TECTONIC SETTING IN THE SUBDUCTION ZONE

The Indonesia archipelago was formed 300 millions years ago from tectonic activity. Indonesia is located on the border of three main plates namely, Eurasia, India-Australia, and Pacific-Philippine sea.

The geography of Indonesia and the surrounding area shows tectonic boundaries and present volcanic activity. Active volcanoes along archipelago arcs are the result of a series of plate collision activities that form the ring of fire. The red arrows show the vector of subduction plates in some area.

Sumatra Cross Section Schematic (Mobile Oil Corp)

Sumatra arc cross section - trench system is formed by subduction of the oceanic plate (Indian-Australian) under the continental plate (Eurasian). The tectonic system formed is the oblique subduction zone, the Mentawai fault zone, and the large Sumatra fault zone which divides the island of Sumatra from the Semangko bay to Banda Aceh. Continental plates are thick and old consisting of volcanic arcs with Permian, Cretaceous, and Tertiary ages. Magmatic rocks form above the Benioff zone which is mostly marked by silicates and intermediates. The movement of Indian and Australian plates caused the two plates to collide resulting in forearch island and magmatic arc island such as the Mentawai islands.

Java Cross Section Schematic (Mobile Oil Corp.)

In the Java cross section demonstrating that this arc was formed due to subduction of the oceanic plate beneath the continental plate. The continental crust on Java is thinner and has a relatively young age consisting of plutonic volcanic arcs. Magmatic rocks are mostly intermediates type. On the Javanese plateau there are two plate systems that is South China Sea Plate (Eurasia) in the north and the Indian Ocean Plate in the South. The Eurasian Plate has been moving southeast since the oligocene while the Indian Ocean Plate is moving and dipping down the arc systems of the Java and Sumatra island.

The process of forming volcanic arcs (Katili, 1974)

The oblique extension around the subduction zone that is show the mechanical field effects of the tectonic plate. Collisions between oceanic crust sinks beneath the continental crust which causes melting of rock that moves to the surface through fractures becomes magmatic arc. On the Java and Sumatra islands the formation of volcanoes occurred due to the collisions of the Indian Ocean crust with the Eurasian Continent crust. Accretion comes to the surface and becomes an island due to stronger subduction. Beside the formation of volcanoes, tectonic ctivity and geological conditions (volcanology, magmatism, geological structures, etc.) are the main factors as geothermal and porphyry deposits.

Reference

[1] Hall, Robert. 2009. Indonesia Island. Universitas London

[2] Katili, A. John. 1974. Geological environment of the Indonesian mineral deposits:a plate tectonic approach. Geological Survey of Indonesia.

[3] Katili,  A. John. 1974. Volcanism and Plate Tectonics in The Indonesian Island Arc. Elsevier Scientific Publisher Company.

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