Fiji lies in a
tectonically complex area between the
Australian plate and the
Pacific plate. The Fiji Platform lies in a zone bordered by active extension fault lines, around which most of the shallow earthquakes in the area have been centred. These fault lines are: the Fiji Fracture Zone (FFZ) to the north; the
176° Extension Zone (176°E EZ) to the west; and the Hunter fracture zone (HFZ) and
Lau Ridge to the east. The oldest rocks on the island are those formed during the
Eocene and
Early Miocene epochs that belong to the Wainimala
group. The lower portion of the group is made up of
volcanic flows and
volcaniclastics, which grade from
basalt to
trachyte and
rhyolite. Geographically, this group is found south of
Nadi, including on the peaks of Koromba (at 3528 feet high) and Natambumgguto (at 1242 feet high), and down to
Sigatoka. From Sigatoka, the group extends almost all the way to Lodoni, and includes the peaks of Tuvutau (at 3060 feet) and Tikituru (at 3071 feet). Along the southern coast it extends almost to
Nausori. The group is
intruded by the Tholo
Plutonics, which consist of similar-age
stocks of
tonalite,
granodiorite,
gabbro and
diorite.
Mio-
Pliocene sandstones, and
marl, grading into epiclastics and
andesitic Volcanic rocks of the Suva group are found mainly in the river valleys, such as those formed by the
Nadi River and
Navua River. The Plio-
Pleistocene Mba group is found in the northern portion of the island. It consists of
porphyritic basalt flows and volcaniclastics, grading into
greywacke. Geographically, it includes the peaks of Koroyanitu (at 3921 feet), Malua (at 3294 feet), Monavatu (at 3708 feet),
Mount Tomanivi (at 4341 feet), and Ndelamendamu (at 2540 feet). It extends along the eastern coast, almost to Nausori, and includes the Emperor Mine near
Vatukoula. == Geography, economy and ecology ==