The Volcanic Arc of Vanuatu

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The Vanuatu Arc is located in the southeastern Pacific.

The Pacific plate-to-Australian plate convergence is accommodated by two opposing dip subductions: the Pacific plate plunges westward under the Kermadec-Tonga arch, and the Australian plate dips east under the Salomon- Vanuatu.

The region bordered by these two spontaneous sub-divisions is experiencing an important extension, reflected in the opening of ocean basins such as the North-Fijian basin and the Lau basin.

The North-Fijian basin is bordered to the north by the currently inactive pit of Vitiaz. This pit is considered to be the border area between the Pacific and Australia plates between the Upper Eocene and the Upper Miocene.

In the Upper Miocene, the collision between the Ontong-Java plateau carried by the Pacific plate and the

Vitiaz would have resulted in blocking the subduction of the Pacific plate. This blocking would have led to the polarity reversal of the subduction between Papua New Guinea and the Fiji Islands. This portion of the arc, now mainly constituted by the Vanuatu Islands, would have been rotated as the North-Fijian basin opened.

The convergent margin of Vanuatu extends over 1,400 km between the Santa Cruz Islands in the North and the Matthew and Hunter Islands in the South.

The rate of convergence is among the fastest in the world: speeds reach 12 cm / year in the southern part and 16 cm / year in the northern part.

The margin of Vanuatu can be divided into 4 segments:

1 - The northern segment (lat. 11-14 ° S) has a deep pit (up to 8000 m) and a volcanic arc whose floor is 1200 m deep and from which emerge a few islands with extinct volcanism . The convergence speed is the highest of the arc with 16 cm / year.

2 - The central segment (lat. 14-17 ° S) is distinguished by the absence of pit and very low convergence rates of the order of 3.5 cm / year. These characteristics are attributed to the collision with the seismic wrinkle of Entrecasteaux carried by the Australian plate. In this segment the volcanic islands are large and form three parallel chains resulting from three major volcanic episodes:

. The western chain, of upper Oligocene age to the middle Miocene, is linked to the fossil subduction of Vitiaz.

. The eastern chain, of Upper Miocene age to the lower Pliocene, marks the beginning of the operation of the Vanuatu arc.

. The central chain represents the active volcanic zone from the Pliocene.

3 - The southern segment (lat. 17-21 ° S) is characterized by a deep pit (6000 - 7000 m) and a narrow volcanic arc from which emerge a series of volcanic islands directly bordered by extension ditches. The maximum convergence rate is recorded at Tanna with a value of the order of 12 cm / year.

4 - The southern extreme sector (lat. 21 ° S) corresponds to the termination of the subduction zone. The volcanic arc is narrow and two active volcanoes emergeVanuatu's

 

volcanic activity:

The archipelago has more than 80 islands, most of which are of volcanic origin.

The oldest rocks are in the western part of the arc, then the rocks of the central part and finally those of the eastern part which are the most recent.

Ph. Bani et al. (2012) A. Beaumais et al. (2013)

Guy

Docteur en Pharmacie, Biologiste Médical libéral à Marseille, France.

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