Convergent Plate Boundaries, Clase 4

Convergent Margins (Subduction zone)

Clase 4

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Subduction zone

A subduction zone is a convergent lithospheric plate boundary where a tectonic plate underthrusts (se subducta) another plate. In the case of an oceanic subduction zone, an oceanic lithospheric plate underthrusts a continental margin (oceanic-continental convergence) or a volcanic island arc (intra-oceanic convergence), giving rise to tectonic deformations, earthquakes and in many cases important volcanism (Clift et al., 2009).

The two plates are separated by a lithospheric discontinuity, which includes the inter-plate fault or mega-thrust fault.

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Subduction zone

Subduction occurs because of thermal convection in the earth mantle and the differences in densities.

Continental rocks are on average lighter (d= 2.69 and 2.74 g/cm3) than oceanic rocks (d= 3.0 and 3.25 g/cm3), so that an oceanic plate is generally subducted beneath a continent.

In an oceanic plate, the difference in densities occurs because the lithosphere becomes thicker and denser as it ages.

There are three basic types of convergent margins

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Convergent Margins

Convergent plate boundaries come in several flavors, but they share one thing in common - plate collisions!

source: http://geomaps.wr.usgs.gov/parks/pltec/converge.html 4

Continental vs. Oceanic Plate Convergence

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink?

The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a subduction zone is born! Where the two plates intersect, a deep trench forms.

Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.

When the subducting oceanic plate sinks deeper than 100 kilometers, huge temperature and pressure increases make the plate ‘sweat'. Well, not exactly, but the uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses. The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate.

This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.


Continental vs. Oceanic Plate Convergence

Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons (calderos a fuego lento) below.

This type of convergent plate boundary, called a subduction zone, is known for producing historic earthquakes of great magnitudes.

Look for curved volcanic mountain ranges with deep trenches alongside and it's a safe bet that you're looking at a subduction zone.


Oceanic vs. oceanic plate convergence

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, you know that it’s the dense oceanic plate that sinks. What happens when two dense oceanic plates collide? Once again, density is the key!

Remember that oceanic plates are born at mid-ocean ridges where molten rock rises from the mantle, cools and solidifies. Little by little, as new molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created. The farther the plate gets from the ridge that created it, the colder and denser ('heavier') it gets.

When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink.


Oceanic vs. oceanic plate convergence

The rest of the story is a lot like the continental vs. oceanic plate collision we described above. Once again, a subduction zone forms and a curved volcanic mountain chain forms above the subducting plate. Of course, this time the volcanoes rise out of the ocean, so we call these volcanic mountain chains island arcs. The Aleutian Peninsula of Alaska is an excellent example of a very volcanically-active island arc.


Continental vs. continental plate convergence

By this time, you understand enough about plates to guess that when the massive bulk (bulto, masa) of two buoyant continental plates collide there is bound to be trouble!

The Himalayan mountain range provides a spectacular example of continent vs. continent collision. When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant.

It is here that the highest mountains in the world grow. At these boundaries solid rock is crumpled (arrugada) and faulted. Huge slivers (astillas) of rock, many kilometers wide are thrust on top of one another, forming a towering mountain range. The pressure here is so great that an enormous piece of Asia is being wedged sideways, slipping.


The Himalayas: Two continents collide

This immense mountain range began to form between 40 and 50 million years ago, when two large landmasses, India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging (incidente) plates could only be relieved by thrusting skyward, contorting the collision zone, and forming the jagged (dentado) Himalayan peaks

Source: http://pubs.usgs.gov/gip/dynamic/himalaya.html 10

About 225 million years ago, India was a large island still situated off the Australian coast, and a vast ocean (called Tethys Sea) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. By studying the history -- and ultimately the closing-- of the Tethys, scientists have reconstructed India's northward journey. About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.

Source: http://pubs.usgs.gov/gip/dynamic/himalaya.html 11

Oblique convergence

At subduction zones, the relative motion between two plates generally includes an oblique component of motion, as described by Fitch et al. (1972); DeMets et al. (1994).

Oblique Convergent Subduction; a) pure elastic strained fore-arc; b) the fore-arc accommodates part of the trench-parallel component along a strike-slip fault (Hoffmann-Rothe et al., 2006). 12

Oblique Convergence

According to Soofi and King (2002), oblique convergence produces strain partitioning with both compressional and shear components, giving place to a variety of structures, depending on the ratio of compressional to shearing motion.

Subduction zones with oblique plate motion show transcurrent faults (Fitch fault), which are faults involving motion parallel to the strike of the margin and delineating a fore-arc tectonic sliver (e.g., Atacama Fault in Chile (Cembrano et al., 2005); Alpine fault extremities in New Zealand (Schofield, 1960); The Queen Charlotte/Fairweather fault system in Alaska (Doser and Lomas, 2000.)

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This is a schematic cartoons showing the features common to the two basic types of active margin (Clift et al., 2004) : (a) accretionary and (b) erosive. Accretionary margins, such as Cascadia, are characterized by forearc regions composed of thrusted and penetratively deformed trench and oceanic sediments that often develop mud diapirism and volcanism because of sediment overpressuring. Gas hydrate zones are also commonly associated with structures in the wedge. In contrast, Erosive plate margins, such as Tonga, are marked by steep trench slopes, composed of volcanic, plutonic, and mantle rocks. Sedimentary rocks are typically limited to the forearc basin, where they may be faulted but are not strongly sheared in the fashion of an accretionary wedge. In the Marianas, serpentinite mud volcanism is recorded.

(Clift et al., 2004)

Accretionary Margin

Erosive Margin

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a) 3-dimensional seismic data volume from the Nankai subduction margin (from Moore et al., 2007), illustrating the structure of an accretionary prism. Note the inferences of a steepening megasplay fault and numerous accretionary prism thrusts accommodating deformation on localized planes within the prism. b) Cartoon showing the subdivision of an accretionary prism into an inner and outer wedge, separated by a transition zone (after Kimura et al., 2007). 15

Von Huene et al., (2004) 16

Accretionary wedge

A body of deformed sediments, wedge - shaped in

two dimensions or prism-shaped in three dimensions, that has been scraped off the surface of the oceanic lithosphere as it moves downwards beneath continent or island arc. The sediments are added to the continental edge.

Ver video: https://www.youtube.com/watch?v=kXjSQQp_js0

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Map showing the distribution of accreting versus eroding subduction zones considered within this study. Accretionary margins are shown with solid barbs on the plate boundary, while open barbs mark erosive margins (Clift et al., 2004). 18

Deber de la publicación de Clift et al. (2004)

1) Leer el abstract, la introducción y las conclusiones de la publicación. Hacer un resumen a mano. 2) En un mapa similar al anterior, ubique los márgenes convergentes de la figura 4 y 6. 3) Brevemente explique las figuras 7, 8, 9, 10, 11, 12 y 13.

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Deber

• Fosa de la Marianas: hacer un breve resumen del video:

https://www.youtube.com/watch?v=YdZA8YO7QnI

1) Cómo se logró definir la profundidad de la fosa de las Marianas?

2) Cuáles son los principales descubrimientos nombrados en el video?

3) En el mapa del deber anterior, ubicar la fosa de las Marianas.

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References

http://geomaps.wr.usgs.gov/parks/pltec/converge.html

http://www.britannica.com/science/igneous-rock/images-videos/Collision-of-a-continental-plate-with-an-oceanic-plate/2497

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Convergent Plate Boundaries, Clase 4