PLATE TECTONICS

The plate tectonism is a revolutionary idea, which grew up with me. (Started in the 1960ís) (The history part is coming from internet exercise, book.)

Plate Boundaries

1.Mid Oceanic Ridge, or divergent plate boundary

Lots of people in the late last century and early this century realized that in the middle of the oceans there is a deep ridge system, which they called mid-oceanic ridge. It is hard to see how is this ridge system looks like, however in Iceland, which is a little island in the north Atlantic ocean the mid oceanic ridge outcropping. The Mid-oceainc ridge is a so called structurural graben. This is a valley bounded by normal faults in which the central fault block is slipped down deeply along very steep planes.  Basaltic  lava moves periodically upward to fill the space forming as a result of the extension (PILLOW BASALT). The continuous magma development will cause the widening of the ridge system on both side of the basin (parallel magnetic anomalies), and advantually the oceanic drift starts (Red Sea, Gulf of Aden). During oceanic drift the graben or basin is wide enough and usually it has ocean water in it. The edges of the basin will be farther and farther away from the igneous activity and slowly become so called Passive margins (eastern US, Western Europe).

Causes of extension

How do grabens form?

1.In the mantle there are big convection cells, along which there is a heatflow.

2.Along the upwelling side (mantle plumes) the temperature is relative high which starts a partial melting in the mantle.

3.Because of the volume of the liquid phase is larger it needs larger space, which will initiate a triple junction with three radiating cracks the normal faults.

2.Convergent plate boundaries

One plate is subducted under an other and adventually resorbed in the astenosphere. The most exiting character of subduction zones are the deep sea trenches. Earthquakes are characteristic !!!!!! Deep focus, benioff zone!!! There are three different type of convergent plate boundaries:

1. Oceanic -Oceanic boundary

When two oceanic plates converge one of them is subducted under the other one, and we call it oceanic-oceanic boundar

As the subducting plate descends into the mantle it gets hot and starts to melt partially generating an andesitic magma. This magma is less dense and rises to the surface to form a chain of volcanic islands called volcanic island arc.

This arc is nearly parallel with the trenches.

Japan island chain, Philippine island region are all result of this.

2.Oceanic-continental plate boundary

The oceanic plate will subduct underneath the continental (more dense)!!!!!! (Continental plate will never subduct!!!!!!!!)

The magma chain forming here is also called volcanic arc, representing the nonsubducting plate, and also composed of andesitic magma. The presence of granitic plutons are also characteristic.

The Peru-Chile trench, Andes are the resulting volcanic chain

3.Continental-continental or collisional plate boundaries

The continents getting closer to each other as the oceanic crust becomes subducted. When two continents meet none of them can sink into the mantle, because continental is much more less dense, than oceanic. Mountain will form!!

Mountains are the suture zones of ancient plate boundaries!!

Ancient convergent zones

Intensely deformed zones, even after the erosion of ancient mountains

Melange ---- mixed rocks

Slices of oceanic crust (ophiolite)

Appalachians, Alps, Hymalayas

3.Transform plate boundaries

Plates laterally past each other.

Lithosphere is neither forming, nor consumed, because of the intense friction, lots of shallow earthquake.

Best known is San Andreas!!!!!

Wilson Cycle

Scientists figured, that there is a cycle in the plate tectonics called Wilson cycle. Wilson cycle always strats with a supercontinent (Pangea is the last we had), which then brakes up!!!!!

Evidence for this is that there is no older seafloor than 200 million years.

After breaking up, the continents later will get together again, making a new supercontinent with newly formed mountains on it showing the suture of the earlier smaller continents..

Plate tectonics and Mountain building

Orogeny

Episode of intense rock deformation or mountain building. Along subduction and collision zones.

Subsidence of plates and the accumulation of sediments

Areas of exeptionally thick sediments vary greatly in size, shape and tectonic setting, but they are collectivelly referred as sedimentary basins.

The accumulation of sediment in a sedimentary basin requires space, which can be produce either by subsidence or by sealevel rise or both.

Causes of subsidence:

1. Subduction subsidence

Forcible depression of one lithosphere plate as it is subducted beneath another.  This result in the long narrow, deep topographic trench on the sea floor.

2. Thermal or cooling subsidence

This is caused by the cooling of lithosphere plate as it moves away from the hot spreading ridge. The cooling material becames more dense and subside into the mantle. This create space for the accumulating sediment.

3. Sediment loading subsidence

Thick accumulation of sediment can also weigh down a lithosphere plate

 

4. Thrust loading

Lateral shoving of huge slab of sediments or rock over an other one.

Opposite movement after melting ice slab.

 Basins:

Basin type

Char. sed.

Dep. environment

Trench

fine, deepwater sediment, overlying basalt

deep marine

Forearc

heterogeneous gravel, sand mud derived from volcani, granitic metamorphic rocks

nonmarine to marine

Foreland

heterogeneous gravel, sand derived from orogenic belt

Mostly river and deltaic

Intracratonic

Homogeneous q-rich sands and limestone, mud, evaporites, coal

shallow marine with deltaic

Passive margin

q-sand, limestone, seaward shale

shallow marine shelf to deeper marine

Rift or aulacogen

Thick gravel and sand, prob. with evaporites and limestone

rivers and lakes, changing to shallow marine