STRUCTURE OF THE HANMER STRIKE-SLIP BASIN, HOPE FAULT, NEW-ZEALAND

Hanmer basin (10 x 20 km), located in northern South Island, New Zeala nd, is evolving where two major segments of the dextral strike-slip Ho pe fault are projected to converge across a 6- to 7-km-wide releasing step-over. The structural geometry and development of Hanmer basin doe s not conform to traditional pull-apart basin models. The respective f ault segments do not overlap but are indirectly linked along the south west margin of the basin by an oblique normal fault. The Hope River se gment terminates in an array of oblique normal faults along the northw estern basin range front, and east-west-striking normal faults on the west Hanmer Plain. Faulted Holocene alluvial-fan surfaces indicate wes t Hanmer basin is actively subsiding and evolving under north-south ex tension. The Conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces, small pop-up r idges, and graben depressions. Early basin-fill sediments of Pleistoce ne age are being folded, elevated, and dissected as the eastern part o f Hanmer basin is progressively inverted and destroyed by north-south contraction. The north margin of the basin is defined by a series of t opographic steps caused by a normal faulting outside of the area of th e releasing step-over. These normal faults we interpret to reflect lar ge-scale upper crustal collapse of the hanging-wall side of the Hope f ault. New seismic reflection data and geologic mapping reveal a persis tent longitudinal and lateral asymmetry to basin development. Four sei smic stratigraphic sequences identified in the eastern sector of the b asin thicken and are tilted southward, with in-sequence lateral onlaps occurring to the north and east, and also onto basement near the faul t-controlled basin margins. The basin depocenter currently contains >1 000 m of sediment adjacent to the south margin and is disrupted by fau lting only at depth. In the western part of the basin, the sediment fi ll is thinner (<500 m) and is intensely faulted across the entire basi n width. Today the rate of basin deepening under transtension at the w estern end is matched by its progressive inversion and destruction und er transpression in the eastern sector, with the oldest basin fill now being recycled. We propose a hybrid model for Hanmer strike-slip basi n, one in which geometric elements of a fault-wedge basin (downward an d upward tipped, spindle-shaped ends) are combined with those of a pul l-apart basin (step-over region between the major fault segments). We also conclude that changes in fault geometry (releasing and restrainin g bends and step-overs) at a variety of scales and over short distance s control the development of the extensile and contractile parts of th e basin and three-dimensional basin asymmetry. Strain partitioning is complex and cannot be related simply to local reorientation of the reg ional stress held.