Triple Junctions
My Blogs (olelog) are mainly based on my daily
reading of earth science news.
Here on whatonearth.olehnielsen.dk I try to weave
some of the pieces together to a greater whole with added background info.
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Triple Junctions |
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My Blogs (olelog) are mainly based on my daily
reading of earth science news.
Here on whatonearth.olehnielsen.dk I try to weave
some of the pieces together to a greater whole with added background info.
|
||
A triple junction is the point where the boundaries between three tectonic plates, and three plate margins, meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault. Of the many possible types of triple junction only a few are stable through time.
Rifting
often start at a triple junction. See my log of Sunday, 9. October 2005,
09:17:01 Triple
junction and rifting.
The rifting continues in the Red Sea and the African Great Rift Valley
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Image from USGS Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar Triangle (shaded, center) -- a triple junction where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone |
Ridge subduction is inevitable on a world covered with plates: there are six present-day examples and probably hundreds of past instances related to former plate configurations. Ridge subduction means that an oceanic spreading center is subducted beneath a tectonic plate.
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The clearest example of active spreadng ridge subduction can be observed in detail where the Chile Ridge (red line) subducts beneath South America along the Chile Trench (purple line). This ridge subduction has been going on from the Miocene to the present. The Miocene Epoch is a period of time that extends from about 23.03 to 5.332 million years before the present. At ridge subduction spreading stops beneath the overriding plate and the ridge widens into an inter-slab gap called a slab window. This Ridge Subduction was treated in my Blog of Friday, 30 March 2007 at http://my.opera.com/nielsol/blog/2007/03/30/ridge-subduction Prior to the ridge collision the Nazca plate was being subducted at a rapid rate (roughly 80 mm/yr for the past 3 my, and as fast as 130 mm/yr during the late Miocene) in a direction slightly north of east. Following the passage of the triple junction, the Antarctic plate is subducted at a much slower rate, roughly 20 mm/yr for the past 15 my, in a direction slightly south of east.
The 22 May 1960 Chile earthquake generated a 11 metres tsunami causing 61 deaths and $23 million in damage. See more about this triple junction at this USGS site http://walrus.wr.usgs.gov/research/sopac.html |
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On the USGS earthquake map to the left I have marked the three plates involved: the Pacific Plate, the Australian Plate and the small Solomon Sea Plate. The black arrows show the movement of the Solomon Sea Plate and the Pacific Plate relative to the Australian Plate. The Red line is the ridge and the purple line the subduction zone. This Ridge Subduction was treated in my Blog post of Monday, 2 Apil 2007 at http://my.opera.com/nielsol/blog/2007/04/02/ solomon-earthquake and Monday, 13 April 2009 at http://my.opera.com/nielsol/blog/2009/04/13/solomon-quake-2007 |
Since 2007 the Solomon earthquake has been thoroughly studied, and a paper on some of the findings has been published in the journal Science.
The New Georgia Island Group of the Solomon Islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc. The spreading ridge pushing the small Solomon Sea Plate and the Australian Plate apart is being subducted beneath the Pacific Plate. The situation is rather complicated because the two plates descend beneath the overriding plate at different rates and directions.
According to the study the event began in the Australian Plate and moved across into the Solomon Sea Plate and had two centers of energy separated by lower energy areas, which is noticeable as we normally think earthquakes should stop at the plate boundaries. When the earthquake moved from one plate to the other, it quickly changed direction, mimicking the different plate motion directions of the plates involved. The authors are confident that the fault slip in the two main locations are different by 30 to 40 degrees. That behaviour during an earthquake has probably never been observed before, but it most certainly has happened here before, according to the authors.
Before about half a million years ago, the easternmost segment of the Woodlark Basin spreading ridge was subducting beneath the westernmargin of the Solomon Islands, and the ridgetrench triple junction migrated northwesterly at around 110 to 120 mm/year. The differences in plate subduction rates and directions produced a slab window, which today lies beneath the southern New Georgia Islands. (When a mid-ocean spreading ridge subducts, it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within an inter-slab gap called a slab window).
A cartoon in the paper shows how the Solomon Sea Plate and the Australian Plate subduct as two different layers (slabs) beneath the overriding Pacific plate, with a slab window at depth (shown as increasing empty space between the two coloured slabs). The surface subduction boundary is marked by a red line.
Map based on USGS earthquake map.
The Cocos plate is subducted beneath the Montagua transform fault between the
North American plate and the Caribbean plate.
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