One Slab or Two? Arc-to-Continent Collision

Sorry, chocoholics this is not about chocolate or cake 😦 (although, as this is Volcanocafe, it could be!).  No, we are looking at arc-to-continent collision round the Banda Sea 🙂

I continued my plotting of Indonesian earthquakes eastward, this time choosing the Banda Sea where the “swirly bit” on the map looks interesting; and, is it interesting!

Fig 1: Latitude v Longitude plot of earthquakes by the author, superimposed over a map of the area from Google Earth. Copyright for the earthquake plot rests with the author; reproduced here with kind permission.

Here we can see the Banda Arc in eastern Indonesia, a double island arc formed from the collision of the Indo-Australian Plate and the Eurasian Plate.  The outer arc is tectonic (the Banda Forearc above); and, the inner arc, volcanic ^[1].

We’ll look first at each arcs; then take a look at one volcano, Banda Api.

The Outer Banda Arc and the Banda Sea

The outer arc overlies the margin of Australian Pre-Cambrian craton ^[1].  The arc is made up of the following belts, working from the inside of the arc to the outside: an ophiolite belt; a metamorphic belt; a thrust and fold belt with Permo-Triassic and Jurassic sediments from the Australian continental margin; a thrust and fold belt with Late Mesozoic and Tertiary deep water sediments; and, a belt of uplifted Late Neogene basins.  The older rocks occur on the inside of the outer arc and the younger rocks occur on the outer side.  According to the geologists, this is contrary to plate tectonics theory, which would predict an accretionary wedge, composed of younger oceanic sediments, on the inner side of the arc ^[2].

Fig 2: Banda Arc Cross Section by Herman Darman, published under Creative Commons Attribution-Share Alike 3.0 Unported license. Colours denote: red, volcanic arc; green, metamorphosed basement with ophiolite fragments; yellow, thrust and fold belt with Permo-Triassic and Jurassic rock; and blue, thrust and fold belt with Tertiary rock. Source: http://commons.wikimedia.org/wiki/File:Banda-arc-cross-section.jpg

De Smet (1999) attributed the continental basement rocks and Early Mesozoic continental sediments found on the inner side of the arc to inversion during the plate collision between Australia and the Banda Subduction Zone.  The collision compressed the northern rim of the Australian continent, pushing up continental margin sediments on the back of continental basement to form the islands of the Outer Banda Arc ^[2].

The Banda Sea’s origins are also complex.  It may have been formed by either back arc spreading or trapped oceanic lithosphere from the Indian Ocean.  Bird (2003) suggests the presence of a microplate, the Banda Sea Plate ^[3].

So what do the earthquakes tell us?

Earthquake Activity

The area is tectonically very active and home to some large quakes, for example the 8.4 Mw in 1938, The Banda Sea Quake ^[4], and a 7.5 Mw in 2006 ^[5].

Plotting earthquakes from 1994 to June 2014 (earthquake data downloaded from USGS: http://earthquake.usgs.gov/earthquakes/), we see very clear evidence of subduction, most strongly between the south eastern forearc basin descending beneath volcanic arc:

Fig 3: Longitude v Depth (looking north) by the author. Copyright rests with the author; reproduced here with kind permission. The lines for longitude are the same as those in Fig 1.

Fig 4: Latitude v Depth (looking west) by the author. Copyright rests with the author; reproduced here with kind permission. The lines for latitude are the same as those in Fig 1.

A rotating plot of the earthquakes shown above can be viewed here:

We can see the subduction of the Australian Plate in the south and what appears to be a second subduction zone in the north.  The latter may be either a continuation of the subduction of the Australian Plate (the subduction zone having bent round); and / or, it may be subduction of the Bird’s Head Plate under the Banda Sea Plate.  However, the margin between the Banda Sea Plate and the Bird’s Head Plate is described in Wiki as a convergent margin ^[6].

The Australian Plate is made up of continental crust.  Continental crust is more buoyant than oceanic crust so it is perhaps surprising to see seismicity reaching a depth of 675 km.  If the continental crust has subducted, we may find evidence in the lavas of the volcanic arc. Let’s now look at the volcanic arc.

The Volcanic Arc

The volcanic arc is the inner arc, made up of active volcanic islands. All volcanoes in the arc are stratovolcanoes, and the Banda Archipelago and Nila also have calderas ^[7].

Jezek, P.A et al (1978) noted that lavas are consistent with those of other island arcs, ranging from tholeiitic basalt and dacite on S. W. Ambon and Banda, through low-K calc-alkaline andesites on Manuk and Serua, to high-K calc-alkaline andesites on Nila, Teun, Damar, Gunungapi Wetar, and Romang (which also contains dacite).  Older cordierite dacites (ambonites) on North Ambon must be derived from underlying continental crust, but the younger tholeiitic lavas of S.W. Ambon and Banda may be related to a shallow subduction zone dipping southwards from Seram. ^[8]

Vroon, P.Z. et al (2001) looked at the oxygen isotope (δ¹⁸O) of the lavas in the Banda Arc. They found that the lavas from the Banda Archipelago, Manuk, Nila, Damar, and Romang include subducted continental material. The percentage of bulk subducted continental material tends to increase along the arc from less than 1% in the Banda Archipelago to more than 3% in Romang. This appears to reflect an increase in subducted continental material towards the arc sector near Timor, where the Australian continent collided first ^[7].

And now let us look at a volcano.  I picked Banda Api because she is close to the subduction zone at the north east end of the Banda Sea Plate and she appeared on the map to be associated with a caldera.

Banda Api

Banda Api is a volcanic island in the Banda Archipelago.  The archipelago is made up of ten volcanic islands.  The main islands are:  Banda Neira, or Naira; Banda Api, an active volcano; and, Banda Besar.  Banda Neira was once the global center of the trade in nutmeg and mace ^[9].

Fig 5; Map of the Banda Islands by Lencer, published under the Creative Commons Attribution-Share Alike 3.0 Unported license. Source: http://upload.wikimedia.org/wikipedia/commons/5/5c/Banda_Islands_en.png

According to GVP, Banda Api, herself, is a basaltic-to-rhyodacitic volcanic island, located in the SW corner of a 7 km wide caldera.  Her setting is described as subduction zone, oceanic crust (<15 km).  Her major rock types are listed as dacite; andesite / basaltic andesite; and, basalt / picro-basalt ^[10].

There have been two or more episodes of caldera formation.  Bandar Besar and Banda Neiro are possibly remnants of pre-caldera volcanoes.  Gunung Api is the 640 m high conical peak in the center of Banda Api island.  27 eruptions have been recorded since 1586, mostly strombolian activity from the summit crater and occasional lava flows. Of the 27 eruptions, 13 were VEI 2; and, 8 were VEI 3 ^[10].  The last large eruption occurred in 1988, leading to the evacuation of 10,000 people ^[10].

Fig 6: The 1988 eruption of Banda Api. Photo by Willem Rohi, 1988 (Volcanological Survey of Indonesia). Source: http://www.volcano.si.edu/Photos/full/008036.jpg

Hope you enjoyed this whistle-stop tour of the Banda Arc.  The usual caveats apply: “not an expert, etc… “

KarenZ, 28 June 2014

References:

1. “Banda Arc”, Wikipedia:  http://en.wikipedia.org/wiki/Banda_Arc
2. “The Geology of Indonesia/Banda Arc”,  http://en.wikibooks.org/wiki/The_Geology_of_Indonesia/Banda_Arc
3. Bird, P. (2003) An updated digital model of plate boundaries, Geochemistry Geophysics Geosystems, 4(3), 1027, doi:10.1029/2001GC000252 http://peterbird.name/publications/2003_PB2002/2003_PB2002.htm
4. 1938 Banda Sea Earthquake, Wiki: http://en.wikipedia.org/wiki/1938_Banda_Sea_earthquake
5. 2006 Banda Sea Earthquake, Wiki: http://en.wikipedia.org/wiki/2006_Banda_Sea_earthquake
6. Bird’s Head Plate, Wiki: http://en.wikipedia.org/wiki/Bird%27s_Head_Plate
7. Vroon, P.Z., Lowry, D., van Bergen, M.J., Boyce, A.J., Mattey, D.P. “Oxygen Isotope Systematics of the Banda Arc: Low δ¹⁸O Despite Involvement of Continental Material In Magma Genesis”,  Geochemica et Cosmochimica, Vol 65, No 4, pp. 589 -601, 2001.
8. Jezek, P.A., Hutchison, C.S.  “Banda arc of Eastern Indonesia: Petrology and geochemistry of the volcanic rocks”, Bulletin Volcanologique, 1978, Volume 41, Issue 4, pp 586-608 http://link.springer.com/article/10.1007/BF02597389
9. Banda Islands, Wikipedia:  http://en.wikipedia.org/wiki/Banda_Islands
10. Banda Api, GVP: http://www.volcano.si.edu/volcano.cfm?vn=265090

 

A new season of Friday riddles starts today, this week under the aspect of the soccer world cup!  

1) In 1791, hope came to the land of men, but then France took it away. Answer: The Marquesas hotspot.  The Marquesas are known as the “Land of Men” in the native language.  An American fur trader arrived on the ship “Hope” in 1791 and claimed it for the USA, but France later took over the islands. Sissel, 2 points.

2) Another one of Darwin’s stops.  Can you hear the drums? Clue: Base of operations for rescue and recovery for a famous plane crash. Answer: Fernando de Noronha. Darwin briefly stopped here.  “Can you hear the drums, Fernando?” is a line from “Fernando,” one of ABBA’s hits. RenatoRio, 1 point. 

3) The island is named after it’s inspiring castaway, who helped save his rescuers recover. Answer: Alejandro Selkirk Island. Alexander Selkirk inspired the book Robinson Crusoe. His rescuers were in bad shape from scurvy, and he helped them get enough vitamin C to recover. Sissel, 2 points. 

4) The treasure of the Maya bursts forth; it can be found on many plates. Partial answer: It has to do with Chocolate…. 🙂 , KarenZ, 1 point. Bonus points may be awarded for Cacao volcano, Cerro Chocolate and Chocolate fountain 🙂 . Answer: Molten Chocolate lava cake, although KarenZ stated in the beginning that this post was not about chocolate or cake 😉 .  

MATT