Strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field.
This article has been cited by other articles:
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  H. Rother, H. M. Jol, and J. Shulmeister Stratigraphy and tectonic implications of late Pleistocene valley fill in the Hope Valley, Canterbury, South Island, New Zealand Geological Society of America Special Papers, January 1, 2007; 432(0): 155 - 167. [Abstract] [Full Text] [PDF]
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 W. D. Cunningham and P. Mann Tectonics of strike-slip restraining and releasing bends Geological Society, London, Special Publications, January 1, 2007; 290(1): 1 - 12. [Full Text] [PDF]
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P. Mann Global catalogue, classification and tectonic origins of restraining- and releasing bends on active and ancient strike-slip fault systems Geological Society, London, Special Publications, January 1, 2007; 290(1): 13 - 142. [Abstract] [Full Text] [PDF]
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 P. M. Barnes, R. Sutherland, and J. Delteil Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand Geological Society of America Bulletin, March 1, 2005; 117(3-4): 411 - 435. [Abstract] [Full Text] [PDF]
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J. W.F. Waldron Anatomy and evolution of a pull-apart basin, Stellarton, Nova Scotia Geological Society of America Bulletin, January 1, 2004; 116(1-2): 109 - 127. [Abstract] [Full Text] [PDF]
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J. Reijs and K. McClay The Salina del Fraile pull-apart basin, northwest Argentina Geological Society, London, Special Publications, January 1, 2003; 210(1): 197 - 209. [Abstract] [PDF]
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