The Mid-Cenozoic Challenger Rift System of Western New Zealand and Its Implications for the Age of Alpine Fault Inception

The mid-Cenozoic Challenger Rift System of western New Zealand and its implications for the age of Alpine fault inception PETER J. J. KAMP Department of Earth Sciences, University of Waikato, Hamilton, 2001, New Zealand ABSTRACT finger-like re-entrants of the Late Cretaceous New Caledonia basin and Bounty Trough, which both protrude into the subcontinent. The objective Analysis of the structure and sedimentary geology of western of this paper is to document evidence for another rift system, of Eocene- New Zealand has identified a middle Eocene to early Miocene conti- Oligocene age, named here the "Challenger Rift System," which trended nental rift system, 1,200 km long and 100-200 km wide, named here north-south through western New Zealand from the Norfolk basin to the the "Challenger Rift System." Four phases of rift development oc- Solander Trough (Fig. 1). This rift, by dissecting the subcontinent, induced curred: (1) infra-rift subsidence, (2) active axial trough subsidence, (3) subsidence of its most interior parts. It also indicates that the foundering expanded rift subsidence involving collapse of the rift shoulders, and was associated with at least two periods of rifting, one in the Late Creta- (4) incipient sea-floor spreading. The spatial and temporal distribution ceous and another in the mid-Cenozoic. of these phases identifies a North Island and a South Island rift seg- The Challenger Rift System is named after the Challenger Plateau, a ment and shows that rifting propagated toward the center of the rift shallowly submerged portion of the New Zealand subcontinent (Law- from both ends. The northern segment shows a simple pattern of rence, 1967) that lies west of two-thirds of the rift system (Fig. 1). rifting that is comparable with Vink's model of rift propagation; the The Challenger Rift System is now dextrally dislocated 480 km southern segment, with locked zones and rift nucleation segments, is across the Alpine fault. This suggests that inception of the Alpine fault, comparable to Courtillot's model of rift propagation. The sea- and thus propagation of the modern Australia-Pacific plate boundary floor-spreading history of the southwest Pacific shows that the north- through New Zealand, did not occur until the early Miocene. In a strict ern rift segment probably linked with a sea-floor-spreading center in sense, therefore, the modern Australia and Pacific plates were not discrete the Norfolk Basin, and the southern segment linked with the Southeast entities until the early Miocene, as compared with the present understand- Indian Ridge. This is corroborated by the good correlation between ing that this was probably achieved by the late Eocene (Molnar and others, the ages of sea-floor magnetic anomaly lineations that are aligned with 1975; Weissel and others, 1977), or possibly as early as the Late the rift and the biostratigraphic ages of rifting. Cretaceous (Stock and Molnar, 1982). The probable continuity of the rift system in its early develop- The essential evidence for this rift system is the former occurrence ment precludes pre-Miocene transcurrent displacement on the Alpine through onshore and offshore western New Zealand of a 100- to 200-km- fault; an early Miocene (23 m.y. B.P.) age of Alpine fault inception is wide zone of interconnected normal-fault-bounded troughs and half gra- indicated by the age and pattern of rift disruption attributed to com- bens, which show many of the structural features that are characteristic of pression that originated at the Australia-Pacific plate boundary. The modern rifts and of continental margins in the early stages of passive modern Australia and Pacific plates were not discrete entities, there- margin development (Fig. 2). Paleogeographic considerations, and notably fore, until the early Miocene. the spatial pattern of lateral differences in the degree of rift development, suggest that the rift system had a northern and a southern segment, and INTRODUCTION that rifting propagated toward central New Zealand from both the north and the south. At the late Oligocene peak of its development, the northern The New Zealand subcontinent occupies an intra-oceanic position in segment linked with the southern segment in northwest Nelson, then a the southwest Pacific Ocean (Fig. 1). A distinctive characteristic of this region of shallowly submerged plateaus with deeper basins to the north continental mass is the small proportion above sea level. That any part is and south. emergent is due mainly to the effects of the late Cenozoic convergence Near the Oligocene-Miocene boundary, the tectonic style and sedi- across the Australia-Pacific plate boundary; the widespread occurrence mentation patterns changed in the southern segment of the rift system; it is onland of thick middle Cenozoic marine successions shows that the pres- inferred that movement began on the Alpine fault, causing the rift system ent landmass was formerly even less extensive. to become dislocated and overprinted by obliquely compressional tecton- The origin of this foundering has received little attention. It has been ics. Crustal thinning, which characterized the Paleogene setting, changed to suggested that it may have followed the Late Cretaceous separation of crustal rethickening, which was manifest in the South Island by reverse New Zealand from eastern Gondwanaland (Carter and others, 1974). movement on pre-existing normal faults, and thus rift basin eversion, Models of passive margin development show that subsidence is certainly to uplift, and erosion. Farther from the Alpine fault, and notably in the be expected about the margins of a rifted continent, but the problem with North Island segment, extension persisted until the late middle Miocene, New Zealand is the extent of subsidence in the interior parts of the subcon- and the rift system may have developed into the early stages of a passive tinent. This may be partly overcome by post-rift subsidence about the margin with incipient sea-floor spreading. Geological Society of America Bulletin, v. 97, p. 255-281, 15 figs., March 1986. 255 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/255/3444925/i0016-7606-97-3-255.pdf by guest on 01 October 2021 150'E 160 E 170 E 40° S now Strike-slip plate boundary with relative motion (cm/yr) Convergent plate boundary with relative motion (cm/yr); barbs on upper pi ate Active spreading ridge ^ Absolute plate motion (cm/yr) Challenger Rift System 160 E 70° E 170 W Figure 1. Map of the southwest Pacific, showing the tectonic character of the Australia-Pacific plate boundary and Alpine fault disloca tion of the Challenger Rift System through western New Zealand. Directions and rates of plate motion from the Plate tectonic map of the circum-Pacific region—Southwest quadrant (copyright: The American Association of Petroleum Geologists, Tulsa, Oklahoma). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/255/3444925/i0016-7606-97-3-255.pdf by guest on 01 October 2021 OLIGOCENE N PACIFIC OCEAN TASMAN . -V '. -\ Eastern Western .:•.•'.'• \ Basin ' «y Platform / N.W. Nelson shields •¿V £•/ Canterbury Basin a / Figure 2. Map of the late Oligocene distribution of basins and structural highs comprising the Challenger Rift System in relation to the trend of the Alpine fault. The outline of New Zealand is based on Walcott and Mumme (1982). T rep- resents Te Anau basin; L-T, Long- wood-Takitimu High; and T-P-H, Tongaporutu-Patea-Herangi High. The distribution of geophysical anomalies is after Hatherton and others (1979). ssr Tuhua Orogen- -Rangitata Orogen- 100 200km Land Submarine Geophysical anomalies I Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/3/255/3444925/i0016-7606-97-3-255.pdf by guest on 01 October 2021 258 P.J.J. KAMP Continental lift systems commonly connect with oceanic spreading of the Alpine fault (Fig. 3). The second map covers southwestern South centers. This is net obvious when the Cenozoic rift system is compared Island south of the Alpine fault (Fig. 4). with current plate-tectonic syntheses of the southwest Pacific (for example, Figure 3 shows that a 100- to 200-km-wide zone of north- Molnar and others, 1975). Consequently, a new synthesis of the sea- south-trending basins occurs in the onshore and near offshore pa rts of floor-spreading history has been developed (Kamp, 1986a), based on the western New Zealand and that this is separated from an eastern basin constraints that (1) there has been only 500 km of dextral displacement on province by a thin strip of outcropping basement. Prior to the early Plio- the plate boundary through New Zealand and (2) that the Alpine fault cene, the zone of outcropping basement was much wider, and therefore the was not initiated until the early Miocene. The result of this reconstruction separation between the eastern and western basinal provinces was much is that the northern segment of the rift system was aligned with a backarc greater; the subsidence in the Taupo Volcanic Zone is Quaternary in age Norfolk basin spreading center, and the southern segment was aligned (Grindley, 1960), and the Wanganui basin originated in the early Pliocene with the Southeast Indian Ridge. (Fleming, 1953). From the magnitude of the negative gravity anomalies The interpretation of the origin of the Eocene-Oligocene depocenters (Fig. 3), it seems that the Wanganui basin subsidence has not yei: been in western New Zealand which is advanced here differs from other current isostatically compensated at depth. A persistent reduction in depth to models. Other workers have viewed these basins as originating in an basement southward through the western basins to the Alpine fault is obliquely extensional continental transform setting concurrent with, and another feature of Figure 3. This feature is examined in the following indeed caused by, Alpine fault movement (McQuillan, 1977; Norris and subsections together with evidence for the continuity of mid-Cenozoic others, 1978; Noiris and Carter, 1980, 1982; Knox, 1982), whereas I basins across the modern plate boundary.

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