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

Home , Alpine Fault

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

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 commonly 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

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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

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garnet-oligoclase zone are found on the southeast side of the fault; metamorphic grade decreases away from the fault with zone boundaries 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 mylonitic 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 cataclasites. 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). The rate of horizontal right- 1971). Seismology, therefore, has had little lateral movement is uncertain but may be as to add in terms of focal mechanism solutions large as 1 cm per yr (Clark and Wellman, 1959). regarding the question of the present tectonic These rates lend credence to the idea that the role of the Alpine fault. Southern Alps owe their elevation to uplift The central section of the fault bounds the along the fault. High-grade schists of the high part of the Alps for which the evidence

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MICROEARTHOUAKE SURVEY SOUTH ISLAND, NEW ZEALAND FEB.-APR. 1972

41' STATIONS: • LDGO-COR SITE

A DSIR PERMANENT

A DSIR TEMPORARY

42"

CHRISTCHURCH

43°

T A S M A N SEA 44"

ALPINE FAULT AWATERE FAULT WHITE CREEK FAULT LYELL FAULT GLASGOW FAULT 45" AHAURA FAULT CLARENCE FAULT HOPE FAULT KAKAPO FAULT BRUCE FAULT HARPER FAULT ESK FAULT PORTERS PASS FAULT MOONLIGHT FAULT SHOTOVER FAULT HOLLYFORD FAULT LIVINGSTONE FAULT LAST COVE FAULT KELLY FAULT

WAIRAU FALjLT 166" 167" 168° 169° 70° 171" 172° Figure 3. Instrument location map. For explanation of station abbreviations, see Appendix.

for vertical movement is greatest. North and on the Macquarie Ridge indicate easterly south of this relatively inactive region, the underthrusting in vicinity of lat 49° to 52° S. level of seismicity increases abruptly. To the (Johnson and Molnar, 1972). This is associated north of Arthur's Pass (Fig. 3) where the fault with the Puysegur trench, a typical circum- zone splays into several traces, most notably the Pacific trench (Hatherton, 1969) which extends Hope, Awatere, and Clarence faults, moderate- as far north as Fiordland. Farther south, earth- sized earthquakes are common and are scattered quakes on the Macquarie Ridge exhibit ele- across the entire width of South Island. This ments of both strike-slip and normal faulting, activity continues northeast through North in keeping with its complex nature and its Island to Tonga and is accompanied by inter- proximity to the Indian-Pacific pole of rotation mediate- and deep-focus earthquakes which (Hayes and Talwani, 1972). define a westward-dipping Benioif zone (Hamil- The evidence from the seismicity north and ton and Gale, 1968) beneath North Island and south of the Alpine fault is suggestive of a northern South Island (Fig. 2). general northwest-southeast compression be- Only two focal mechanism solutions are tween the Indian and Pacific plates in this available for earthquakes in South Island; both region, with underthrusting in opposite direc- are for events in the northern zone: the West- tions north and south of the fault. These port earthquake of 1962 (Adams and LeFort, inferred stress directions together with geologic 1963) and the Inangahua earthquake of 1968 observations are not consistent with pure (Johnson and Molnar, 1972). Both events Quaternary strike-slip movement on the Alpine occurred to the west of the Alpine fault zone fault, and several authors have pointed out that and showed largely high-angle thrust faulting its strike is also inconsistent with its being a with a component of strike-slip motion—right simple transform fault (Karig, 1970; Johnson lateral in the case of Westport and left lateral and Molnar, 1972). in the case of Inangahua. Faults in this region are more northerly striking than the Alpine OBSERVATIONS fault zone itself, and these mechanisms suggest The data reported here were obtained dur- a regional northwest-southeast compression. ing 10 weeks of field work from February Field observations and geodetic measurements through April 1972. Ten microearthquake for the Inangahua earthquake (Lensen and seismometers were used in a network 50 km in Otway, 1971) indicate high-angle thrusting on diameter. The network was leapfrogged on a a north-northwest-striking plane, with the weekly basis to the south along the Alpine east side upthrown, in agreement with the fault zone. The instrument sites are given in focal mechanism solution. The nearby Mur- Figure 3. Refer to the Appendix for details of chison earthquake of 1929 produced very procedures used. similar movement (Fyfe, 1929). In contrast, the Amuri earthquake of 1888 produced nearly Lewis Pass to Harihari pure right-lateral, strike-slip motion of almost The sharp bend in the Alpine fault just 3 m on the Hope fault (McKay, 1890; Richter, north of where it crosses the Lewis Pass road 1958). The Hope fault is the most easterly marks the northern boundary of the area striking fault of northern South Island, how- investigated in this study. Figure 4 shows the ever, and a northwest-southeast compression seismicity observed in this region. Its general would be inferred for this earthquake, similar aspect is that of scattered activity with little to that of the others discussed above. tendency for clustering on major faults. This is South of the central section of the Alpine similar to the pattern for small earthquakes in fault, in Fiordland, seismicity is also high and southern California (Allen and others, 1965), includes intermediate-depth earthquakes. This whereas events of magnitude > 6 do occur activity is confined to Fiordland and its offing on mapped and usually major faults. If any of and is continuous southward with the seismical- the activity of Figure 4 can be assigned to a ly active Macquarie Ridge complex. The inter- major fault, it is the Hope fault, rather than mediate-depth activity extends 150 km beneath the Alpine fault, which proves to be the most Fiordland but does not appear to continue active. Twelve events lie close enough to the further south. Hope fault to be considered to have occurred Focal mechanisms are not available for the on it, whereas only half that number lie as close New Zealand part of this zone, but solutions to the Alpine fault, which has a trace twice as

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REEFT

42*30'

• • —

A •

ARTHUR'S •

43*00'

171*00' 171*30' 172° 00' Figure 4. Microearthquake epicenters in the Lewis Pass to Harihari region.

long as that of the Hope in this map area. The ularly high seismicity. Most :>f the activity is Hope fault has also been the more active clustered in a zone with a trend roughly parallel historically. The 1888 Amuri earthquake to, but south of, the Hope fault. An earth- (magnitude ~7) ruptured the Hope fault near quake of magnitude 7 occurred in the Arthur's the eastern edge of the area shown in Figure Pass area in 1929. Speight (1933) observed a 4 (McKay, 1890). No large earthquake can be narrow zone of intense landslides approximately assigned to the Alpine fault. The group of 30 km long which he associated with this events near and just south of Reefton may be earthquake. This line of landslides was parallel on a southern extension of the White Creek- to, but 10 km south of, the Hope fault and Lyell group of faults. The Inangahua earth- ended to the southwest, near the point where quake of 1968 and the Murchison earthquake the microearthquake activity described here of 1929 (both events of magnitude 7) were begins. A composite focal mschanism for the associated with these faults, which are just microearthquakes indicates strike-slip faulting north of the map area. with the right-lateral nodal plane parallel to North of Arthur's Pass, earthquakes occur the Hope fault (solution A, Fig. 5). This solu- with nearly equal frequency on either side of tion is not compatible with pure strike-slip the Alpine fault. South of the point where the motion on the Alpine fault, but indicates a Hope fault joins the Alpine fault, earthquakes northwest-southeast P axis, normal to the appear to be confined to the southeast of the Alpine fault and consistent with the focal Alpine fault. This striking asymmetry of the mechanisms of the Inangahua and Westport seismicity continues for 200 km farther south earthquakes (Johnson and Molnar, 1972) and and will be discussed later. with movement during the 1388 Amuri earth- The Arthur's Pass area was a site of partic- quake.

Figure 5. Focal mechanism solutions. All solutions fault at Haast; C, Haast-Lake Wakatipu trend; D, are composite from microearthquakes except D. 1960 Big Bay (magnitude 7) earthquake; E, near coast, Diagrams are lower-hemisphere, stereographic projec- Big Bay to George Sound; F, coastal events, Fiordland tions. Solid points are compressions; open, dilatations. south of George Sound; G, inland Fiordland; H, inter- Smaller points are less certain, and crossed points are mediate events west of Lake Te Anau; I, intermediate nodal readings. A, Arthur's Pass area; B, near Alpine events east of Lake Te Anau.

The depths of the microearthquakes in (Eiby, 1971; Evison, 1971). Our observations Figure 4 range from 2 to 20 km, with a max- indicate that this area is active at the micro- imum frequency at about 10 km. There do earthquake level, although at a somewhat not appear to be any spacial trends to the lower level than the areas to the north and depths. south (Fig. 6). One cannot make a quantitative comparison based on the activity indicated in Harihari to Haast Figure 6, because poor weather forced us to operate our instruments at lower than normal The central section of the Alpine fault, gains while in this area. A comparison of the where it fronts the highest part of the Southern several instruments that did operate at normal Alps, has had very low seismicity in the past gains, however, does suggest distinctly lower

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43-15'

43'45'

Figure 6. Microcarthquake locations, Harihari to Haast.

activity than the Arthur's Pass or Haast Pass There appears to be some activity associated areas. with a group of prominent faults which trend The activity shown in Figure 6 is spacially north-northeast from Lake Wakatipu (for continuous and is confined to the southeast example, the Shotover and Moonlight faults). side of the Alpine fault. The data does not This activity may be construed as forming a indicate any activity to the west. Depths are vague trend extending in a shallow arc from in the normal range observed elsewhere and do Lake Wakatipu to Haast. A second diffuse not show any spacial pattern. trend lies in a band within 15 km to the east of the Alpine fault. The most distinctive Haast to George Sound feature of the seismicity of this region when The Haast Pass road roughly marks the compared to that farther north is the presence beginning of the southern seismically active of offshore activity. This activity, which zone of New Zealand, and this is reflected in begins in the vicinity of Haast, marks the the seismicity we observed during two weeks abrupt beginning of activity west of the fault of recording in this area (Fig. 7). The trace of and is reflected in the macroseismic record the Alpine fault is straight in this region and (F.iby, 1971). The activity off Big Bay was goes out to sea at Milford Sound. The Waiau probably in the epicentral zone of an event of syncline, with a narrow band of ultramafics and magnitude 7 that occurred in the area in 1960 blueschists and bounded by the Hollyford and (Hamilton, 1966). Livingston faults, trends north through this Depths of onshore events range from 3 to area and is truncated by the Alpine fault south 15 km and do not show any systematic varia- of fackson Bay. This line separates the meta- tions over the region. The activity was largely volcanic rocks of the Fiordland complex on the concentrated at depths of 8 to 14 km, typical west from the lower grade metasedimentary for the activity observed elsewhere in this rocks to the east and is probably a relic of a study and for shallow seismicity in continental Paleozoic- Mesozoic subduction zone (Landis regions in general. However, the offshore and Bishop, 1972). hypocenters were systematically deeper, rang- The seismicity, although more intense here ing from 15 to 30 km in depth. than farther north, continues to occur mainly Seismicity was again not directly associated as a diffuse band of activity lying to the east with the Alpine fault except for a group of of the Alpine fault (Fig. 7). South of th: events near Haast which occurred directly latitude of Milford Sound, this band widens below the trace of the fault. These events are into the broad zone of shallow activity that aligned along the strike of the Alpine fault and covers most of Fiordland. It is also south of this range in depth from 6 to 13 km, but their com- latitude that intermediate-depth earthquakes posite focal mechanism (solution B, Fig. 5) occur. is not consistent with slip on a vertical fault.

44*00'

44-30'

168* 30' 169* 00' Figure 7. Microearthquake locations, Haast to George Sound.

Instead, the solution indicates predominantly The computed ratio of vertical to horizontal thrust faulting, with a northwest-southeast P movement on this plane would be 1:1, in axis. One nodal plane has a strike similar to close agreement with the ratio determined that of the Alpine fault but a dip of 22° SE. for the Alpine fault in the Haast area from the This does not agree with the spacial distribution offset of glacial and postglacial features (Sug- of the microearthquakes, which occurred in a gate, 1963, 1968; Wellman, 1955b). nearly vertical zone. The consistency of the Although the dip of the Alpine fault is composite solution, however, suggests that the observed to be quite gentle near Haast, it events share a common thrusting mechanism. steepens at shallow depths. Wellman (1955b), The disparity between the spacial distribution pointing out the straight trace of the Alpine of hypocenters and the nodal planes of the fault, has suggested that it dips vertically, and solution, however, argues that the events share has explained the often observed shallow dip of a common response to a regional stress field the fault as a surficial phenomenon caused by (northwest-southeast compression) rather than slumping of the schists in response to rapid slip on a common fault plane. uplift on the southeast side of the fault. A If one supposes that the gently dipping shallow refraction profile running southeast nodal plane is the fault plane, the solution from Haast, however, suggests that the fault indicates overthrusting by the southeast side dip may be as small as 12° and does not steepen with a small component of right-lateral motion. with depth (R. Garrick and T. Hatherton,

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oral commun., 1972). Our solution fits this All of the data thus indicate a southeast- to latter interpretation better; however, the east-trending compression in this region, with lack of evidence for a clustering of hypocenters thrust faulting dominating the motion. This along a gently dipping fault plane leaves the stress direction is nearly perpendicular to axial question of dip unresolved. planes of folds and the schistosity of the Haast Two other composite focal mechanism solu- schist group east of the Alpine fault and is tions were determined from shallow events in inconsistent with pure strike-slip motion on the area; one for the Lake Wakatipu-Haast the Alpine fault. These motion directions are zone and one for events within 15 km east of consistent with uplift of rhe Southern Alps the Alpine fault from Big Bay to George Sound along such a zone, but a concentration of (solutions C and E, Fig. 5, respectively). activity is not observed along the fault itself. Solution E is similar to the Haast solution, exhibiting predominantly thrust faulting, Fiordland although with more northerly striking nodal Relatively high seismicity was encountered planes. The other solution shows a predomi- in Fiordland during two weeks of recording nance of strike-slip faulting. The nodal planes there; more than 10 locatable events were re- of these solutions do not correlate with major corded daily. Over two-thirds of these were faults in the area, nor do they correlate with observed to be subcrustal, with depths ranging the trends of the seismic zones. Hence, we to 140 km. again interpret these data as indicative of the Shallow Activity. The locations of events regional stress field. The data are consistent with depths less than 15 km are shown in when considered in this way—all three solu- Figure 8. They again appear scattered over the tions yield a similar northwest-southeast to east- region with little correlation either among west P axis. The focal mechanism for the themselves or with major geologic structures. Wakatipu-Haast zone differs from the others The activity, however, is largely confined to simply in the orientation of the intermediate the area west of Lake Te Anau, and there is a and minimum principal stress directions, which sharp decrease of activity south of Doubtful have switched positions. Sound, neither of which c£.n be accounted for A first-motion diagram is also given in by insufficient instrumental coverage. Although Figure 5 for the 1960 Big Bay earthquake the short time sample involved might lead one (solution D, Fig. 5). The earthquake of mag- to question the significance of this localization nitude 5.6, which occurred in the same location of seismicity, a similar pattern has existed in on March 8, 1964, had an essentially identical the seismicity of this area aver the past 10 yr pattern of first motions. We read polarities (Eiby, 1971; Smith, 1971). As will be pointed from the original long-period seismograms out below, subcrustal activity is also limited for the 1960 event and from World Wide to the same region. Offshore activity persists Standard Seismic Network film clips for south as far as Doubtful Sound, although only the 1964 event, supplemented by some New one event is shown in Figure 8 because such Zealand and Australian readings. The avail- events could be only poorly located with the able data are insufficient to yield a firm Fiordland network. Considerable offshore focal mechanism for either event but do define activity is shown by Smith (1971) and Eiby one of the nodal planes as nearly vertical and (1971). striking due north. The data also rule out pure Composite focal mechanisms for shallow strike-slip movement and indicate that thrust events in Fiordland show close similarities to faulting must have been dominant. The main those for events farther north. Solution F observation is that the P axes for these events (Fig. 5) is for the group of events near the are in close agreement with those determined coast between George and Doubtful Sounds. from the microearthquake composite focal This solution indicates nearly pure thrust fault- mechanisms. We suggest that the simplest ing with nodal planes striking north to north- interpretation of these events is pure thrusting northeast and is very similar to solutions from with an east-west P axis. The gently dipping coastal or offshore events to the north (solu- nodal plane, and hence the P axis, can be shifted tions B, D, and E). The strike of the shallow- considerably within the constrants of the data, dipping nodal planes for solutions D, E, and F but the horizontal direction of the slip vector are poorly determined, hence the P axis for is well determined and is east-west. these solutions could well be northwest-south-

east, more in agreement with the other solu- Benioff zone such as observed in other areas of tions. The solution for events farther east intermediate-focus earthquakes but rather a shows pure strike-slip motion with a N. 80° W. very steeply dipping zone nearly 50 km wide. P axis (solution G, Fig. 5). This is again con- It does not appear that errors in location are sistent with inland events farther north (solu- responsible for this spread of locations. tions A and C). All eight focal mechanism Earthquakes were observed to depths of 140 solutions obtained for shallow events thus km, with a peak of activity from 50 to 120 km. agree in indicating a west-northwest to north- There appears to be a distinct minimum of west-southeast compression, with thrusting activity in the depth range from 15 to 50 km, dominating along the coast and strike-slip although it cannot be demonstrated that this is faulting dominating farther inland. a definite gap in seismicity. Intermediate Activity. The locations of Two composite focal mechanisms were ob- events with depths greater than 15 km are tained for the intermediate-depth events (15 shown in Figure 9. They occur primarily in the km < depth < 140 km), one for the main same region as do the shallower events but are cluster of events between Lake Te Anau and somewhat more restricted laterally, forming a the coast (solution H, Fig. 5), and the one for northeast-striking band parallel to and 25 km events farther east (solution I, Fig. 5). The inland from the coast. solution for the main cluster contains data from Cross sections are given in Figure 10. The more than 50 events and shows a surprisingly three-dimensional configuration of the activity strong correlation. One might expect that, if closely reproduces that observed by Smith this activity is occurring in a downgoing (1971). The activity does not define a narrow lithospheric slab, a considerable variety in focal

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/ / y*TE ANAU \ /

STATIONS A 3/Z8- 4/5 -• 5 •• V 4/5 - 4/IT • 4/6-4/tB lOOMaL ISOSTA PUYSEGURF COIiroUR(afl«r WOODWARD. I9TZ)

Figure 9. Epicenters of events with depths greater than 15 km, Fiordland. mechanisms would occur with the P or T exist in the vicinity of Fiordland; intermediate- (tensional) axis as their only point of similarity depth earthquakes, paired gravity anomalies, a (Isacks and Molnar, 1969). Under those condi- Quaternary volcano (Solander Island), and a tions, a composite focal mechanism solution trench. The relative positions of these features, would not be possible. The strength of solution however, are more confused than in the usual H, however, indicates that nearly all of the case. The maximum intensity of the paired events have identical solutions. gravity anomalies (Fig. 9) occurs south of the If one makes the reasonable suggestion that most intense, intermediate-depth seismic zone, these events are occurring in a downgoing slab and the trench proper doe:; not appear farther (although the term "slab" is not particularly north than Puysegur Point. Solander Island apt here because of its inference to a more is several hundred kilometers southeast of planar geometry), then the stress directions Puysegur Point. for solution H do bear some relation to the It is perhaps misleading to suppose geometry of the slab. Thus, the T axis of the that a descending lithospheric slab be- solution is nearly along strike of the slab; the neath Fiordland is limited to the small B, or null, axis is down its dip; and the P axis region of most intense, intermediate-depth is normal to the slab. This can be interpreted seismic activity. There are two well-located as a flattening of the slab or as an extension events at 90-km depth just east of Puysegur along its strike. Point (Fig. 9), and it cannot presently be Solution I, for the few subcrustal events east determined if events farther south are of of the main seismic zone, is poorly determined normal dep:h (Smith, 1971). Therefore, the but does indicate a different mechanism from seismicity would indicate that the slab extends solution H. The significance of this is not as far south as Puysegur Point and perhaps understood. considerably farther. The liigher seismicity to Several workers (Hamilton and Evison, the north may be due to higher stress in the 1967; Smith, 1971) have noted that all the slab as it approaches its termination at the primary features of an active subduction zone Alpine fault junction.

PUYSEGUR DOUBTFUL p POINT SOUND

• «•• • • • • * • • • I • I • • • • • •

J I I I I L I I ' I I I i_

Figure 10. Cross sections of the seismicity of Fiord- C.L., coast line; H-L FZ, Hollyford-Livingston fault land. The position of the section lines are given in zone; W.S., Waiau syncline. Figure 9. Abbreviations are AFZ, Alpine fault zone;

Bathymetric data also do not preclude north as Big Bay. No great earthquake has underthrusting off the Fiordland coast. Barker occurred off this coast in recent years, but a (1967) shows that a shallow trough occurs off large event was reported to have produced the coast as far north as George Sound. This several meters of uplift on the coast in 1826 trough, which strikes in a more northerly (Eiby, 1968). Such coastal uplift is typical direction than the Alpine fault, could mark of underthrusting events (Fitch and Scholz, a site of underthrusting. Profiles 8 and 9 of 1971). Christoffel and van der Linden (1972) also Thus, the data do not preclude the simplest show this feature. The earthquake focal explanation of Fiordland intermediate-depth mechanisms reported here would support the activity—underthrusting of the Tasman Sea view that underthrusting is occurring as far floor beneath Fiordland—although there are

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certainly unexplained complications. Further- properly described as a zone of oblique con- more, since our results indicate that the slip tinental convergence with most of the move- vector for the relative plate motions in this ment taking place on the Alpine fault. The area is not parallel to the Alpine fault, it may tectonic style of this region is similar to a con- not be necessary to invoke unduly complicated tinental collision boundary, the difference models for underthrusting in Fiordland such as being that here the continents were never apart those proposed by Christoffel and van der but had previously shared a transcurrent plate Linden (1972) or Hayes and Tahvani (1972). margin, the Alpine fault. The focal mechanisms indicate that a large component of thrusting CONCLUSIONS must be occurring on the Alpine fault north The over-all pattern of seismicity we ob- of Haast. It is just at this point that a marked served is in accord with that recorded over the change in tectonic style occurs: the rapid uplift past 15 yr by the New Zealand Seismic Net- of the Southern Alps on the Alpine fault be- work (Eiby, 1971), although the more exten- gins at Haast. This uplift can thus be likened sive coverage and accurate hypocenter loca- to the uplift of the Himalayas, except that most tions obtained in this study allow a more of the uplift takes place on a pre-existing fault detailed discussion of the seismicity than had and since the convergence is oblique, a con- previously been possible. siderable amount of strike-:>lip motion is probably still prevalent. The general seismicity pattern is illustrated schematically as the shaded region in Figure 11. North of Arthur's Pass, the Clarence, Although the seismicity is diffuse, without a Awatere, and Hope faults (the Marlborough direct correlation with major faults, a subtle faults) lie more nearly parallel to the present relation of seismicity with the Alpine fault slip vector, and consequently the movement does appear. North of Arthur's Pass, where the is more nearly strike-slip on these faults. The Alpine fault splits into several traces, seismicity strike of the Hope fault, which is a pure strike- is scattered over the width of South Island. slip fault, is taken as the piesent slip direction. South of Arthur's Pass, however, all seismicity These faults take up much of the motion from occurs directly to the east of the Alpine fault. the Alpine fault; hence the Southern Alps end South of Haast Pass, activity occurs offshore at Arthur's Pass. to the west of the fault as well, and this trend The evidence for large strike-slip movements continues along the Fiordland coast. on the Alpine fault indicates that the Indian- The focal mechanisms indicate northwest- Pacific slip vector must have been parallel to southeast compression over the entire region. the Alpine fault at one time but that it has South of Haast Pass, the offshore seismicity, now rotated to a direction oblique to the Alpine thrusting mechanisms, and intermediate-depth fault, producing convergence (and thrusting) activity all suggest underthrusting of the along it. This change does not necessitate a Tasman Sea floor along the Fiordland coast. major change in the relative motion of the The strike-slip mechanisms for inland events two plates. Because of the proximity of the also indicate northwest-southeast compression. Indian-Pacific pole of rotati on to South Island, This pattern of underthrusting offshore and minor changes in plate motion, reflected as strike-slip faulting inland with the same P minor changes of the pole pasition, can produce axis is similar to that observed in Japan large changes in the slip vector for South (Ichikawa, 1965). This is interpreted as Island. The Le Pichon (1968) pole (Fig. 2), for regional deformation inland in response to the example, implies thrusting an the Alpine fault, same compressive stress system that drives whereas the Christoffel (1971) pole implies the underthrusting. The termination of offshore strike-slip motion. activity in the vicinity of Haast Pass correlates We can attempt to date the migration of closely with the conjunction of the Lord Howe the pole position from geologic information. Rise with New Zealand (Fig. 2). The Lord The uplift of the Southern Alps is a good Howe Rise is composed of continental crust marker for the change in plate motions. Their with a thickness of at least 25 km (Shor and uplift occurred in the Kaikoura orogeny, which others, 1971; Glomar Challenger Scientific began in mid-Miocene time and accelerated in Staff, 1972); hence, underthrusting would be the Pliocene (Suggate, 1963). The development inhibited north of this point. The plate bound- of the Awatere, Clarence, and Hope faults was ary north of this point must then be more also during the Kaikoura orogeny. Their

40',-, T r T 'If , , 'If

LIMIT OF NORTH ISLAND BENIOFF ZONE

SEI5MICALLY ACTIVE AREAS

ZONE OF RAPID UPLIFT OF ALPS

Figure 11. Summary diagram showing seismically solutions are also given, as well as the 1964 Big Bay active areas and focal mechanism solutions for shallow earthquake. Solid quadrants are compressional, open seismicity. In addition to the focal mechanisms shown dilatational. Bathymetry in fathoms from Hayes and in Figure 5, the 1962 Westport and 1968 Inangahua Talwani (1972) and Brodie (1964).

development can also be explained by the the Hikurangi trench in order for the trench change in motion direction: they are transform to link up with the offshore extension of the faults which formed successively in response to faults. Strong evidence for this migration is the changing slip vector, as it changed from given by the continuous shoaling of the depth parallel to the Alpine fault to parallel to the of the North Island Benioff zone from 400 km Hope fault. If this is true, then it might be at lat 38° S. to less than 100 km at its southern- expected that the Clarence and Awatere faults most extremity beneath South Island (Hamil- (which have strikes intermediate to the Alpine ton and Gale, 1968). This shoaling is more and Hope faults) would be slightly older than rapid than predicted by a linear decrease in the Hope fault and would have an early history subduction rate as the pole of rotation is of strike-slip motion followed by increasing approached. Thus, if the depth of the Benioff amounts of vertical motion. zone is proportional to the age of underthrust- There is abundant evidence for significant ing (Isacks and others, 1968), then the age of movement on these faults. The Hope fault underthrusting of the Hikurangi trench must has had at least 20 km of right-lateral move- be progressively younger to the south. The ment since the Miocene (Freund, 1971; Clay- southernmost extremity of the Hikurangi ton, 1966). The transferral of motion to these trench now abuts the Chatham Rise, which, faults, if they are to be considered, broadly like the Lord Howe Rise, is also continental; speaking, as constituting part of a plate bound- hence, it appears that migration of the sub- ary, would require a southerly migration of duction zone farther south is inhibited.

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In summary, our results suggest that the occur scattered over the region south of the tectonic patterns of the South Island of New big bend (Allen and others, 1965). This is Zealand are rapidly evolving in response to probably a result of regionally high stresses changing plate-motion directions in that developed as a result of th: lack of congruity region. The evidence for large right-lateral between the plate boundary and the slip vector. movements on the Alpine fault suggests that In areas where the plate boundary is con- prior to mid-Miocene the slip vector was gruent with the slip vector, such as near the parallel to the Alpine fault, and it was be- San Andreas system in central California, having as a transform fault. Subsequently, the microearthquake activity is strongly con- slip vector has rotated to its present position, centrated along the faults; (Lee and others, assumed to be parallel to the Hope fault. The 1972). simplest change in the plate-boundary con- This interpretation of Alpine fault move- figuration which would result from this would ments in terms of plate tectonics is similar be a migration of underthrusting south along to the plate-tectonic model of the evolution of the east coast of New Zealand and north along the San Andreas fault (McKenzie and Morgan, the west coast, with a transform fault forming 1968; Atwater, 1970), and involves the same parallel to the slip vector and connecting the major problem: data on sea-floor spreading two. The inhibiting effect of the Lord Howe would imply fault movements several times and Chatham Rises on this migration resulted greater than actually observed. If we assume instead in convergence within the continent, that the slip vector was parallel to the Alpine which was concentrated on the old plate fault at 15 m.y. B.P., that the slip vector boundary, the Alpine fault. This model ex- rotated at a constant rate to its present position plains two of the major features of the Kaikoura parallel to the Hope fault, and that the Alpine orogeny: the uplift of the Southern Alps and fault dips 30° SE., the slip rate necessary to the formation of the Marlborough faults. produce the estimated 20 km of uplift of the The present tectonic situation of the South Alps would be 1 cm per yr. This is several times Island of New Zealand can be compared with less than predicted from data on sea-floor the situation existing in southern California. spreading (Hayes and Talwani, 1972). Further- There, in its big bend through the Transverse more, 135 km of strike-slip motion would Ranges, the San Andreas fault strikes obliquely occur during the same period of time, which to the slip vector as does the Alpine fault. To is nearly equal to the inferred total Cenozoic compare South Island with southern California, strike slip (Wellman and Cooper, 1971). one should rotate Figure 11 and look at it The underthrusting off Fiordland may have with west up, which corresponds with a map been going on before the change in slip direc- of southern California viewed in the normal tion. The recent bathymetric data show that way with north up. The central, seismic-gap this is not readily explained by underthrusting section of the Alpine fault corresponds there- at the Puysegur trench (Fig. 11). One pos- fore with the San Andreas fault in the big sibility is that this underthrusting occurred bend between Tejon Pass and San Gorgonio at the trough described by Barker (1967), Pass. In both areas, the faults are oblique to the which we will call the Fiordland trough be- slip vectors causing compression in a con- cause of some uncertainty in the nomenclature. tinental area. In New Zealand, this is accom- The bathymetric appearance of this trough is modated by overthrusting of the Southern not unlike a buried trench,. The map of Hayes Alps; in California, by uplifting and thrusting and Talwani (1972), which, does not show this of the Transverse Ranges. Similarly, the trough, shows that the Puysegur trench is cut Marlborough group of faults (Hope, Awatere, off at its northern end by the northeast- and Clarence) correspond to the Elsinor and striking Resolution Ridge. The Fiordland San Jacinto group of faults in southern Califor- trough runs north-northwest from the Resolu- nia. These faults may possibly be attempts tc tion Ridge beginning at a point off Puysegur cut off the Alpine and San Andreas faults where Point east of the Puysegur trench. Thus, the they are not parallel to the slip vectors, in Fiordland trough is oriented and located order to produce lower energy, pure transform properly to have provided the underthrust plate boundaries. The seismicity of southern slab defined by the Fiordland intermediate- California is also similar to that of South depth earthquakes; the Resolution Ridge be- Island: small- and moderate-size earthquakes tween the Fiordland trough and the Puysegur

trench could be a short trench-trench trans- These latter units proved useful for the more remote form fault. Considerably more detailed marine sites and in some cases were run in tandem, allow- geophysical work in this area will be required ing up to 8 days unattended recording. All of these to test this hypothesis. units have similar gains and frequency responses (see Ward and others, 1969) and record on smoked In summary, our results suggest that the paper. Timing was achieved with crystal clocks tectonics of the South Island of New Zealand set with radio time signals from the Australian are rapidly evolving in response to a recent station VNG. Clear P-wave arrival times are usually change in the plate-motion directions of that accurate to +0.1 sec. area. In the southern part, underthrusting of The ten units were normally set out in a network the Tasman Sea beneath Fiordland seems to with a diameter of roughly 50 km approximately have recently begun. North of the junction of centered on the fault. After about one week of the Lord Howe Rise with New Zealand, a observations, the network was leapfrogged to the continental convergence boundary exists in south, leaving the southernmost several stations in which motion is dominated by the old plate place, thus providing overlap and a continuity of observations between succeeding network margin, the Alpine fault. North of Arthur's positions. For example, during the first week, Pass, several faults, more favorably oriented, stations 1 to 10 were occupied in the region south have been created which spread the relative of Lewis Pass (Fig. 3), then for the second week the plate motion over a wide area, forming a broad network was composed of stations 8 through 17 zone of deformation which transfers the motion farther to the south. This procedure was followed to underthrusting at the Hikurangi trench. down the entire length of the fault zone, thus providing fairly uniform coverage. The last two ACKNOWLEDGMENTS weeks were spent in Fiordland where, because of the presence of intermediate-depth earthquakes, the We thank R. Adams, T. Hatherton, and W. network was spread over a larger area (125-km Arabasz of the Geophysics Division, DSIR, G. diam.) and operated for the full two weeks. Lensen and R. P. Suggate of the New Zealand A total of 72 sites was occupied during the Geological Survey, and F. F. Evison of Victoria study, with an average recording time of 8 days University for their logistical assistance, en- per site. The average gain obtained in the field was couragement, and discussions. E. Annear, D. 2 x 106 at 20 Hz, although this varied appreciably Williams, and K. Murray of the New Zealand with site location and weather conditions. Normal- Geological Survey provided field assistance. ly, this gain is sufficient to yield a perception of The New Zealand National Parks, the Elec- most events of magnitude greater than 0. In the tricity Board, the Alpine Club, and many region from Franz Josef glacier to Haast, however, local citizens were helpful in many ways, in detectibility was somewhat lower because of poor weather conditions. particular the Wallaces of Haupiri, the Prout- Readings from permanent stations of the New ings of Mesopotamia, the Cooks of Waitiri, and Zealand Seismic Network, operated by the Geo- J. Aspinal of Mt. Aspiring. Funds were physics Division, Department of Scientific and provided by the U.S. National Science Industrial Research (DSIR, see Fig. 3), were used Foundation Grants GA-30383 (LDGO) and to supplement our data wherever possible. The GA-30366 (Cornell). I.W.D. Dalziel, D. E. stations used were Mt. John (MJZ, see Fig. 3), Hayes, P. G. Richards, B. Isacks, and J. Milford Sound (MSZ), Monowai (MNW), and Oliver read the manuscript. Roxburgh (ROX). In addition, the Geophysics Division operated a portable microearthquake unit at Puysegur Point (PPZ) during the time we were APPENDIX. EXPERIMENTAL recording in Fiordland. These records were used in PROCEDURES our analysis of that area. Field Methods Data Analysis The seismic data were obtained during ten weeks of field work from February through April 1972. Arrival times of P and S waves were read and Ten portable microearthquake seismographs were hypocenters computed using both arrivals with a operated nearly continuously during this time at modified version of the HYPO 70 program (Lee, 72 different locations (Fig. 3). The instruments 1970). A 35-km-thick crust was assumed, composed consisted of four LDGO backpack mounted units, of a layer 12 km thick with a Vp of 5.5 km per sec one Kinemetrics microearthquake unit, and five overlying a layer of 6.5 km per sec velocity. The new LDGO designed units equipped with large upper mantle velocity was assumed to be 8.1 km drums and capable of running for four days at per sec. This structure is an average of the measured 60 mm per min drum speed and 1-mm line spacing. velocities of Pg, P*, and Pn for New Zealand (Eiby,

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1958; Hamilton, 1966). The corresponding S-wave Christoffel, D. A., 1971, Motion of the New Zea- velocities assumed for the three layers were 3.1, 3.7, land Alpine fault deduced from the pattern and 4.6 km per sec, respectively. of sea-floor spreading, in Collins, B. W., and Magnitudes were determined using the scale Fraser, R., eds., Recent crustal movements: devised by Brune and Aelln (1967) and extended Royal Soc. New Zealand Bull. 9, p. 25-30. by Gumper and Scholz (1971). There was no Christoffel, D. A., and van cier Linden, W.J.M., observed trend in magnitude residuals with 1972, Macquarie Ridge-New Zealand Alpine distance, indicating that the distance corrections fault transition, in Hayes, D. E., ed., Antarctic originally devised for the western United States oceanology II, the Australian-New Zealand are suitable for New Zealand as well. sector: Washington, D.C., Am. Geophys. Focal mechanisms were determined by the com- Union, Antarctic Research Ser., v. 19, p. posite method. Well-located microearthquakes 235-242. were grouped into subsets on the basis of their Clark, R. H„ and Wellman, H. W„ 1959, The spacial distribution, and a composite focal mech- Alpine fault from Lake McKerrow to Milford anism plot was made for each subset by plotting the Sound: New Zealand Jour. Geology and polarities of first arrivals for all events of the subset Geophysics, v. 2, p. 590-601. on the same stereographic projection, assuming all Clayton, L., 1966, Tectonic depressions along the events of the subset have similar focal mechanisms Hope fault, a transcurrent fault in North (Scholz and others, 1969; Gumper and Scholz, Canterbury, New Zealand: New Zealand 1971). Jour. Geology and Geophysics, v. 9, p. 95-104. The accuracy of locations is somewhat variable, Eiby, G. A., 1958, The structure of New Zealand and depends largely on the station distribution. from seismic evidence: Geol. Rundschau, v. 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