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The Alpine Fault at Gaunt Creek, Westland, New Zealand

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The Alpine Fault at Gaunt Creek, Westland, New Zealand Anatomy, structural evolution, and slip rate of a plate-boundary thrust: The Alpine fault at Gaunt Creek, Westland, New Zealand ALAN F. COOPER RICHARD J. NORRIS j- Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand ABSTRACT South Island, New Zealand, is one of Earth's ture over a 40-km section southwest of Gaunt major transpressional structures. It has a Creek, and we have found a similar pattern of Minimum slip rates calculated for plate-vec- documented dextral strike-slip displacement alternating thrust and strike-slip sections in tor-parallel slickenside trends in cataclasite on of480 km (Wellman, 1955), and as much as 70 the Alpine fault zone 85 km farther south, the sole of the Alpine fault at Gaunt Creek, km of convergence by reverse oblique slip north of Haast River. Westland, New Zealand, range from 18 to 24 (Walcott, 1979; Allis, 1986). Transpression The best exposure of the Alpine fault in mm/yr. Between half and two-thirds of the to- has resulted in the exhumation of amphibo- Westland is at Gaunt Creek, Waitangi-taona tal relative motion between the Pacific and Aus- lite-facies rocks from depths of 20-25 km River, which lies on the longest thrust seg- tralian plates is being accommodated by move- (Wellman, 1979; Cooper, 1980), most of ment mapped to date (Fig. 1). This paper de- ment on a single structure, the Alpine fault. which has occurred in the past 7 m.y. (Kamp scribes details of this outcrop and discusses During the past 14 ka, the leading edge of and others, 1989). Physiographically, the Al- the implications of the inferred tectonic his- the Alpine fault has changed from a moder- pine fault is best expressed in central West- tory on the mechanics of thrusting, the gen- ately southeast-dipping, oblique reverse fault land, where it separates the 3-km-high moun- eration of overthrusts, and the geomorphic to a shallowly dipping thrust. The hanging wall tains of the Southern Alps in the east from the evolution of the Alpine fault plate boundary. (Pacific plate) is composed of a gradational se- topographically subdued terrain of the quence from basal gouge, through pseudo- coastal apron to the west. On satellite REGIONAL GEOLOGY tachylite-bearing cataclasite, to progressively imagery, the fault trace is remarkably linear, more coherent schist-derived mylonite, which striking northeastward (055°). In central The regional geologic history and tectonic has been faulted against subhorizontally bed- Westland, however, the linearity of the fault development of the Alpine fault plate ded, fluvio-glacial gravel in the footwall (Aus- trace is illusory, and mylonitized schist from boundary were reviewed recently by Norris tralian plate). During uplift the hanging-wall the Southern Alps has been transported west- and others (1990). The rocks exposed to the sequence has been internally sheared and im- ward in a series of napplets over the West northwest of the Alpine fault in the Waitangi- bricated, producing duplex structures, and Coast sequence for distances of up to 2.5 km taona River area are composed of foliated retrogressively veined and altered by pervasive from the Alpine front (Bowen, 1954). These granitoid and quartzofeldspathic gneiss of hydrothermal fluid flow. overthrusts have been attributed to gravity probable Mesozoic age (Kimbrough and oth- Erosion of the exhumed fault zone produced collapse of the range front (Wellman, 1955; ers, 1994; Rattenbury, 1991) overlain by Qua- angular, cataclasite- and mylonite-derived, ta- Suggate, 1963) or tectonic shortening (Norris ternary moraine and fluvio-glacial gravel lus-fan breccias, building a west-dipping apron and others, 1990). (Fig. 1). Southeast of the Alpine fault, the beneath the fault scarp. Wood fragments from Recent detailed field mapping between the Southern Alps are made up of the Haast near the base of the talus breccias have been Fox and Whataroa rivers has shown that the Schist, which reaches a metamorphic grade 14C dated at 12,650 ± 90 yr B.P. Progressive fault zone is far from simple, comprising al- of amphibolite facies in the zone of maximum tectonic shortening resulted in 180 m of over- ternating segments that have average strikes uplift adjacent to the fault. thrusting of a schist-derived nappe across an ranging from 010° to 050° and 070° to 090°, irregular talus fan surface composed of its own respectively (Norris and others, 1990). East- GAUNT CREEK SECTION erosional debris. The structural history of the trending fault segments dip steeply but con- Alpine fault at Gaunt Creek illustrates the im- tain gently plunging slickensides and shear- On the south bank of Gaunt Creek, the Al- portance of the interaction between fault-in- sense indicators that suggest predominantly pine fault is exposed in an outcrop almost 700 duced topography and erosion, and the control dextral strike-slip displacement. In contrast, m long and >100 m high (Figs. 2, 3). The these processes exert on the continued tectonic, the more northward-striking segments are fault, marked by a zone of cataclasite in the geometric, and geomorphic evolution of the characterized by thick zones of cataclasite hanging wall, dips 40° southeast at creek fault zone. overlying a gently to moderately east-dipping level, but, 200-300 m northwest, isolated fault surface. Slickenside lineations indicate patches of cataclasite in the middle sections INTRODUCTION oblique thrusting. Fault segments range from of the face define a shallow, south-dipping hundreds of meters to several kilometers in thrust. In the southeastern part of the expo- The Alpine fault, forming the boundary be- length (Norris and others, 1990, Fig. 2). Seg- sure, cataclasite overlies fluvial gravel com- tween the Pacific and Australian plates in the mentation dominates the Alpine fault struc- posed of rounded schist clasts, but farther Geologica) Society of America Bulletta, v. 106, p. 627-633, 5 figs., 1 table, May 1994. 627 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/5/627/3381983/i0016-7606-106-5-627.pdf by guest on 01 October 2021 COOPER AND NORRIS Figure 1. Locality maps. A. Plate boundary structures in the South Island, New Zealand, with localities referred to in the text. Heavy arrow represents the direction of motion of the Pacific plate, determined from the pole of rotation given by DeMets and others (1990). B. Geology of the Waitangi-taona River catchment, Westland, illustrating the segmented nature of the Alpine fault trace. Outcrop of the Alpine fault described in Figures 2 and 3 is labeled as Gaunt Creek Slip. The shading shown on the Waitangi-taona River thrust segment of the Alpine fault indicates, somewhat diagrammatically, the distribution of cataclasite. northwest, the thrust has ridden over angular melt structures, suggests that deposition oc- cross-beds, developed between individual mylonite debris. Structurally above the fault, curred during a glacial period, probably the units of breccia. A large wood fragment, in- a varied sequence of cataclasite passes gra- last glaciation. In a zone as much as 2 m wide corporated in fine-grained sediments near the dationally into cataclastic mylonite and ulti- beneath the fault surface, platy schist tablets base of the sequence (Fig. 2), has yielded a mately into intact or coherent mylonite. in the gravel have been rotated into a subver- 14C age of 12,650 ± 90 yr B.P. (Table 1). In- tical orientation by fault drag. cluded in the debris are blocks of the pale Fluvio-glacial Gravel green, basal cataclasite, which—based on Mylonite-Derived Gravel observed present-day erosional behavior— In the southeast part of the Alpine fault survives fluvial transport for distances of only outcrop, the footwall is a sequence of crudely Subhorizontal bedding in the fluvial gravel a few or, at most, tens of meters. The largest bedded gravel composed of rounded, schist- is truncated to the northwest by a west-dip- blocks of mylonite rest on top of the dipping derived clasts. Coarse schist plates are com- ping erosional surface marking the base of a sequence and have an internal foliation that monly imbricated with an inferred flow direc- younger sequence of lenticular breccias and dips eastward. Gravel units likewise have tion similar to that of the present-day creek. gravels. In contrast to the fluvial sequence, crude layering that also is back-tilted due to Sag structures in bedding may have origi- these younger sediments predominantly rotational slumping. nated by collapse of the creek bed following composed of angular and unsorted clasts of The predominantly mylonite-derived melting of underlying "dead" ice. The age of mylonite. Grain size is highly varied, ranging gravel passes abruptly up section into a thick the gravels is unknown, but the absence of from blocks many meters in diameter, to fine unit of subhorizontal to shallow west-dip- wood material, together with possible ice- sandy or silty seams, exhibiting fine-scale ping, poorly imbricated and cross-bedded 628 Geological Society of America Bulletin, May 1994 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/5/627/3381983/i0016-7606-106-5-627.pdf by guest on 01 October 2021 PLATE-BOUNDARY THRUST, NEW ZEALAND SE NW 140° 320° QUARTZOFELDSPATHIC AND AMPHIBOLITIC MYLONITE CUT BY GOUGE-FILLED SHEARS FLUVIAL OUTWASH/FAN GRAVEL WITH AND VEINS OF PSEUDOTACHYLITE. DUPLEX MIXED SCHIST-MYLONITE PROVENANCE. STRUCTURES COMMON. BLACK ULTRACATACLASITE AND WOOD: 10,300 yrs B. P. WHISPY PSEUDOTACHYLITE. LENSES OF MYLONITE-DERIVED, TALUS FAN PALE GREEN CATACLASITE COMPOSED OF BRECCIA. LOCALLY LARGE. COMMONLY BACK-TILTED, FRAGMENTS OF CHLORITIZED MYLONITE SLUMP BLOCKS. AND VEIN QUARTZ. SCHIST-DERIVED, FLUVIO-GLACIAL GRAVEL WITH SUBHORIZONTAL BEDDING AND STRONG PEBBLE IMBRICATION. 100m Figure 2. Map of the Gaunt Creek slip face, compiled from horizontal photographs corrected for scale variations by radial-line methods (Ailum, 1966).
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