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Detailed Seismicity of the Alpine Fault Zone and Fiordland Region, New Zealand

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Detailed Seismicity of the Alpine Fault Zone and Fiordland Region, New Zealand CHRISTOPHER H. SCHOLZ 1 Lamont-Doherty Geological Observatory of Columbia University, JOHN M. W. RYNN J Palisades, New Yor{ 10964 ROBERT W. WEED \ „ , CLIFF FRÖHLICH I Department of Geological Sciences, Cornell University, Ithaca, New Yor\ 14850 Detailed Seismicity of the Alpine Fault Zone and Fiordland Region, New Zealand ABSTRACT Island of New Zealand by the formation of the transcurrent Alpine fault, which cuts the A study of the Alpine fault zone and the geosyncline obliquely and has an inferred Fiordland region of the South Island of New dextral post-Jurassic displacement of 450 km Zealand from February through April 1972 (Wellman, 1955a). The Alpine fault has been indicates high but diffuse microearthquake traced more than 500 km on land from Milford activity. Composite focal mechanism solutions Sound to Cook Strait (Fig. 1). Along much of show that a regional northwest-southeast com- its course, the fault trace is nearly straight, pression dominates the tectonic pattern. This bounding the western escarpment of the direction is nearly normal to the Alpine fault, Southern Alps, and is easily identified by its indicating that the Alpine fault is now under- juxtaposition of dissimilar lithologies and by going a large component of thrust faulting. its commonly well-developed mylonite zone. This agrees with geologic data for uplift of the To the north of the Alps, the Alpine fault Southern Alps along the Alpine fault beginning branches into several subparallel faults and in mid-Miocene time and accelerating in the bends sharply. Some efforts have been made Pliocene, the time of the Kaikoura orogeny. to correlate the Alpine fault with faults across Before the Kaikoura orogeny, the Alpine Cook Strait on North Island (Lensen, 1958) fault apparently was a transcurrent fault. This and to extend it along the Fiordland coast major change in the New Zealand tectonic south of Milford Sound (Brodie and Dawson, pattern could have been produced by a rela- 1965; Barker, 1967). tively minor migration of the nearby Indian- The significance of the Alpine fault and its Pacific pole of rotation. Incipient underthrust- relation with the Southern Alps was recognized ing of the Tasman Sea appears to be occurring by Henderson (1929), but Wellman was off the Fiordland coast, terminating at the largely responsible for the modern concept of point where the Lord Howe Rise intersects the the Alpine fault as one of the major tectonic coast. To the north is a zone of oblique con- features of New Zealand (Wellman and Willett, tinental convergence, with the Southern Alps 1942; Wellman, 1952, 1953, 1955a, 1972). being rapidly uplifted along the Alpine fault. Wellman demonstrated the dominant role North of the Alps, much of the motion is that large right-lateral movement on the fault transferred to several faults that have more has played on the post-Jurassic geologic easterly strike; these formed in the Kaikoura evolution of South Island. The Alpine fault is orogeny and constitute a new transform fault now widely regarded as being one of the major system. transcurrent faults of the world and is com- monly compared with the San Andreas fault GEOLOGIC BACKGROUND of California (Richter, 1958; Allen, 1965; The late Paleozoic to Mesozoic New Zealand Hatherton, 1968). geosyncline was the site of active subduction On a larger scale, the Alpine fault is thought until disrupted by the Late Jurassic to Creta- to form a sector of the Indian-Pacific plate ceous Rangitata orogeny (Landis and Bishop, boundary (Le Pichon, 1968), filling the gap 1972). The orogeny was marked on the South between the westward-dipping underthrust Geological Society of America Bulletin, v. 84, p. 3297-3316, 11 figs., October 1973 3297 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3297/3432993/i0016-7606-84-10-3297.pdf by guest on 26 September 2021 3298 SCHOLZ AND OTHERS MESOZOIC GRAYWACKE AND ARGILLITE UPPER PALEOZOiC GRAYWACKE FIG.8, 9 LOWER PALEOZOIC METAMORPHIC ROCKS C'TAGO, HAAST, AND MARLBOROUGH SCHISTS F'RECAMBRIAN (?) GRAYWACKE »ASIC AND ULTRABASIC GRANITIC AND DIORITIC STEWART IS. Figure 1. Generalized geology of the South Island of New Zealand. zone of the Hikurangi and Kermadec-Tonga slip feature. There are a number oi observa- trenches to the north and the Macquarie tions, however, that point to a significant Ridge complex to the south (Fig. 2). The north component of vertical motion on the fault end of the ridge is terminated by the Puysegur during Quaternary time. trench (Hayes and Talwani, 1972). McKenzie and Morgan (1968) have therefore described Evidence of Fault Movement the Alpine fault as a trench-trench transform A right-lateral shift of 450 km on the Alpine fault. Quantitative calculations using this fault (Wellman, 1955a) is suggested by the model suggest rates of movement of 5 to 6 apparent offset of the major stratigraphic units cm per yr on the fault (Christoffel, 1971). of the New Zealand geosyncline (Fig. 1). The Indian-Pacific pole of rotation lies just Wellman's hypothesis, although not rigorously south of New Zealand in the vicinity of !at documented, is widely accepted. He originally 51° S. to 58° S„ long 160° E. to 180° E. (Hayes suggested that movement has been con- and Talwani, 1972), depending on the model tinuous since the Jurassic, but more recently chosen. As a result of the proximity of the he has argued that the entire movement was pole to South Island, the direction of motion post-Miocene (Wellman, 1964). Suggate (1963), on the Alpine fault has a sensitive influence on on the other hand, maintains that most of this the position of that pole, and vice versa. Thus, offset occurred during the Late Jurassic-Early the present direction of motion on the Alpine Cretaceous Rangitata orogeny. The offset of a fault plays a key role in our understanding of set of mid-Cretaceous lamprophyre dikes the tectonic pattern in this part of the south- indicates a total Cenozoic displacement of 120 west Pacific. Workers unfamiliar with the to 150 km (Grindley, 1963; Wellman and details of South Island geology have often Cooper, 1971). assumed that the Alpine fault is a purely strike- The difficulty in establishing the history of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3297/3432993/i0016-7606-84-10-3297.pdf by guest on 26 September 2021 ALPINE FAULT ZONE AND FIORDLAND REGION, NEW ZEALAND 3299 garnet-oligoclase zone are found on the south- east side of the fault; metamorphic grade decreases away from the fault with zone bound- aries nearly parallel to the fault (Grindley, 1963). K-Ar radiometric ages of the schists decrease toward the fault (Hurley and others, 1962). All of these observations are consistent with late Cenozoic uplift on the fault of as much as 20 km (Suggate, 1963). The mylonitic rocks in the fault zone also suggest significant vertical motion. The mylo- nitic zone is about 1 km wide and is composed of rocks varying from fault breccias and cata- clastites to mylonites and blastomylonites in concentric zones of successively increasing grade (Reed, 1964). Mylonitic rocks on active transcurrent faults such as the San Andreas normally range ¡n grade no higher than cata- clasites. The higher grade mylonitic rocks are normally observed only on deeply eroded, ancient faults, such as the Moine thrust of Scotland, and probably represent mylonitiza- tion at great depth. Their present outcrop along the Alpine fault is probably a result of Figure 2. Regional tectonic setting of New Zealand. large vertical motion on the fault which has Trenches are black, major plateaus and rises are cross- hatched. The Indian-Pacific pole of rotation of Le exposed previously deep-seated fault segments. Pichon (1968) is given by the open circle; that of Gunn (1960) has suggested that minor struc- Christoffel (1971), by the closed circle. tural features indicate that the Alpine fault is a thrust fault with a maximum principal com- movement on the fault lies in the lack of suit- pressive stress nearly normal to it. able offset reference lines. Other than the Thus, there is some doubt as to the sense offset of upper Paleozoic and Mesozoic rocks, and rate of motion on the fault since the there is no record of movement except that Cretaceous. There is no doubt that vertical provided by offset Quaternary morphologic as well as horizontal motion has been important features. These features are of primary interest during the late Cenozoic; however, it is prob- here, because we are concerned with the able that earlier movements were largely present tectonic role of the fault. horizontal. The indications of a significant Studies of offset river terraces and courses vertical component of motion pose a puzzle of the Alpine fault north of Arthur's Pass (Fig. concerning the present role of the fault as a 3) and of the Awatere and Wairau faults show major plate boundary. We will attempt to systematic right-lateral movements as well as clarify this puzzle from our detailed seismicity vertical movements (Suggate, 1963; Wellman, studies. 1952, 1964; Lensen, 1964, 1968). Along the Alpine fault where the fault bounds the South- Historical Seismicity ern Alps south of Arthur's Pass, horizontal The central section of the Alpine fault, south movement is less sure; and vertical motion, of where it splits into several separate traces with the southeast side up, appears to dominate. and north of Milford Sound, constitutes a Observations at the Hokitika and Paringa prominent seismic "gap" in the sense that no Rivers suggest postglacial uplift rates of 1 cm major earthquakes have been associated with per yr on the fault (Wellman, 1955b; Suggate, it during historical times (Eiby, 1971; Evison, 1963, 1968).
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