CLEMENT G. CHASE Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92037

Tectonic History of the Fiji Plateau

ABSTRACT or the new global tectonics. This theory started with the notion that the sea floor was spreading The Fiji Plateau is a high, hot area of young from rifts and that new oceanic crust was being oceanic crust. It is bounded on the north by a created at the mid-oceanic ridges (Hess, 1962; Cretaceous Pacific archipelago, and to the east Dietz, 1961). The first important elaboration and west by the Tonga and New Hebrides is- was that stripes of linear magnetic anomalies land arcs which go back to the Eocene. The Fiji found on the sea floor recorded reversals of Islands are an Eocene and younger continental polarity of the earth's magnetic field frozen into mass formed within the ocean basin. Plate tec- the oceanic crust as it was created at the ridge tonics provides the key for understanding the crests (Vine and Matthews, 1963). Rates of mo- area. Marine geological and geophysical data tion had become measurable if the reversal from Scripps Institution of Oceanography expe- time scale were known. The idea of transform ditions, especially Nova, and published seismic, faults (Wilson, 1965) explained the peculiar gravity, and island geologic information pro- offsets of the ridge crests and also provided a vide the basis for the interpretation. way of determining the directions of relative Fiji is now flanked by three active sea-floor movement. spreading centers which are part of a very com- These methods transcended their application plicated transform linking the Tonga and New in studying the sea floor and gained global Hebrides crustal consumption zones. Extension power with the addition of a unifying concept in the Lau Basin is also taking place. Magnetic based on geometry and earthquake seismology. anomalies and seismicity permit six small blocks This concept was that large areas of the earth's and the large Pacific and Australian plates to be surface behave as rigid plates on the surface of distinguished, and some idea of their relative a sphere, move as a unit with some of the upper motions to be gained. mantle, and that these plates are created, de- From published magnetic anomaly and frac- stroyed, or significantly deformed only along ture zone data, a detailed history of the Tertiary their boundaries (McKenzie and Parker, 1967; motions of the Pacific and Australian plates Morgan, 1968; Isacks and others, 1968). The with respect to Antarctica has been deduced. boundaries of the plates are defined by bands of By a suitable choice of plate boundaries, hori- seismic activity; directions of motion are given zontal movements of the larger tectonic units of by earthquake mechanism solutions and trans- the Fiji Plateau region can be worked out for form fault trends, and rates come from seafloor the entire Tertiary. This reconstruction success- spreading and geodetic measurements. The fact fully accounts for many hitherto unexplained that the instantaneous relative motion of any bathymetric and geologic features of the area. two plates can be expressed by an angular The history proposed for the Fiji area is proba- velocity vector makes possible the quantitative bly unique among the world's oceans. power of the theory. From extensional velocit- INTRODUCTION ies measured at the oceanic spreading centers, The Fiji Plateau is not a large region on a rates of transform faulting and crustal consump- global scale (Fig. 1), but it is wonderfully com- tion on the other plate boundaries can be de- plex. Within an area 10" of latitude by 15° of duced by using these vectors. longitude are islands, trenches and island arcs, In this report the ideas of plate tectonics will transform faults, spreading centers, deep ba- be applied in two ways: (1) We will attempt to sins, a large extent of abnormally shallow make the complex tectonics of the Fiji area un- oceanic crust, and a budding continent. The derstandable in terms of the motions of a num- history of such an area promises to be no less ber of small blocks by applying the hypothesis complex, and is described in this paper within of plate tectonics to phenomena on a small the framework of the theory of plate tectonics scale, involving blocks a few hundred kilome-

Geological Society of America Bulletin, v. 82, p. 3087-3110, 11 figs., November 1971 3087

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MELANESIAN BORDER ROM™ PLATEAU Wollis Is

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25° Soulh Fiji Bosin

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NEW ZEALAND

I 40° 150° 155° 165° 170° 175° 180° 175° 170° Figure 1. Index map of southwest Pacific. Area of lines represent axes of deep ocean trenches. detailed bathymetry (Fig. 2) shown by inset box. Dashed

ters or less on a side; (2) We will try to explain other older geophysical data. The large plate both the recent detailed motions and the gen- motions are deduced from published magnetic eral Tertiary geologic and tectonic history of data on the Pacific-Antarctic Ridge (Pitman and the region in terms of the motions of two of the others, 1968) and the mid-oceanic ridge be- very large tectonic plates of the world. In this tween Australia and Antarctica (Le Pichon and context, the Fiji area has been generated in a Heirtzler, 1968). complicated transform involving oppositely fac- ing trenches separating the Pacific plate and the DESCRIPTION OF THE AREA plate containing Australia. These two appro- The large shallow hot areas of the world's aches prove very successful in reconstructing oceans behind island arcs are not generally well horizontal movements. understood. The Fiji Plateau is one such area The data used in the synthesis come from a for which a development is proposed in this variety of sources. Especially useful in inter- paper that may make it unique in the recent preting the marine geology of the region have history of the earth. In this section geological been the results of the 1967 Expedition Nova and geophysical data are presented as a basis for of the Scripps Institution of Oceanography and discussion of the origin, age, and boundaries of

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the tectonic elements of the Fiji Plateau and its Oceanography data, shows the extent of the environs. most up-to-date coverage used in this report. Figure 3 also shows the location of seismic Bathymetry, Reflection Profiles, and reflection profiles obtained during Expedition Sediment Distribution Nova. A uniform value of 2 km sec -l is as- The Fiji Plateau is an elevated area of sea sumed for the velocity of sound in sediments floor, with an average depth of 2 to 3 km in throughout this report. Thus a travel time of 1 contrast to the deeper ocean basins around it, sec on the records in Figure 4 is assumed which have average depths of 4 to 5 km. It is equivalent to a thickness of 1 km. almost completely surrounded by submarine In general, the sediment thickness on the Fiji ridges and island chains (see Figs. 1 and 2). To Plateau is small or undetectable, except near the north it is bounded by the Melanesian Bor- obvious sources of volcanic ash and transported der Plateau (Fairbridge and Stewart, I960; debris. In profile A-A' (Fig. 4) a thickness of Fairbridge, 1961), on the west by the Santa about 100 m at the eastern end pinches out to Cruz and New Hebrides Islands, on the south the west against the presumed spreading center by the Hunter fracture zone, and to the east by along long 174° E. The closely spaced reflectors the Lau Basin. apparently draped over the underlying topog- There are several significant bathymetric fea- raphy suggest that the sediment is of pelagic or tures within the Fiji Plateau itself (Fig. 2), al- hemipelagic origin. On a segment of profile though the general relief is less dramatic than near lat 20° S., long 175° E. no sediment can be that of the boundaries, and tends to be rough detected. The short profile B-B' has a max- on a small scale only. A series of ridges and imum 60 m of cover over acoustic basement troughs trends south of west from Hazal where sediment is visible at all, except slight Holme and Horizon Banks on the edge of the ponding in topographic lows. These small Border Plateau. This feature is hereby named amounts of sediment in an area well above car- the Hazel Holme fracture zone, and is inter- bonate dissolution depths and near volcanic preted as the trace of a transform fault for rea- centers are strong evidence that the western Fiji sons that will be given in the following section. Plateau is quite young. The Fiji Plateau west of the Fiji platform rises Likewise, the Lau Basin appears very young. very gently to a crest in its central portion, Karig (1970) has discussed its sediment distri- along 174° E. long. It is probably an active bution. He shows that the small marginal fans fast-spreading center. Likewise, the conclusion and unsedimented central area are the result of of Sclater and Menard (1967) that the ridges material transported from the surrounding and troughs immediately west of Fiji are a rifted ridges into an active extensional zone. spreading center will be strengthened. Another The western margin of the Fiji Plateau is area of interest is the series of ridges and deeps similar to the edges of the Lau Basin, though on just north of the continental Fiji platform, a larger scale. Profile C-C' (Fig. 4) shows the which are the locus of intense active faulting. sediment fan coming off" the New Hebrides The Lau Basin is also an area of shallow crust, complex and spilling out onto the plateau. 2 to 2.5 km deep. At its northern end there is There is 1.2 km of acoustic penetration at the a shallower area embracing Niua Fo'ou Island base of the ridge, decreasing to 400 m east- and a sharp ridge running northwest. This ward. The numerous reflectors are mostly in ridge, which rises to a depth of 1 km from a conformity. A number of faults are visible in basin with similar trend, has been named Peggy the original record, with a small graben at the ridge. A bathymetric survey of the ridge has foot of the ridge and a series of step faults been published by Karig (1970, p. 247). downthrown to the east toward the Fiji Plateau. Some of the bathymetry in Figure 2 is poorly This pattern suggests that the edge of the Fiji controlled through lack of data. For example, Plateau has been sinking with respect to the the contours with a northeast trend around lat New Hebrides arc concurrently with or since 15° S., long 175° E. are rather arbitrarily most of the deposition. A less striking sediment drawn, and the area between the Fiji Islands pile is seen in profile D-D' (Fig. 4), which is and the Border Plateau is not well known. based on records of poorer quality. Here the Other areas, such as the Hunter fracture zone best penetration is less than 400 m. The sedi- and most shallow regions are better surveyed. ments in the western and central parts of the Figure 3, a track chart of Scripps Institution of profile seem similar to those in C-C', although

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170' 180° 175° Figure 3. Location map of Scripps Institution of data. Selected profiles are labelled and indicated by Oceanography bathymetric, reflection profiling, and brackets on the track. Dots are sample locations, num- bottom sampling data. Light lines are bathymetric data, bered according to Table 1. Contour interval 2 km. heavy lines are bathymetric plus reflection profiling

the layers are less horizontal and parallel. East the Border Plateau is revealed by profile F-F of the sharp peak in D-D' the sediments appear across its southern edge. About 500 m of pene- to be covered by 100 m, or less, of semi trans- tration into Tuscarora Bank was achieved. Im- parent pelagics. The triangle of crust north of mediately to the south there is a basin with over the Hazel Holme fracture zone may be older a kilometer of fill shed by the bank to the north. than that to the south. The basin abuts against a ridge to the south The Melanesian Border Plateau is quite a which apparently consists of an upfaulted block different kind of boundary. The deep-sea sedi- of sediments. Further south, sediment is only ments of the Pacific rise up the slope to the visible where ponded in topographic lows. drowned atolls of the Border Plateau (Fair- The inference drawn from the evidence is bridge and Stewart, I960) without apparent that the Border Plateau is in fact a Pacific ar- discontinuity. There is no conspicuous tectonic chipelago, and older western continuation of boundary between the shallow banks and the the chain formed by the Samoa and Wallis Is- deep Pacific east of the Vitiaz Trench (profile lands. This conclusion is strengthened by pa- E-E', Fig. 4). Fairbridge (1961) concluded that leontological, petrological, and gravity data the Border Plateau has been subjected to exten- discussed later. sive left-lateral shear. If this is correct, it has not included great movements along the northern Bottom Samples side of the Border Plateau. The en-echelon pat- Few samples are now available from the sea tern of ridges and troughs on which his conclu- floor of the Fiji Plateau, and those that do exist sion was based is much less evident in the more do not in general penetrate older strata. Table recent contouring of Figure 2. Profile E-E' 1 contains a list of the Scripps Institution of looks like a crossing of an archipelagic apron Oceanography samples used in this study. For (see Menard, 1956). Some of the structure of paleontological determinations I am indebted

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TABLE 1. AGES OF SELECTED SCRIPPS INSTITUTION OF OCEANOGRAPHY BOTTOM SAMPLES IN THE FIJI PLATEAU AREA

SIO Depth Recovery 1 Type Identifier (m) (m) Comments

1 C Proa 67G 2895 1.65 Quaternary (R) 2 C Proa 66G 4265 0.43 Top Quaternary, bottom Quaternary + few Miocene (R) 3 C Proa 59P 5083 4.1 Rare (R) may be Quaternary (R) 4 C Proa 60P 5038 4.81 Top Quaternary + rare Miocene (R) 5 C Proa 60PG 5038 1.17 Top Quaternary + Miocene, Cretaceous, middle Quaternary, bottom Miocene + Cretaceous (R) 6 C Proa 61 PG 5151 0.64 Top Quaternary, bottom Miocene + Cre- taceous (R) (9° 58' S, 173° 52' 1} 7 C Nova H28V 3395 _ _ Pliocene-Pleistocene (A) 8 D Nova H27D 1585 u. Pliocene ? (A) 9 0 Nova H31D 2000 u. Pliocene-1. Pleistocene (A) 10 C Nova H24V 2658 0.05 Pliocene-Pleistocene (A) 11 C Cap 8BP 2560 7.6 All mid-Pleistocene (M) 12 C Nova H34GO 2629 1.21 u. Pliocene (A) 13 D Nova H22D 1300 u. Pliocene,!. Pleistocene (A) 14 C Proa 586 2996 _ _ Quaternary + some Eocene (R) 15 C Nova H35V 2845 - _ u. Pliocene-1. Pleistocene (A) 16 C Nova H2060 3063 1.08 Pliocene-Pleistocene (A) 17 C Nova H39V 2679 - - u. Pliocene-1. Pleistocene ? (A) 18 C Nova H38V 3527 _ _ u. Pliocene-1. Pleistocene (A) 19 D Nova HI 60 500 post-Eocene (A) 20 D Nova HUD 1425 Holocene (A) 21 C Proa 57G 3012 0.10 1. Pleistocene + reworked rare Pliocene- Miocene (M), Quaternary (R) 22 C Nova HI 9V 2783 - - Pliocene-Pleistocene (A) 23 C Nova H40V 3396 - - 1. Pleistocene + reworked Miocene (M), Pleistocene (A) 24 C Nova H18G 3582 - - Pliocene-Pleistocene (A) 35 C Nova H49GO 3096 - - Pliocene-Pleistocene (A) 26 C Nova H46GO 2629 1.08 Bottom Pleistocene (A) 27 C Nova H50V 2891 _ _ Pleistocene (A) 28 C Proa 52G 3140 0.78 Quaternary (R) 29 C Proa 53G 3250 0.2 1. Pleistocene (M), Quaternary (R) 30 C Proa 54G 3075 0.96 All 1. Pleistocene (M), Quaternary (R) 31 C Proa 556 3345 1.52 All 1. Pleistocene (M), Quaternary (R) 32 C Cap 11HG 4150 0.3 1. Pleistocene + reworked 1. Pliocene (M) 33 C Cap 10HG 2120 0.6 Pleistocene (M) 34 C Nova H41V 3039 _ _ u. Pliocene-1. Pleistocene (A) 35 C Cap 9HG 3000 0.9 All m.-l. Pleistocene + reworked Mio- cene-Pliocene (M) 36 C Proa 56G 2260 0.83 Top, prob. Quaternary, bottom undated {R) 37 D Nova H64D 1000 Miocene-Pliocene ? (A) 38 D Nova H63D 1100 1. Miocene (A) 39 D Nova H61D 825 Tertiary (A) 40 C Nova H53V 4321 - _ u. Pliocene-1. Pleistocene (A) 41 C Nova H52V 3134 _ - Pleistocene (A) 42 C Nova A37G 3125 1.27 1. Pleistocene (M), Pleistocene (A) 43 C Nova H54V 4207 _ - Pliocene-Pleistocene (A) 44 C Nova H60V 2945 - _ Pliocene-Pleistocene (A) 45 D Nova H58D 2400 Volcanic rocks with ropey surfaces 46 C Nova H57GO 4275 0.35 m. Miocene-1. Pliocene, prob. m.-u. Miocene (A) 47 C Nova H51V 3191 • ~ Pliocene-Pleistocene (A)

In type column, C = core samples; D = dredge haul. Unpublished paleontology: (M) = calcareous nannoplankton examined by Dean Milow; (R) = radiolarians by William Riedel; (A) - ftraminifera by Edward Allison. Published cores: (1) Ross and Riedel, 1967; (2) Riedel and Funnel, 1964. Locations of samples are shown in Figure 3.

to E. D. Milow, W. R. Riedel, and E. C. Alli- over the whole area before Holocene age, the son. Sample locations are shown in Figure 3- basement underlying the thin (60 to 100 m) The usefulness of the core samples has been sediment of the central Fiji Plateau should be mainly in determining that the sedimentation young. The rest of the cores from the Fiji Pla- rate has been high for the past few million teau were all too short to derive a rate from our years. dating methods. The longest core available, sample 11 of Ta- Curiously, all the cores from the Fiji Plateau ble 1, recovered 7.6 m of sediment. The time are of similar age and composition. They are range represented is probably no more than 0.5 calcareous-siliceous oozes with 5 to 10 percent m.y. (E. D. Milow, 1970, oral comm.). A mini- ash and mineral content. They all contain Quat- mum sedimentation rate is thus around 10 m ernary Radiolaria, middle or early Pleistocene m.y. -!. If this rate was reasonably uniform calcareous nannoplankton. Some of the cores

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100 200 300

Figure 4. Selected reflection profiles. Locations are sound velocity of 2 km sec -'. Bars along base of profiles shown in Figure 3- Thickness of sediment is based on show extent of profiler records. contain reworked, broken, and corroded older slope of the Fiji platform (core 35); the older microfossils. Core 32, located in the rift valley material easily could be derived from it. of the presumed spreading center just west of Two cores, 21 and 23, located north of the Fiji, consists of Pleistocene and early Pliocene Fiji platform contain a few broken Miocene and forms, both of which may be transported. The Pliocene forms among the Pleistocene forms. magnetic evidence suggests that the rift is only The older fossils may represent transport from 1 m.y. old. Thus both ages of sediment may the Fiji area or the Border Plateau. Reworking come from slumping in the newly created of the older material from the immediate topography. Miocene-Pliocene ooze is mixed vicinity by tectonic activity is more likely in the into early Pleistocene ooze just below the west case of core 21. If so, the north central part of

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the Fiji Plateau probably would be established these, Alexa Bank, is 20 to 40 m below sea level as older than the western portion. Core 14 and is covered with dead coral boulders and located south of Alexa Bank contains a mixture foraminiferal mud with a few living coral heads of Quaternary and Eocene Radiolaria. The (Fairbridge and Stewart, I960). Other banks nearby island of Rotuma is not a likely source are at similar depths. The Wallis Islands, how- due to its recent origin (Gardiner, 1898). A ever, protrude above water and provide a bet- more probable source for the Eocene fossils is ter look at their geology, which has been the south flank of Alexa Bank of the Border studied by Stearns (1945). A roughly circular Plateau. As will be discussed further on, there barrier reef supports 22 small islets and the is good reason to believe that the Border Pla- main island Uvea. The high islands are vol- teau is of pre-Terdary origin. canic, built of olivine basalt flows and pyroclas- Cores 1 to 6 (Table 1) taken just to the north tics. The low islands are volcanic remnants or of the Melanesian Border Plateau penetrate, composed of calcareous sand; most of the vol- beneath the upper few decimeters, a calcareous canic rocks are assigned to the mid-Pleistocene turbidite bed with a mixture of Pliocene and by Stearns (1945). In their petrography the possibly Miocene calcareous nannoplankton lavas of the Wallis Islands clearly belong to the and Pliocene, Miocene, and Cretaceous Radio- alkaline suite typical of the volcanoes of the laria (Riedel and Funnell, 1964, p. 321; Ross central Pacific (MacDonald, 1945). Thus, their and Riedel, 1967, p. 288). Riedel and Funnell origin is related to that of the Pacifc ar- (1964, p. 322) point out that the turbidite bed chipelagoes rather than to island arc tectonics. thickens and coarsens to the south, indicating The island of Rotuma is younger than the an origin somewhere on the slope of the Bor- subaerial portion of the Wallis Islands. It is der Plateau. Since there is no major fault elongate east northeast-west southwest and lies boundary between the banks of the Border Pla- on a submarine ridge which may be a continua- teau and the Pacific, perhaps a Pacific ar- tion of the Hazel Holme fracture zone. Accord- chipelago of Cretaceous or older age has been ing to Gardiner (1898) Rotuma is composed of brought into its present position by motion of recent olivine basalts, virtually uneroded and the Pacific plate. It would then be of analogous scarcely weathered. There is a narrow fringing origin to other island chains such as the Toke- reef. No evidence is available that suggests laus and Gilbert Islands. The cause of these Rotuma is any older than Holocene age. No features is one of the unsolved questions of data are available about the islands west of marine geology in which plate tectonics has not Rotuma, Anuda and Mitre Islands, which ap- yet proved useful. If this interpretation of the pear to consist of volcanic rock; we do not know Border Plateau is correct, it may be the oldest whether they and Pandora Bank and Charlotte crust in contact with the Fiji Plateau. Bank belong geologically to the Border Pla- The scattered dredge haul samples available teau. in the Fiji area are also listed in Table 1. None The Tonga Islands are a long linear group of them showed any great age on the basis of lying atop a north-northeast-trending subma- foramineriferal dating, and most were probably rine ridge. There are over 150 islands in two late Pliocene or early Pleistocene. Sample 45 parallel chains. To the west a chain of purely contained volcanic rocks which have not been volcanic islands, morphologically young and analyzed. lacking major reef development, extends from Ata (22.5° S.) to Tafahi (15.8° S.). Many of Geology of the Islands these western islands are currently volcanically For many of the smaller islands in this remote active, and eruptions are frequent. This island part of the world little is known of geological chain is the locus of the current andesite vulcan- structure and history. Fortunately, some of the ism associated with the Tonga arc. The eastern large islands have been of sufficient economic chain consists of older uplifted islands either and scientific interest for reconnaisance map- composed entirely of coral reef limestone or of ping. The available information that has most volcanic material, and in some cases covered by direct application to our problem of tectonic elevated reefs. Eua Island is the most exten- history will be summarized here. sively studied geologically of the Tonga Islands A portion of the Border Plateau, which and may be taken as representative of the older would be islands in a more normal part of the volcanic islands. Pacific, consists of drowned atolls. One of The geological history of Eua Island may be

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summarized briefly from Hoffmeister (1932). west part of the pattern. Outcropping volcanic There is no evidence of pre-Cenozoic rocks on basement rock of the islands is composed of the island; the oldest dated beds are foraminif- andesitic flows and pyroclastics of unknown eral limestones of late Eocene age, which con- age; the oldest dated strata overlying the base- tain occasional shallow-water reef corals. These ment are extensive early Miocene limestones. overlie deformed andesitic tuffs, demonstrating Subsequent to deposition these limestones that activity on the Tonga arc started no later were uplifted and subjected to erosion and than the Eocene, and that Eua Island was close karsting. After the islands were emerged, there to sea level then. The Oligocene section is ap- was a second period of vulcanism (agglomer- parently missing. In the early to middle Mio- ates and flows mainly of olivine basalt erupted cene, volcanic activity was greater and the on some of them). A period of submergence island had subsided, so that tuff beds with a followed in parts of the Lau Island group, with mixture of Foraminifera were deposited. Dur- concurrent formation of reef limestones, as- ing the late Miocene or Pliocene, Eua Island signed to the late Miocene by Ladd and was again elevated to form reef-rimmed ter- Hoffmeister (1945). Since then, there has been races. Karig (1970) has correlated the brec- intermittent uplift and formation of fringing ciated pre-late Eocene basement and reefs. Eocene-Pliocene strata of Eua Island with units It has been proposed (Karig, 1970) that the visible on a reflection profile to the south, Tonga and Lau Ridges were once adjacent, and which may imply that the eastern part of the that their current separation is due to opening Tonga Ridge has had a similar history to that of of the extensional Lau Basin behind what is Eua Island. now the frontal arc. Age of the opening is not Three isolated islands can be associated tec- known, but is probably post-Miocene (Karig, tonically with the Tonga arc. One is Niua 1970, oral commun.). Because the chains of Fo'ou, or Tin Can Island, a historically active andesitic volcanoes in island arcs occur at a defi- basalt volcano. It is not evident from the com- nite distance behind the trench (Dickinson and position of the lavas whether they belong to the Hatherton, 1967), the Lau andesite vulcanism, central Pacific alkaline suite or the calc-alkaline like that of the Tongas, was a result of subduc- trend associated with island arcs (MacDonald, tion of the Pacific lithosphere at the Tonga 1948). The location is near the north end of the Trench. The locus of activity now is the western active Tonga Trench, but there is also reason to chain of Tonga volcanoes, a new feature since suspect that another tectonic environment may the Lau Basin began to open. be involved in producing its magmas. The The New Hebrides and Santa Cruz Islands chemical affinities of the other two islands, form another roughly linear group atop a sub- Futuna and nearby Alofi, are evident. The core marine ridge. North of lat 17° S. the islands of Futuna is olivine andesite which is overlain form western and eastern chains, with the pre- by limestones, tuffs, and conglomerates. The sent volcanic activity down the middle and ex- sediments contain both Miocene fossils and tending to the south where the chains are not clasts derived from the andesitic core (Aubert separate. This situation may be analogous to the de la Rue, 1935). As will be seen later, Futuna breakup of the Tonga and Lau Ridges (Karig, and Alofi (the Home Islands) may be a rem- 1969, oral commun.). There are two indica- nant of the northern end of the Tonga arc tions of the age of the New Hebrides-Santa which has been faulted to its present position Cruz island system. Preliminary work in the well back of the Tonga lineament. Torres Islands yielded a radiometric age of 39 The Lau Island group is situated on the m.y. or late Eocene for a sample of andesite north-south Lau Ridge, which turns west at its (Coleman, in press). Coleman (1969) has also northern end to join with the Fiji platform. Of found derived clasts of late Eocene age in an the hundred small islands and atolls to the north early Miocene conglomerate on the Island of on the ridge, Ladd and Hoffmeister (1945) Maewo (lat 15° S., long 168° E.). He speculates have investigated 26 in detail. They propose that Fiji may have been the source area for the that location of the islands is controlled by a older sediment. However, the reconstruction grid of northeast-southwest and northwest- given here never has Fiji very close to that par- southeast lines, probably due to faulting. In ticular part of the New Hebrides. another section of this paper we will see a rea- What is known of the post-Eocene history has sonable origin for at least the northeast-south- been summarized by Williams and Warden

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(1964) and Coleman (in press). That history by a gabbro stock that gave a K-Ar date of 50 has been complicated by apparently independ- m.y. (Rodda and others, 1967), consistent with ent vertical movements of a number of fault the Eocene fossil age. Few Oligocene fossils blocks elongated parallel to the arc. In general, have been found. Over the oldest strata there however, the oldest rocks seen are andesite is a thick series ranging from basaltic conglom- tuffs and basalt and andesite lavas with scattered erates and pillow lavas up to andesites at the diorite and gabbro plugs. These are overlain by top; limestones and sediments become more lower Miocene sediments, both reef limestones frequent upward. Age of this series is from and volcaniclastic rocks. The sedimentary se- early to possibly middle Miocene. This series quence apparently carries through the Mio- and the older series are inferred to be marine cene, occasionally interbedded with andesite (Dickinson, 1967). They are exposed along the flows. The upper Miocene does not seem to be southern side of Viti Levu. Both these older well represented, or at least not well reported. sequences are folded, faulted, mildly metamor- On Pentecost (15.6° S., 168.2° E.) there is an phosed, and intruded by plutonic stocks, which intriguing occurrence of ultrabasic rocks. They range from 50-m.y.-old gabbro to lO-m.y.-old are found near pillow lavas and marine sedi- tonalites (Rodda and others, 1967). Rodda ments in which the oldest fossils detected are (1967) infers that the youngest stocks are con- late Miocene (Coleman, in press). Unfortu- temporaneous with folding of the older strata in nately, the contact relation is not exposed. In a single episode. the Pliocene, vulcanism again becomes promi- Above an intra-Miocene unconformity are nent, together with reef formation on some of upper Miocene to lower Pliocene calc-alkaline the islands. The central chain of currently active andesitic volcanic rocks, volcaniclastic rocks, volcanoes may have developed in the Pliocene. and limestones, overlain in turn by Pliocene The Pleistocene has seen more formation of basaltic shield accumulations. They seem to reefs and some uplift. have been deposited in a series of isolated ba- Aubert de la Rue (1956) has emphasized the sins developed on the older landscape (Dickin- episodic nature of vulcanism in the New Hebri- son, 1967); the older portions are somewhat des. As far as can be distinguished, the middle- folded. Vanua Levu, the other large island, con- late Miocene was a time of low activity. It will sists of similar andesitic and basaltic rocks that be proposed below that such diminished vigor are contemporaneous with similar rocks in Viti of volcanic activity was due not just to a slowing Levu and somewhat overlapping in time (Ibbot- in the rate of overthrusting on the New Hebri- son and Coulson, 1967), and a dacitic volcanic des Trench, but to cessation of all horizontal group also overlying the andesites. The elon- movement on the arc during that time. gate island of Taveuni just southeast of Vanua The Fiji group consists of two large islands Levu is composed of a line of basalt volcanoes, rising from the shallow, flat Fiji platform and probably Pleistocene in age. The Quaternary many smaller ones. The largest island, Viti history of the islands of the Fiji Plateau, like Lau Levu, has been extensively studied. Bouguer Island, has been marked by intermittent uplift gravity anomalies suggest that the crust under and deep erosion (Dickinson, 1967). Viti Levu is on the order of 30 km thick (Rob- ertson, 1967). On the basis of a marine gravity Magnetic Anomalies and Spreading profile, Solomon and Biehler (1969) extend Centers the thick crust right to the edge of the Fiji plat- form (see Fig. 8). Thus the gravity data imply Correlation and recognition of magnetic ano- that the crust under the Fiji platform is conti- malies proves difficult and uncertain in this nental. The presence of plutonic bodies and complicated region. The sparseness of the cov- especially tonalites on Viti Levu also indicate a erage makes the problem worse. However, continental origin. when taken together with the other geophysical There is a fair amount of literature on the information, the present magnetic data serve to geology of Fiji. This summary will follow resolve three loci of active spreading. Signifi- mostly Rodda (1967), who includes a bibliog- cance of the spreading centers as plate boundar- raphy of previous work. The oldest known ies will be discussed in the next section. rocks of Viti Levu are andesitic volcanic rocks The magnetic data were obtained on Expedi- with foraminiferal limestone lenses of late Eo- tion Nova of the Scripps Institution of Ocean- cene to early Oligocene age. They are intruded ography. Total field values were digitized and

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reduced to anomalies by removal of the Inter- Ridge are shown in profile group A of Figure national Geomagnetic Reference Field (I AG A 6. Over the ridge, the magnetic anomalies are Commission 2 Working Group 4 Analysis of lineated parallel to the topographic and seismic the Geomagnetic Field, 1969). Figure 5 shows lineations. A slow spreading half-rate of 0.4 to the anomalies plotted as profiles normal to 0.6 cm yr -l accounts for the anomalies. If this track. In Figure 6 selected profiles are displayed rate has remained constant, the roughly 100- and compared to models calculated on the basis km-wide basin in which the ridge is located of the sea-floor spreading hypothesis. The geo- would have opened in approximately 10 m.y. magnetic reversal time scale is from Cox The sharp, narrow topographic and seismic (1969) for reversals to 4 m.y. B.P.; older rever- lineations are both characteristic of very slow sals are from Heirtzler and others (1968). spreading. Models are calculated from a program written There is less bathymetric and seismic expres- by D. P. McKenzie. sion of the supposed spreading center in the Peggy Ridge (17° S., 177° W. to 15.5° S., south-central Fiji Plateau and this lack of data, 178.5° W.) has already been mentioned in this which is typical of the rapid spreading half rate, section. In addition to the distinct linear bathy- 3.9 cm yr-1, is deduced from the magnetic metric feature, there is a clear, narrow linear profiles in group B of Figure 6. This rate is of band of shallow earthquake epicenters along the right order of magnitude to have generated the ridge crest (Fig. 7). Without knowledge of the western portion of the Fiji Plateau within existence of a ridge, Sykes and others (1969) the last 10 m.y., but the spreading direction, have already identified the feature as a plate now 096°, has probably changed within the last boundary by seismic lineation. Some of the million years (see below). It is not hard to un- magnetic profiles crossing the crest of Peggy derstand, given this kind of complication, why

i ^'^ff V,( i " , , I 170'

Figure 5. Magnetic anomalies plotted as profiles selected profiles shown in Figure 6. Anomalies are posi- along track where space permits. Dashed lines give ex- tive as viewed from bottom, or from right for tracks tent of Scripps Institution of Oceanography magnetic within 10° of due north. Contour interval 2 km. coverage, solid lines in labelled areas indicate location of

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/11/3087/3442938/i0016-7606-82-11-3087.pdf by guest on 26 September 2021 PROFILE GROUP A PROFILE GROUP C

500 GAMMAS

100 KILOMETERS

MODEL

v = 0.50cm/yr 0=048°

PROFILE GROUP B

J my = 3.9cm/yr 9=084 f v= = 3.8cm/yr e = v=3.9cm/yr 9=084

Figure 6. Selected observed and model magnetic lei to the spreading direction given as 0 for each profile anomaly profiles. Observed profiles are projected paral- group, v is the spreading half rate.

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170° 175° 180° 175° Figure 7. Other geophysical data. Shallow earth- Hebrides Trenches has been omitted. Location of quake epicenters (open circles) and mechanisms from gravity profiles in Figure 8 given by heavy lines. Seismic Sykes and others (1969), Isacks and others (1969). Paral- refraction stations (dashed heavy lines) from Shor and lel arrows indicate strike-slip motion, arrows radiating others (1971). Heat-flow values (solid dots) in 10-'' from open circles indicate thrusting. Where sense is am- cal/cml /sec from Sclater and Menard (1967) and Sclater biguous, both arrows are shown, as on Hunter fracture and others (1968). Contour interval 2 km. zone. Intense shallow activity near the Tonga and New the older anomalies are difficult to identify. suggested, crustal consumption under the edge Some possible correlations are shown in Fig- of the Fiji platform would have to be invoked ure 6. to account for crust generated on the east side. The third locus of spreading is the one From the bathymetry (Fig. 2), the ridge ap- thought by Sclater and Menard (1967) to lie pears to have offsets to the left which should be just west of the Fiji platform, near 18° S., right-lateral transform faults and could be 176° W., where the bathymetric expression is checked by earthquake first-motion studies. sharply rifted topography and seismicity is rela- How the spreading center connects to other tively intense, but somewhat diffuse in location. plate boundaries to the south is not clear. Of four magnetic profiles crossing the area (profile group C, Fig. 6), two have the charac- Earthquake Seismology teristic central anomaly signature of a spreading The Tonga-New Hebrides area is the site of ridge. The deduced half rate is 3.0 cm yr-1; the world's most intense concentration of shal- spreading direction is 115°. The direction is not low, intermediate, and deep earthquake ac- well controlled, as the profiles are too close tivity. Many of the shallow earthquakes and together. On the basis of the smooth magnetic presumably all of the intermediate and deep profiles outside the central zone, I conclude ones are the result of underthrusting of the li- that this spreading center is very young, about thosphere to the west under the Tonga Trench 1 m.y. old. This conclusion is confirmed by (Isacks and others, 1969) and to the east under gravity model studies and by the closeness of the New Hebrides Trench, as revealed by the the ridge to the continental Fiji platform. If the eastward dip of the Benioff zone there. We are ridge had been spreading much longer than not directly concerned with the trench activity

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in this paper, so the compilation of shallow cluster of epicenters north of the Hunter frac- earthquake epicenters in Figure 7 includes only ture zone represent. The two earthquake epi- those involving crustal motions in the Fiji Pla- centers near lat 21° S., long 174° E. studied by teau and the Lau Basin. Sykes and others (1969) that showed strike-slip In addition to the epicenters associated with mechanisms were interpreted by them as left- spreading centers discussed above, inactive lateral, parallel to a northeasterly trend in the areas are also important, for example, the Me- epicenters (Fig. 7). However, if they are con- lanesian Border Plateau, the Vitiaz Trench, and sidered to be right lateral, as plotted in Figure the scarp running from the Border Plateau to 7, they could represent two events on a fault the north end of the Tonga Trench. The sharp striking northwest. This might be a transform bathymetric relief on the scarp and in the Vitiaz fault offsetting the spreading center in the cen- Trench imply that these areas have become tral Fiji Plateau. It could also be part of more inactive recently. complicated motions near the Hunter fracture The band of seismicity extending west along zone. lat 15° S. from the north end of the Tonga Trench represents an active transform fault Gravity and Seismic Refraction Results (Isacks and others, 1969). It is the boundary The combined gravity and seismic refraction between the Pacific plate and the crust behind data available on the Fiji Plateau shows an the Tonga Trench. An earthquake focal mech- unusual structure. Interpretation of the gravity anism near lat 15° S., long 179° W. (Fig. 7) is data also confirms the reality of some of the interpreted as left lateral on a bearing of 096° proposed tectonic boundaries. Two seismic re- (Isacks and others, 1969), consistent with the fraction stations, 3-4 (22 S., 172 E.) and C5 (14 transform faulting proposed. Farther west, S., 175 E.) of Figure 7, (from Shor and others, along the north side of the Fiji platform, the 1971) demonstrate that the crustal structure is band of seismicity trends slightly more to the typical of oceanic crust, even though the area is south. A mechanism solution of left-lateral elevated 2 km above normal oceanic depths. In shear with bearing of 076° (Sykes and others, order to fit the density model shown to gravity 1969, p. 1112) is located in this zone. The profile A-A' (Figs. 7 and 8), Solomon and apparent bend in the faulting direction is proba- Biehler (1969) concluded that the mantle un- bly due to motion of the small plate behind the der the Fiji Plateau is of very low density. Pro- north end of the Tonga Trench. file B-B' (Figs. 7 and 8) shows that the gravity There are two clusters of epicenters extend- pattern is uniform over the plateau, implying ing with a northwesterly trend from the west that the low-density mantle underlies the whole end of the band by Fiji. These will be related area. Despite the low-density mantle, the seis- to motions between two smaller plates and the mic velocity measured below the Moho is nor- Pacific plate, as will the group of earthquake mal. Perhaps the unusual elevation is caused by epicenters on and near the Hazel Holme frac- influx of material from the lithosphere which ture zone (see below). The scattered epicenters was thrust rapidly under the area from each just south of Fiji may be related to slow move- side. ment of the Fiji platform, causing it to thrust The density model for profile A-A' (Fig. 8) south along the Hunter fracture zone. One shows the crust as thinnest in the central Fiji earthquake epicenter near lat 20° S., long Plateau, where a fast-spreading ridge is 178° E. (Fig. 7) shows thrusting with an un- proposed. From there it thickens to both sides, determined amount of strike-slip motion (Sykes thinning again just west of Fiji. The proposed and others, 1969). On the Hunter fracture structure may indicate a recently initiated zone itself, the shallow seismicity continues spreading center next to Fiji. A similar thinning around from the south end of the New Hebri- is seen to the west, under the inner ridge of the des Trench, following the bathymetric trends to New Hebrides. Karig (1969, oral commun.) as far east as long 175° E. From this longitude has suggested that another inter-arc basin is be- to Fiji the fracture zone is presently inactive. A ginning to open in that area. focal mechanism solution at lat 23° S., long The gravity signature of the Melanesian Bor- 173° E. shows thrusting, again with an indeter- der Plateau (profile B-B', Fig. 8), a free-air minate strike-slip component (Sykes and oth- anomaly maximum surrounded by two minima, ers, 1969). is distinct from the uniform free-air high of It is not clear just what tectonic boundary the about 40 mgal encountered on the Fiji Plateau

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/11/3087/3442938/i0016-7606-82-11-3087.pdf by guest on 26 September 2021 o 100 i- Figure 8. Free air gravity profiles. Profile A-A' and structures are taken from Solomon and Biehler (1969) with permission, (see Solomon and Biehler for comparison of observed and computed gravity.) Profile across Viti Levu (Bou- guer anomalies) from Robertson (1967). Profile B-B' gravity values from Worzel (1965). Bathymetric profile for B-B' con- structed from contours in Figure 2.

Vertical Exaggeration 12 ,5x

200 400 600 800 1000 1200 1400 Distance Km

100 i—

B' Hunter Border Plateau Q |— Fracture Zone Fiji Plateau

10 200 400 600 800 1000 1200 1400 1600 1800 Distance Km

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and tends to confirm the hypothesis that the Fiji platform is not clear, though some of it is Border Plateau is a separate tectonic province. probably Miocene. Its origin is probably com- plex; it is bounded to the north by inactive Heat Flow faults and to the south is separated from Fiji by The high heat flow of the Fiji Plateau is fur- an active transform fault. ther evidence for the presence of crustal spread- The central and western Fiji Plateau is ing (Sclater and Menard, 1967). All but two of young, having been formed within the last 10 the values plotted in Figure 7 for the central Fiji m.y. by crustal spreading. There are at least two Plateau are much higher than the oceanic aver- spreading centers active. The portion north- age value for older crust of about 1.1 heat-flow west of the Hazel Holme fracture zone may be units. As with the mid-oceanic ridges, some of older and is in transform fault contact with the elevation may be due to thermal effects. In younger crust to the south. The southern contrast to the Fiji Plateau, heat-flow values in boundary of the Fiji Plateau is the Hunter frac- and around the Border Plateau range from ture zone, only parts of which seem to be active. high to remarkably low. The high values can be The Lau Basin is also young, the result of exten- ascribed to the type of recent vulcanism that sion behind the Tonga arc which has moved formed the island of Rotuma; the prevalence of apart the Lau and Tonga Ridges. normal values is due to the older age of the crust there, and the very low-heat flow mea- LATE CENOZOIC TECTONICS AND sured in several places might be due to slump- SMALL BLOCKS ing or other recent geological effects. In studying the horizontal crustal motions of The very high values between the Border the Fiji Plateau, we can distinguish small blocks Plateau and the Fiji Islands are puzzling; in the for very recent time only. We must rely mostly proposed tectonic model no spreading in that on the seismicity to define them. Figures 10 and area is required. However, the existence of ex- lla illustrate the proposed tectonic develop- tensive strike-slip faulting through the region ment of the larger blocks over a period of 10 ceasing about 1 m.y. B.P. will be suggested. m.y. B.P. The Pacific plate has been moving Such faulting might involve enough vulcanism nearly due west in relation to the Australian to explain the hot spots. Conversely, the Lau plate at a relative velocity of 8 cm yr -1. The Basin does not show as high a heat flow as Pacific plate has thrust west under the Tonga might be expected from the extension Ridge, which is the leading edge of the Aus- proposed by Karig (1970). There are, how- tralian plate, and the Australian plate has thrust ever, higher values reported by Sclater and oth- east under the New Hebrides arc which is more ers (1968) to the south of the area (see Fig. 7). or less attached to the Pacific plate. Given this situation and the latitudinal overlapping of the Summary: Ages, Origins, and Boundaries Tonga and New Hebrides arcs, it is geometri- of Tectonic Elements cally necessary that extension occur between The oldest piece of crust with which we are them. The beginning of this extension, which concerned is the Melanesian Border Plateau, a has generated most of the Fiji Plateau, is shown Cretaceous Pacific Ocean archipelago. It has in Figure lla. ridden into the area attached to the Pacific plate The Hazel Holme fracture zone is shown as and is in strike-slip fault contact with the Fiji the northern limit of the spreading zone (Fig. Plateau. Pleistocene to Holocene volcanic ac- lla). Thus only the crust to the south would tivity has permitted some islands to survive its have been created since the late Miocene, and general shallow submergence. Its boundary to the crust north of the Hazel Holme fracture the west may be the Vitiaz Trench. zone could be considerably older. Since it Next oldest, no younger than late Eocene, seems to slope gently down to the deep Pacific, are the Tonga and New Hebrides island arcs, it may be tectonically continuous with the crust the Fiji Islands, and presumably the Lau arc. there and, therefore, as old as Mesozoic. Bot- The island arcs have resulted from underthrust- tom sampling in the area should be able to pro- ing of lithospheric plates, active under Tonga vide a test of the idea. and New Hebrides Islands. The older parts of Although the Hazel Holme fracture zone the continental Fiji Islands, also related to is- and the south side of the Border Plateau were land arc tectonics, may have a special history. once the Pacific-Australian plate boundary, Age of the part of the Fiji Plateau north of the they are not at present. The seismicity shows

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this clearly. The most reasonable explanation is 9. Their boundaries are consistent with the pat- that the Tonga Ridge, before opening of the tern of earthquakes plotted in Figure 7. Those Lau Basin began, continued to the north of its boundaries that are more speculative have been present termination (Fig. 11 a). As the Tonga labelled with question marks (Fig. 9). Superim- Ridge and Tonga Trench began to move away posed on the large plate movements are those from the Australian plate, it shed portions of its of the six smaller plates distinguished in the Fiji north end; the transform shifted south toward area. These are the units which, I assume, are its current location (Fig. 9). The two small acting as rigid plates on the earth's surface. deeps near lat 13.5° S., long 177.5° W., and Since three spreading centers have been de- 14° S., 175.7° W. (Fig. 2) are remnants of the tected between the smaller blocks, the relative Tonga Trench. The northwest-trending scarp motions of four blocks can be determined. The west of them is a combination of short segments small size of the blocks and the sparse data make of island arc, including the island of Futuna, it difficult to measure proper poles of relative and inactive transform faults running east-west. rotation, therefore, a flat earth approximation The crust west of the scarp has been effectively with translation only is used to describe their transferred to the Pacific plate. Reasonable movements. The small block vectors are related rates of opening of the Lau Basin suggest that to the large plate motions by a Pacific-Aus- the northern transform became inactive about tralian relative velocity vector deduced in the 1.5 m.y. B.P. and that the intermediate fault next section and by the assumption that motion was active from then to 1 m.y. B.P. It was then between the Fiji block and the Australian plate cut off as the faulting shifted to its present loca- is small enough to be ignored in this model. tion along the north side of the Fiji platform. Thus, in Figure 9 motions of the plates are The proposed present pattern of small blocks shown by arrows representing their velocities and some of their motions are shown in Figure relative to the Fiji (Australian) block. The inset

Figure 9. Late Cenozoic tectonics of the Fiji Plateau. gram. Contour interval 2 km. The large plates are la- Current known and speculative block boundaries are belled P for Pacific and AUS for Australian. The small shown by heavy solid lines, fossil boundaries by heavy blocks are F for Fiji, N, T, M, A, and the smallest and dashed lines. Arrows show motion of plates relative to most speculative, H, or Hunter block. Contour interval Fiji to the same scale as resultants in inset vector dia- 2 km.

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contains a vector diagram of the various result- The small plate behind the Hunter fracture ants. zone is problematical. It may be that southward The southward shift of the Pacific-Australian extension is moving the ridge bearing Matthew transform fault suggested above probably has and Hunter Islands out over the Australian much to do with the currently complex situa- plate. This is consistent with the thrusting tion. It coincides with the start of the spreading mechanism shown in Figure 7. center separating blocks F and. A. This center HISTORY OF THE LARGE PLATE may take over the entire spreading between the Pacific and Australian plates in the next few MOTIONS million years, and the other small plate boun- We are fortunate in studying the Fiji area in daries in the western Fiji Plateau may become that the two large plates involved, the Pacific inactive. Now there is too much spreading in and the Australian, both border on Antarctica, the Fiji Plateau to be accounted for by large and each is separated from it by a spreading plate motion. This is understandable if it is al- ridge. From the magnetic anomalies and frac- lowed that plate M and the New Hebrides ture zone trends on these two spreading ridges, Ridge which is its leading edge continue to we deduce the history of motion of both plates move during the reorganization of boundaries. relative to Antarctica and to each other. Thus when the transform shifted south from the Large-scale magnetic surveys of the Pacific- Hazel Holme fracture zone, the cut-off segment Antarctic Ridge (Pitman and others, 1968) and of ridge in the northwest and central Fiji Pla- of the Australian-Antarctic Ridge (Le Pichon teau did not stop spreading immediately, but is and Heirtzler, 1968) divide each ridge into 10 doing so now. Such a gradual transition within m.y. intervals by recognizing key anomalies; a period of 1 m.y. could lead to the present spreading velocities are deduced for each inter- complications. val. The time-scale is from Heirtzler and others The proposed model (Fig. 9) may be justified (1968); the anomaly numbering system follows by such arguments, but it also may entail conse- Pitman and others (1968). The intervals are: quences that can be tested. The easiest tests are Stage I (anomaly 0-5), 0 to 9.9 m.y. B.P.; Stage earthquake focal mechanism solutions. North II (anomaly 5-6) 9.9 to 21.2 m.y. B.P.; Stage III of lat 15° S. the New Hebrides Trench reflects (anomaly 6-9) 21.2 to 30.9 m.y. B.P.; Stage IV, Pacific-Australian plate motion. Shallow earth- 30.9 to 40 m.y. B.P.; Stage V, 40 to 45 m.y. quakes on the Benioff zone should be consist- B.P.; Stage VI, before 45 m.y. B.P. Because it ent with underthrusting to the east northeast. is difficult to identify the older anomalies, the South of 15° S., shallow earthquakes should rates for Stages IV and V for both ridges were reflect motion of plates M and F and show measured on anomalies 12-13 (35 to 38 m.y. thrusting east southeast. New Hebrides shallow B.P.), which are very distinctive. The same rate mechanisms are not complete, but the inter- was assumed to hold for Stage VI on the Pacific- mediate depth earthquakes show a change at Antarctic Ridge. The Australian-Antarctic the proper point (Isacks, 1970, written com- Ridge began to spread at 45 m.y. B.P., or mun.). anomaly 18 time (Le Pichon and Heirtzler, Other testable consequences are: (1) oblique 1968; Smith and Hallam, 1970). For my calcu- compression on the reactivated western trace of lations the above ages were used, but for fur- the Hazel Holme fracture zone (M-P bound- ther discussion the stage boundaries are ary); (2) extension on the isolated segment of estimated to fall on even 10-m.y. lines. Table 3 ridge crest south of there; (3) strike-slip on the includes the simplified stage boundary times. remaining portion of the M-P boundary; (4) Patterned in this way, the spreading rates oblique compression on the A-P boundary; (5) represented averages over each stage, and use strike-slip between Fiji and the Pacific plate (see of the plate motions derived from them re- Fig. 7); (6) extension associated with the three quired the assumption that spreading was con- spreading centers. Obviously, a good deal of stant during the stage. Where possible, rates work must be done to test the model. Although were measured over smaller intervals within the excess spreading is large, on the order of 6 the stages in order to check this assumption. For cm yr ~~!, the time during which it has acted is both ridges, spreading did appear constant for short. Thus only about 50 km of actual displace- Stage I, and show an abrupt change at its lower ment is required, which would be hard to de- boundary. Stage II satisfied the assumption also. tect in large-scale bathymetry. It seems likely that Stages III to VI are more

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arbitrary divisions, and the rates are probably These poles were then vectorially averaged to averages of a slowly varying function. Spread- find the Australia/Antarctica and Pacific/An- ing was assumed to be symmetrical, which was tarctica poles presented in Table 2. To preserve true for at least the Pacific-Antarctic Ridge a consistent frame of reference, all older poles where it could be checked. were rotated by use of the younger poles into Spreading directions proved harder to meas- a coordinate system fixed to Antarctica, and are ure than rates. Fracture zones, and especially quoted in that system. In this way the problem their older parts, are not well mapped for either of finite rotations is avoided (McKenzie and ridge. For the Pacific-Antarctic Ridge, the ten- Morgan, 1969). The Pacific/Australian poles tative fracture zone trends of Pitman and others given in Table 2 are the sum of the Pacific/An- (1968) were used. On the Australian-Antarctic tarctic and Antarctic/Australian poles for each Ridge for Stage I, earthquake mechanism solu- stage. tions by Banghar and Sykes (1969) were used In this way we have arrived at a history of the to guide choice of directions, and for the older relative movements of the Pacific and Aus- portions inferred offset locations in the data of tralian plates for much of the Cenozoic. The Le Pichon and Heirtzler (1968). Where these Stage I Pacific/Antarctic pole essentially was methods failed, the spreading direction was identical to that of Le Pichon (1968), so his taken as normal to the trend of the magnetic value was adopted. Evidence for the rapid shift anomalies. It is likely that the most serious of the Australia/Antarctica pole (Table 2) may source of error in the final results is the uncer- be seen in Le Pichon and Heirtzler (1968). tainty in the direction of the spreading velocit- Anomaly 5 converges with the ridge crest to ies. In view of this difficulty, the Euler poles of the west in Figure 5 of Le Pichon and Heirtzler Table 2 should be taken to be first approxima- (1968, p. 2111), anomaly 6 converges toward tions only. anomaly 5 to the east, and the older anomalies Euler pole is a new term for the pole of rela- converge again to the west. These conver- tive rotation between any two plates. A Euler gences indicate that the equator of spreading pole is stated with a positive angular speed of has shifted quickly back and forth, and is re- relative motion and a convention imposed on flected in the location of the poles. the choice of pole or anti-pole: viewed from A check is possible on the validity of the above the pole given for the motion of plate X motions proposed for the Australian plate rela- with respect to plate Y, plate X is moving coun- tive to Antarctica. The total effect of all the terclockwise relative to plate Y. Any two rotations since the Eocene listed for Australia spreading velocities on the boundary of two (Table 2) is rotation about a pole that differs in plates can be used to determine the Euler pole location by less than 10° from the net pole of for the plates. Four or five spreading velocities Smith and Hallam (1970) and differs in magni- were deduced for each period and for each tude by less than 4°. This agreement with a pole ridge, and taken in pairs to derive Euler poles. derived from shorelines and selected contours

TABLE 2. EULER POLES DERIVED FROM SPREADING VELOCITIES, QUOTED IN CO-ORDINATE SYSTEM FIXED RELATIVE TO ANTARCTICA*

Austral ia/Antarotica Pacific/Antarctica Pacific/Australia (Aust/Ant) (Pac/Ant) (Pac/Aust=Pac/Ant-Aust/Ant)

Angular Angular Angular Rate Total Rate Total Rate Total Lat Long 7 Rotation Lat Long 7 Rotation Lat Long 7 (xlO" °/ (xlO' °/ Rotation Stage (xlO" °/ (°) (°) yr) (°) (°) (°) yr) (°) (°) (°) yr) (°)

I 7S 44E 5.86 5.8 70S 118E 10.8 10.7 585 172W 11.1 11.0 II 20N 28 E 5.69 6.5 68 S 114E 5.11 5.8 50S 172W 8.65 7.6 III 20S 62E 6.52 6.3 73S 125E 5.94 5.7 32S 134W 6.55 6.3 IV 26 S 71 E 8.21 7.2 77S 150E 7.92 7.2 29S 123W 8.27 7.5 V 26S 71 E 8.21 3.6 77S 150E 7.92 3.9 29S 123W 8.27 2.1 VI - - - - 77 S 150E 7.92 77S 150E 7.92

Locations and rate are average for stage; total angles are angular displacements, positive counterclockwise, for pole operating for duration of stage. These poles should be considered as first approximations only.

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is certainly adequate. It is the kind of error that Stage I tectonics have already been discussed. can be expected, especially in the older poles of The main effect of the motions has been to open Table 2. the Fiji Plateau and the Lau Basin, increasing TECTONIC HISTORY OF THE FIJI the separation of the Tonga and New Hebrides PLATEAU arcs. The more complicated motions, now exist- ing, are too recent to have significant effects at Now we are ready to explain the tectonic the scale of Figure 11. One testable conse- development of the Fiji area as the result of quence of the Stage I motions is that portions interactions of the Pacific and Australian plates. of the sediment apron on the back side of the The motions are fixed; the only freedom of New Hebrides Ridge should have been left choice is in the placement of the boundaries. behind north of Fiji when the western Fiji Pla- We must assume that there were no intervening teau began to open. plate motions, that is, opening of the Tasman A model for Stage II tectonics was more diffi- Sea, and South Fiji Basin took place before 45 cult to construct. The relative motion of the two m.y. B.P. This assumption is uncertain, but rea- plates remains roughly east-west (Table 3), and sonable, I think, in the absence of contrary data. if the New Hebrides islands were attached to The simplified outlines of the tectonic ele- the Pacific plate, they would have to start from ments are shown in Figure 10. To arrive at the a position atop Tonga and Lau Ridges at 20 situation at the beginning of Stage I (Fig. 1 la) m.y. B.P. To avoid this impossible situation, the Lau Basin was first closed by rotation of the the following assignment of tectonic units to Tonga Ridge back against the Lau Ridge about plates was made: Pacific-Border Plateau; Aus- an arbitrary pole. Then the Lau and Tonga tralian-New Hebrides, Fiji, Tonga, Lau, Vitiaz, Ridges, and Fiji were considered to be attached Cape Johnson. There are two notable results for to the Australian plate and rotated about the Stage II (Figs. 1 la and 1 lb). One is that during appropriate pole. New Hebrides island arc, this time the New Hebrides island arc would be Cape Johnson Trough, the Vitiaz Trench, and an inactive feature, stranded on the Australian the Melanesian Border Plateau were rotated plate and not representing an overthrusting about the Stage I Pacific/Antarctic pole. The plate boundary. This conclusion is not incon- tectonic units were then plotted in their posi- sistent with the geology discussed previously. tion 10 m.y. B.P. relative to Antarctica. The The other result is that the Pacific-Australian vectors of relative velocities shown in Figure 11 plate boundary must bend to the north to clear were calculated from the Pacific/Australian the New Hebrides Ridge. Thus it is proposed poles. The relative velocities at the location of that the Vitiaz Trench was the locus of oblique the Fiji Islands are displayed in Table 3; the crustal consumption from 20 to 10 m.y. B.P., older directions of relative motion in Table 3 and that the Cape Johnson Trough is the trace also have been rotated back to the present loca- of a transform fault terminating its north end, tion, to indicate how they would appear if pre- as shown in Figures lla and lib. served in fracture zone trends. If the Vitiaz Trench was an east-facing over- thrust, impingement of the Border Plateau, a large feature difficult to thrust down, may ex- plain why its activity stopped and activity on the New Hebrides island arc resumed. Essentially, a corner of the Australian plate was torn off along an old line of weakness. One seismic re- flection profile across the Vitiaz Trench (Ewing and others, 1969, p. 2487) suggests that the downgoing lithosphere there has separated and the east side of the trench rebounded to some extent. Perhaps this lost piece of lithosphere is the source of the strange horizontal region of very deep earthquakes near the north end of the New Hebrides arc (Barazangi and Dor- Figure 10. Fiji Plateau, at present, showing sim- man, 1969). Also, if this interpretation is cor- plified tectonic elements to be used in following illustra- rect, Anuda and Mitre Island and Charlotte and tions, and current and fossil tectonic boundaries. Pandora Banks may not be related in origin to

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BEGINNING OF NEW STAGE ENO OF PREVIOUS STAGE

Figure 1 la. Fiji Plateau at 10 m.y. B.P. Maps for Fig- Figure lie. Fiji area at 30 m.y. B.P. Heavy solid lines ures lla-d are plotted in coordinate system fixed with are Stage III; dashed lines are Stage IV. respect to Antarctica. Heavy solid lines are Stage I boun- daries; heavy dashed lines are Stage II.

Figure lid. Fiji area at 40 m.y. B.P., assuming no intervening motion in the South Fiji Basin or Tasman Figure lib. Fiji Plateau at 20 m.y. B.P. Heavy solid Sea. Heavy solid lines are Stage IV; dashed lines are Stage lines are Stage II; dashed lines are Stage III. V.

the Border Plateau, but to the peculiar local Hunter fracture zone. It is reasonable to expect tectonics. However, the deep earthquakes may concurrent northeast-southwest faulting in the represent lithosphere detached more recently north end of the Lau Ridge, as previously men- from the New Hebrides down-going slab. If so, tioned. The present apparent alignment of the the total length of the New Hebrides and east end of the Fiji platform with the north end Tonga seismic zones is more consistent with the of the Lau Ridge is probably the result of Stage amount of underthrusting in the last 10 m.y. I rifting after the two features had been brought Between Stages II and I, the directions of nearly into juxtaposition during Stage III. The motion did not change much, but the rates in- plate boundaries as drawn in the illustrations creased. Conversely, between Stages III and II are obviously much simplified from their real the rates are roughly constant and the direction occurrence. Note that Fiji is for Stage III and shifts sharply (Table 3). The Border Plateau, older stages an island of the Pacific plate, and Fiji, and the New Hebrides islands have been that the Border Plateau has not yet arrived in rotated with the Pacific plate and Tonga and the area to set off complications. Lau remain with the Australian plate (Fig. 1 Ic). For Stage IV the motions remain much the The effect is to bring the Tonga and New Heb- same, as does the assignment of tectonic units rides island arc, still facing in opposite direc- to large plates. The eastern portion of the Fiji tions closer together. The transform fault Islands and the north end of the Lau Ridge are linking their ends is probably the ancestral dropped from the last plot, Figure lid, because

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TABLE 3. MOTION OF THE PACIFIC PLATE WITH RESPECT TO THE AUSTRALIAN PLATE AT THE FIJI ISLANDS, GIVEN FOR THE tory of the Fiji Plateau and its environs. Al- BEGINNING AND END OF EACH STAGE DIRECTIONS ARE though the theory of plate tectonics does not MEASURED POSITIVE CLOCKWISE FROM NORTH. FURTHER EXPLANATION IN TEXT. explain all the geology of the crust involved, it does provide a framework in which to consider Approxi- Direction Direction the more detailed aspects. Using only the mo- mate time Lat Long then now Rate Stage (m.y.B.P.) (°) (") (°) (°) cm/yr tions given for the Pacific and Australian plates, the history of the area proposed in Figures 10 -16 S — 179 E — 263 263 — 8.1 and 11 is not necessarily unique, but the most reasonable that could be constructed. It may also help in understanding Melanesia. 264 269 7.4 -21 S — 179 M - 261 266 4.8 Implications of this model for areas outside the Fiji Plateau, although important, have not

261 270 4.3 been discussed. There is, however, one striking - 20 24 S — 179 W - 202 212 4.7 consequence. The southward continuation of the transform terminating the north end of the New Hebrides neatly coincides with the posi- - 30 25 S — 174 W - 200 209 4.2 197 207 6.4 tion of the island of New Caledonia on the Australian plate during Stages V to III. The fault could have connected to another consum- - 40 26 S — 167 W — 194 203 — 5.7 ing plate boundary to the west. If so, it could correlate with the Tertiary geologic history of

.4 444 2il7 Sb — 16Ib3J Et 193 201 5.3 thrusting on New Caledonia (E. L. Winterer, 282 289 ?>2 1970, oral commun.). ACKNOWLEDGMENTS I would like to thank H. W. Menard for ad- there is not much evidence that they existed at vice, encouragement, many hours of discus- that time. The Tonga Ridge and New Hebrides sion, and much help in the course of my island arc continue to return toward alignment graduate education. into a straight feature. By the time the motions The development of this paper has been have been continued back to the beginning of aided by discussions with H. W. Menard, E. L. Stage V (not shown), the two arcs are end to Winterer, D. E. Karig, R. L. Larson, T. Atwa- end. ter, and many others. R. L. Parker guided me That situation is the key to the origin of the in the geometry of plate motions, without Fiji Islands, for which the breakup of Australia which the reconstruction of the tectonic history and Antarctica is probably responsible. In Stage would not have been possible, and provided VI, with Australia attached to Antarctica, the programs to plot the moving plates. E. D. Mi- Pacific-Australia relative motion is simply the low was kind enough to date core samples at my Pacific-Antarctic motion. That motion is an request, and E. C. Allison and W. R. Riedel east-west convergence of the two plates (Table provided me with unpublished paleontological 3). I assume that the convergence was taken up results. J. G. Sclater provided plots of the heat on a single east-facing arc, the ancestor of both flow. the Tonga and New Hebrides Trenches. The Data on which the study was based was taken drastic change in direction of relative motion by many people on R/V Horizon and R/V Argo on the boundary as Australia began to move during Expedition Nova, and previous Scripps away from Antarctica could have caused what is Institution of Oceanography cruises. T. E. now the New Hebrides Trench to reverse di- Chase provided the bathymetric base map of rection and evolve away from the Tonga the area, and S. M. Smith and G. F. Sharman Trench. The initial vulcanism that built the Fiji wrote the programs to manipulate the data. platform would then occur in the complex This manuscript was improved by comments oblique tear between the two arcs. from H. W. Menard,J. R. Curray, V. Vacquier, E. C. Bullard, R. E. Houtz, and B. Isacks. CONCLUDING REMARKS Support for the scientific effort was provided Application of plate tectonics has led us to an by the Office of Naval Research, the National interpretive explanation of the Cenozoic his- Science Foundation, and the Naval Oceano-

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