Session n°1 (Monday June 10th) : Plate Tectonics

Origin of plate tectonics (thermal status of the Earth) / Plates : boundaries, characterization, nature, velocity vectors and velocity diagrams. Thermal profiles at plate boundaries and consequences. Hotspots and triple junctions. Discussion around maps showing present and past plate distribution and characteristics.

Plates

The surfical Earth is made of « rigid » plates moving with respect to each others.

What are their compositions ?

- Some plates are made of mafic rocks (basalt, gabbro : oceanic crust) only, rich in iron, rather dense (mean density : ~ 3.0) : this is the case of the Pacific plate, and also Nazca, Cocos and Philippine plates (Figure).

- A few are made of felsic rocks (granites in a wide sense : continental crust) only, rich in silicium (Si), aluminum (Al) and lighter (mean density : ~ 2.7) : this is the case of the Eurasian, Anatolian and Iranian plates.

- And some are made of both compositions, partly made of basalt, partly made of granite s.l. : this is the case of the Africa, America (North and South) and Antarctica plates.

What are their limits ?

- If the plates are rigid, and if they move with respect to each others … they must have boundaries along which they move past with respect to each other : the plate-boundaries. According to the type of mouvement between 2 contiguous plates (characterized by a velocity vector -vv- attached to one plate with respect to the other), three types of plate boundaries can be defined : convergent, divergent and transcurrent.

- Convergent (vv pointing toward the other plate) : one plate (the denser one, the « oceanic plate ») moves under the other (subducts) along a subduction zone, at a trench, and plunges into the underlying mantle, like the peri-Pacific subduction zones.

- Divergent (vv pointing out of the other plate) : both plates move away from each other … therefore material (basalt), coming from the mantle, is created at the ridge and spreads on both sides of it : for example the Atlantic and Pacific ridges, the East-African Rift.

- Transcurrent (vv is parallel to the boundary) : plates move past each other, parallel to a transform fault (TF). These TF belong to two kinds : - the small TF participate to the sphericity of the Earth, they troncate the 1D plate boundaries into ~ 50 km segments (ridge segmentation) ; the strike-slip faults along which plates move parallel to each others (Levant Fault, Gibraltar faut Zone, N90°E Fault in the Indian Ocean …).

- Usually, plates meet themselves along 1D boundaries (lines), but (because the Earth is a sphere, not a ribbon …), necessarily there are places where 3 plates are in contact : these places are called triple junctions. It is rather easy to show that junctions > 3 are unstable.

Date: 2015-01-29; view: 821