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PLATE TECTONIC HISTORY OF FIJI

Structure of the Earth

The Earth is made up of three concentric zones : the crust, mantle and core (Figure 1). The crust is the thin outer layer forming the continents and the ocean floor. Oceanic crust is thinner and more dense than continental crust. Under Viti Levu the crust is about 15-20 km thick. The crust is enriched in O, Si, Al, Ca, Na and radioactive elements. The mantle is the middle layer, rich in Mg, Fe and silicates (combinations of mainly silica and oxygen) and the core is the dense inner layer composed of iron and nickel. The inner part of the core is liquid and extremely hot. The lithosphere is the solid part of the Earth, up to 120 km thick, composed of rocks and minerals, which in turn, compose the crust and part of the upper mantle. Geologists study the lithosphere because it is the most easily accessible part of the Earth.

Figure 1. Diagram showing the concentric zones of the Earth.

Plate Tectonics

Our planet Earth is under constant change, and evidence that the Earth is still active can be seen whenever a volcano erupts or an earthquake shakes the land. The lithosphere or outermost shell of the Earth, which includes the crust and upper mantle, is broken up, like a cracked egg, into a number of rigid plates which move around because of forces in the upper mantle (Figure 2).

Figure 2. The Pacific lithospheric plates.

The boundaries of these plates are zones of intense activity, and associated with them are many of the large-scale geological features produced by such processes as mountain-building, volcanism, creation and destruction of seafloor, and earthquake activity. The movement and deformation of the plates is known as plate tectonics. Tectonic movements, which indicate instability of the earth's crust, produce faulting (fracture and displacement), folding, subsidence and uplift of rocks. Over millions of years plate tectonics has been responsible for the formation of many great mountain ranges and ocean basins.
Plate Boundaries Plate boundaries are where plates meet and move against each other (Figure 2). They are usually the locations of many violent earthquakes and volcanoes. Earthquakes and volcanoes tend to occur in belts (for example, the "Ring of Fire" on which Fiji is located, surrounding the Pacific Ocean). These belts are the locations of plate boundaries. There are three types of plate boundaries - divergent, convergent and transform (Figure 3).

Figure 3. Sketch showing three major types of relative plate motion:

(a) divergent;

(b) convergent,

(c) transform fault,

(d) block diagram showing all three types of plate boundary motion.

Divergent boundaries (constructive boundaries) are where adjacent plates move apart and new crust is created (Figure 3a). In the major oceans, there are long underwater mountain ranges called mid-ocean ridges. Mid-ocean ridges develop at edges of diverging plates and are the sites of volcanism and shallow earthquake activity. Along the centres of the ridges, new rock is forming from molten magma coming up from the upper mantle, along fissures. The rocks produced are basaltic in composition. The addition of new rock from below helps to push the plates apart and widen the oceans. This process is also known as seafloor spreading. An example of a mid-ocean ridge is the Mid-Atlantic Ridge in the Atlantic Ocean.

Convergent boundaries (destructive boundaries) are where two adjacent plates move towards each other and crust is destroyed (Figure 3b). Along convergent boundaries are crumpled mountain ranges, deep-sea trenches, shallow and deep earthquakes and volcanoes. The edge of the thin plate usually sinks under the other plate at an angle of between 30ฐ and 70ฐ, usually forming deep trench. This process is called subduction, and the movement of the plates as they adjust produces earthquakes. As a plate edge sinks into the hot mantle, the rocks will be squeezed and heated and parts of it begin to melt. The rock melt, or magma, thus formed would float upward, some of it reaching the surface as lava erupting from volcanic vents. The resultant volcanism often forms a chain of volcanoes along the plate boundary, known as a volcanic island arc. The plate boundary can be seen at the Earth's surface as a deep depression or ocean trench. The islands of Fiji, Vanuatu, Lau and Tonga were formed as volcanic island arcs by subduction-related processes; however, Fiji and Lau are no longer active island arcs.
Transform boundaries are where plates slide along each other in opposite directions, and no new crust is created or destroyed (Figure 3c). The famed San Andreas fault of California is such an example. There the Pacific Plate slides past the American Plate in a northwesterly direction at a rate of 10 mm per year. When arcs are stretched they split, initially forming a rift and eventually a back-arc spreading centre. The weakest area of the upper plate is near the volcanic front (where volcanism is occurring), and the plate is both the hottest and has the thickest crust. Therefore the arc splits within about 50 km of the volcanic front. The basin that usually opens up behind the volcanic island arc is known as a back-arc basin or a marginal basin and it usually becomes filled with lava and sediments. The North Fiji Basin and the Lau Basin are examples of back-arc basins.

The Tectonic History of Fiji

In the southwest Pacific two large plates, the Pacific Plate and the Indo-Australian Plate, are moving towards each other, that is, converging (Fig. 2). This area of the South Pacific was formed in the last 65 million years of Earth's history by the growth of volcanic island arcs and associated marginal and oceanic basins. Fiji's tectonic history is very complex but it can be summarised in four major stages.

Stage 1 : 40-28 million years ago

During this period it is generally thought that there was a continuous volcanic island-arc system, known as the Vityaz Arc, which included New Hebrides (Vanuatu), Fiji, Lau and Tonga. The exact orientation of this arc is not known but a possible interpretation is shown in Figure 4a. This arc was formed by westward subduction of the Pacific Plate beneath the Indo-Australian Plate at the Vityaz Trench, parts of which are still visible today. In Fiji, rocks which are remnants of the old Vitiaz Arc are found only in western Viti Levu in the Yavuna Group rock sequence, which is 40 to 28 million years old .

Stage 2 : 28-12 Million years ago

The tectonics of the area in this stage were very similar to the previous stage, the key difference being a change in relative plate motions. The Pacific/Indo-Australian Plate boundary was reorganised in this period as the movement of the Pacific Plate changed direction from a westward motion to a more northwestward motion. This led to oblique or diagonal northwestward subduction of the Pacific Plate under the Indo-Australian plate at the northern part of the Vityaz Trench and increased stresses in this region. The subduction was therefore at a particularly shallow angle to the plate boundary in the area to the northwest of Fiji, whereas to the south there was little change.
In Fiji, another volcanic island arc was formed, along with associated sedimentary basins, that produced volcanic and sedimentary rocks of the Wainimala Group which are exposed today in southern Viti Levu, and in the Yasawa and Mamanuca groups of islands. There was continued volcanism on the Lau-Tonga Arc due to the northwestward subduction of the Pacific Plate under the Indo-Australian Plate along the Vityaz Trench. Fiji was therefore caught in an area of increasing stresses during this period. Intrusions of plutonic
rocks were common, and the earlier-formed volcanic and sedimentary rocks were weakly folded, faulted and metamorphosed. Uplift near the end of this period may have produced the first significant land masses of Fiji, mostly on Viti Levu.

Figure 4. Tectonic reconstruction of the southwest Pacific region. (Modified from Hathway 1993).

View Fig. 4c (35Kb)

Stage 3 : 12-7 Million years ago

During this period, the Pacific Plate continued to obliquely subduct in a northwest direction under the Indo-Australian Plate south of Fiji, while the Indo-Australian Plate probably moved northeast. The new stresses resulted in a major change in the nature of the plate boundary with the end result being splitting of the Vityaz Arc and the formation of a transform boundary north of Fiji called the Fiji Fracture Zone (Figure 4b). At this boundary the two major plates were moving past each other. This marked the end of continuous subduction at the Vityaz Trench, and the eventual break-up of the Vityaz Arc.

Stage 4 : 7 Million years ago - Present

Following the breakup, the movements of the plates led to the formation of a northeastward-dipping subduction zone along the New Hebrides Trench (Figure 4b). Previously, the Pacific Plate was subducting westwards under the Indo-Australian Plate on the Vityaz Trench, but now the Indo-Australian Plate began to subduct northeastward under the Pacific Plate at the New Hebrides portion of the Vityaz Arc. This reversed subduction eventually led to renewed volcanism on the New Hebrides Arc. At about the same time, the New Hebrides Arc rotated clockwise and Fiji rotated anti-clockwise away from the Vitiaz Trench. The divergence of Fiji and New Hebrides was accompanied by seafloor spreading and the opening up of the North Fiji Basin in between New Hebrides and Fiji (Figure 4b). The North Fiji Basin is a back-arc basin related to the New Hebrides Arc.
Another major event during this period was the splitting up of the Lau-Tonga portion of the old Vityaz Arc (Figure. 4c). The Lau-Tonga Arc split apart into the Lau Arc and the Tonga Arc as volcanism and later seafloor spreading formed the Lau Basin. The Lau Basin is a back-arc basin related to the Tonga Arc. Much of the Lau Basin developed in the last 2.5 million years. As a result of the seafloor spreading in the Lau Basin, the Lau Arc became isolated from effects of subduction at the Tonga Trench portion of the old Vityaz Trench. Therefore volcanism stopped in the Lau Arc but continued in the Tonga Arc. At present there is active island arc volcanism on the Tofua Arc on the western side of the Tonga Arc. This is related to westward subduction of the Pacific Plate under the Indo-Australian Plate at the Tonga Trench.

Summary

Fiji, therefore, represents a portion of the old Vityaz Arc, which was split up, and rotated anti-clockwise to its present position. It forms a mini-plate, known as the Fiji Plate (Fig. 2). The breakup of the Vityaz Arc is probably reflected in the strong faulting and folding of 12 to 7-million-year-old rocks in southwest Viti Levu. This was also a period of great volcanic activity in Fiji and the whole region. Fiji is located at the Indo-Australian and Pacific plate boundary between two opposite-facing subduction zones (Figure 5) and hence has had a very complex tectonic history.

Figure 5. Schematic cross-section showing the plate, ocean, island-arc relationship in the southwest Pacific area, and Fiji’s location between two opposite-facing subduction zones. Present-day volcanism occurs in the Tonga and New Hebrides Arcs.

The stresses created by the opposing plate movements have resulted in the formation of transform faults such as the Fiji Fracture Zone to the north and the Hunter Fracture Zone to the south. Seafloor spreading resulted in divergence and

opening up of the North Fiji Basin and the Lau Basin. Fiji and Lau are no longer active volcanic island arcs, as there is no subduction or rifting within the Fiji islands, but they formed from similar processes in the past. There is, however, active seafloor spreading occurring at the present day in the Lau Basin to the east and in the North Fiji Basin to the west of Fiji - these are both examples of back-arc or marginal basins. However, Vanuatu and Tonga are still active volcanic island arcs, as subduction is still occurring under these arcs, and therefore volcanism occurs within these islands.

SUGGESTIONS FOR FURTHER READING

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(All articles are available in the MRD Library).
Auzende, J. M., Eissen, J. P., Lafoy, Y.,Gente, P. & Charlou, J. L. 1988. Seafloor spreading in the North Fiji Basin(southwest Pacific). Tectonophysics, 146, pp. 317-351.
Colley, H. & Hindle, W. H. 1984. Volcano-tectonic evolution of Fiji and adjoining marginal basins. In: Kokelaar, B. P. & Howells, M. F. (eds) Marginal Basin Geology. Geological Society, London, Special Publication, 16, pp. 151-162.
Hathway, B. 1993. The Nadi Basin: Neogene strike-slip faulting and sedimentaion in a fragmented arc, western Viti Levu, Fiji. Journal of the Geological Society, London. Vol 150, pp. 563-581.
Louat, R. & Pelletier, B. 1989. Seismotectonics and present day relative motions in the New Hebrides-North Fiji Basin region. Tectonophysics, 167, pp. 41-55.
Parson, L. M., Pearson, J. A.,Murton, B. J., Hodkinson, R. A. & RRS Charles Darwin Scientific Party. 1990. Role of ridge jumps and ridge propagation
in the tectonic evolution of the Lau back-arc basin, southwest Pacific. Geology, 18, pp. 470-473.
Rodda, P. 1994. Geology of Fiji. In: Stevenson, A. J., Herzer, R. H. & Ballance, P. F. (eds) Contributions to the marine and onland geology of the Tonga-Lau-Fiji region. SOPAC Tecnical Bulletin 8, pp. 131-151.

ISSN 1016-2135

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