Structure of the Hanmer strike-slip basin, Hope fault, New Zealand /l&VWd&Q .ta, G%+\..ip Ø A. Iizsfitiite of Geolo@~laiid Nirckear Scieiices Ltd., P.O. B0.x J320, Welliiigtoii, hhv Zealaiid RAY WOOD I JARG R. PElTINGA Dep;oclrrnreiit of Geolog: Uiziiwsip of Cariterbriiy, Cliristclizircli, AJew Zealaiid SPHENB"ls~R~ Iiistitute of Geological mid Nuclear Scieiices Ltd.., P. O. Box 1320, TVelliiigtoii, Neiv Zealarid *%??%!zORRAN Deyni-tiueiit of Gcologv. Uiiiivrsit)' of CaiiteiSriiy, Cliri~t~hrircl~,New Zcaluird ABSTRACT thicken and are tilted southward, with in- sin (step-over region between the major sequence lateral onlaps occurring to the fault segments). \Ye also conclude that Nannier basin (10 x 20 km), located in north and east, and also onto basement changes in fault geometry (releasing and re- northern South Island, New Zealand, is near the fault-controlled basin margins. straining bends and step-overs) at a variety evolving where two major segments of tfie The basin depocenter currently contains of scales and over short distances control dextral strike-slip Hope fault are projected > 1000 ni of sediment adjacent to the south the development of the extensile and con- to converge across a 6- to 7-kni-wide releas- margin and is disrupted by faulting only at tractile parts of the basin and three-dinien- ing step-over. The structural geometry and depth. In the western part of the basin, the sional basin asymmetry. Strain partitioning developnient of Ilannier basin does not con- sediment fill is thinner ("5ni) and is in- is complex and caiinot be related simply to form to traditional puli-apart basin models. tensely faulted across the entire basin local reorientation of the regional stress The respective fault segments do not width. field. o\*erIap but are indirectly linked :ilong the Today the rate of basin deepening under southwest margin of the basin by an oblique transtension at the western end is matched INTRODUCTION normal fault. The Hope River segment ter- by its progressive inversion and destruction minates in an array of oblique normal faults under transpression in the eastern sector, The Hanmer basin in northern South Ts- along the northwestern basin range front, with the oldest basin fill now being recycled. land, New Zealand, is evolving at a 6- to and east-west-striking normal faults on the We propose a hybrid model for Hannier 7-km-wide releasing step-over between eil west Hanmer Plain. Faulted Holocene allu- strike-slip basin, one in which geometric el- echeloiz segments of the dextral strike-slip vial-fan surfaces indicate west Hannier ba- ements of a fault-wedge basin (downward Hope fault (Figs. 1 and 2). The busin lins sin is actively subsiding and evolving under and upward tipped, spindle-shaped ends) been frequently cited in the international lit- north-south extension. The Conway seg- are combined with those of a pull-apart ba- erature as one of the best examples of a ment along the southeastern margin of the basin terminates in a complex series of ac- tive fault traces, sniall p~p-upridges, and 172"'E 173"'E 174'1 E graben depressions. Early basin-fill sedi- / ments of Pleistocene age are being folded, elevated, and dissected as the eastern part of Hannier 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 nornial faults we interpret to reflect large-scale upper crustal collapse of the hangingwall side of the Hope fault. New seismic reff ection data and geologic mapping reveal a persistent longitudinal Figure L. (A) Neiv Zealand plate boundary setting. IIM, Hikurangi Margin oblique and lateral asymmetry to basin develop- subduction zone; Al?, Alpine fault; B, region of the hlarlborougli fault system depicted in ment. Four seismic stratigraphic sequences Figure lß. ßold arrow is plate motion vector after de Mets and others (1990). (B) Marl- iderilified in the eastern sector of the basin borough fault system and location of Hanmer basin. Hope fault segments: H, Nope River segment; C, Conway segment. Arrow denote sense of relative fiorizontal displacenient, and letters (U = up; D = down) sense of vertical displacenient. Bold arrow represents plate *Present address: OIISTOh4, Nice, France. motion lector (after de hlets and others, 1990). Geological Society of Amcrics l3ulle~in,v. 1015, p. 1459-1473,, 10 figs., Novenibcr 1994. 1450 CI. R. S. T. O. M. Fonds Documentair9 LEGEN0 P localrty __----_--__--- bedding form lus major active foul 1639 spot height ___-- degraded escarpment & - with movement sense .. Figure 2. (A) Structural setting of Hanmer basin at the step-over between the Hope River and Conway segments of the Hope fault. Shading indicates elevated inourilairious terrain. Averaged strike of segments is annotated..ßold arrow indicates relative plate motion vector [after de Mets and others, 1990). LG, Lake Glynri Wye Graben; PG, Popl:irs Graben. (ß) Geologic and geomorphologic map of Ilanmer hasin. GG, Gabi-iels Gully; HP, Hanmer Plain; IS, 1inr;iIin SLation; M, RInrclimont Station; MS, Mctl~vaySt:ition; ‘YI%, po1i-up ridge or bulge; T, glacial outwasl~terrace; J,$’S, Woodbank Station. Dashctl/broken lines indicate projected and/or inferred continuatio~iof structures. HANhIER BASIN, HOPE FAULT, NEW ZEALAND structural depression, conforming closely to Prior to our study, understanding of the The Hope fault changes strike acros! traditional pull-apart basin niodels (Schu- Hanmer basin was based on reconnaissance Hanmer basin from 053" k 10" along tlic bert, 1980; Reading, 1950; Aydin and Nur, geologic mapping (Cotton, 1947; Clayton, Hope River segment west of the basin, tc 1982, 1985; Mann and others, 1983; 1966; Freund, 1971), a few shallow drill about 065" +- 5" along the eastern Conwaj Christie-Blick and Biddle, 19S5; Sylvester, holes (Thompson, 1966), and analysis of segment (Fig. 2). Historically the Hope 19SS). We present here newly acquired seis- gravity anomalies (Anderson, 1957). In this River segment last ruptured coseismieally in mic reflection data and detailed geologic paper we present analysis of newly acquired 1885 (Cowan, 1991), and it is clear from the mapping that reveal a strongly asymmetric seismic reflection data collected by the detailed report by McKay (1890) that rup- longitudinal and transverse basin geometry former New Zealand Departnient of Sci- ture \vas arrested at Hanmer basin. Paleo- and deformation extending \vel1 outside tlie entific and Industrial Research (DSIR) seismic studies completed in recent years immediate step-over width. The new data (Wood, 1991; Bannister and others, 1992.) suggest that the Hope River and Conway reveal a structural geometry coni-licting with and recently conipleted detailed field map- segments are seismically independent publislied interpretations of Hannier basin ping. These new data have allowed us to de- (Cowan, 1990. 1991; Cowan and McGlone, as a "classic" pull-apart. velop an impro\,ed three-dimensional un- 1991; McMorran, 1991; W. B. Bull, 1991, Structurally controlled rhombic and lazy- derstanding of the basin structure and its personal coniniun.), indicating that a struc- z-shaped depressions along major strike-slip evolution. Our results are discussed in the tural break exists across Hanmer basin. fuults are usually interpreted using well-es- light of the alternative niodels for strike-slip The Hope River segment represents a tablished theoretical and empirical pull- basin development, and we briefly consider zone of transtension subparallel to the apart basin models (for example, Crowell: tlie implications of strain partitioning in a muth of the relativc plate motion vector 19743, 197%; Rodgers, 1950; Mann and transpressive plate boundary setting. (26.1." ? 10') (de Mets and others, 1990) and others, 1953; Christie-Blick and Biddle, defines a 30-km-long releasing bend witliin 1985; Aydin and Nur, 19Si). Such basins the Hope fault zone (Cowan, 1991). Several cvolve progressively at releasing step-overs basins liave evolved at self-siniilar releasing or bends between major eli CC/IC~OIZstrike- bends and step-overs along this fault seg- slip fault segments. Basin dimensions are The Hannier basin formed on the Hope ment, ranging in width from several hun- controlled by the perpendicular-to-strike fault. the southern arid most active element dred meters (Lake Glpnn Wye Graben, Pop- step-over \{.idth and the overlap of the of the SO-hn-\vide Marlborough fault sys- lars Graben) to >5 kni (Hanmer basin) bounding fault segments. Secondary inter- tem. This fault system is that part of the Pa- (Fig. 2) (Clayton, 1966; Freund, 1971, 1974; connecting norinal and oblique normal cific-Australia plate boundary zone that Cowan, 1990). In this context Hannier basin faults strike diagonally behveen the master transects the continental crust of tlie north- represents a major segment bounday bounding faults, and it is across these that ern South Island, connecting the oblique- (Cowan, 1991). basins are "symmetrically" extended. Freund slip Alpine fault along the west coast of Tlie main surface trace of tlie Hope fault (1971) (see also Mann and others, 1953) South Island (for example, Norris and otli- is only partially preserved in the Hope- studied several basins along the Hope fault ers; 1990) to the west-directed oblique sub- Waiau river valley soutliwest of Hiinnier ba- and proposed a basin model in which the duction zone oEshore tlie east coast of sin. This is priniarily because a restraining niaster fault segments are not strike-parallel North Island and northeastern South Island bend of about 12" in the fault trace -3 kni but converge across the releasing step-over (for example, Lewis and Pettinga, 1993). west of the basin projects the active trace or bend. In the case of Hanmer basin, The Hanmer basin, as defined by the ex- northeast across the valley fioor and active Freund also noted that the hvo strike-slip tent of Hannier Plain, formed between the flood plain where it is concealed.