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 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 3088 C. G. CHASE—TECTONIC HISTORY OF THE FIJI PLATEAU MELANESIAN BORDER ROM™ PLATEAU Wollis Is 1 . *A o % ^ 'a ; a 25° Soulh Fiji Bosin (AUSTRALIA 30° ' I jf 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 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 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 DESCRIPTION OF THE AREA 3089 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.