Review Lecture One - Plate tectonics and structural geology Flashcards
Show how the Earth convects.
What are the three main kinds of plate boundaries?
Divergent plate boundaries:
Plates move apart, creating new ocean floor.
Convergent plate boundaries:
One plate with oceanic crust at the front collides with another plate with continental curst at the front. The senser oceanic crust subducts under the less dense crust.
Transform plate boundaries:
Two plates move parallel past one another.
Where do the deepest earthquakes occur?
In subduction zones.
As this is where colder crust subducts into the warmer crust, thus cooling this warmer crust and making it more brittle.
So deeper earthquakes can occur, where the crust would have otherwise been ductile.
Describe ocean-ocean convergent plate boundaries.
Ocean-ocean convergent plate boundaries:
Oldest (coldest) plate subducts.
A tranch is formed from the subducting oceanic crust.
Volcanics = rising melt of sediments off subducting plate +water + mantle + plate.
Earthquakes = motion of subducting plate = Benioff Watatti zone.
Island arc = volcanics. E.g. Tonga Islands.
Juvinile continental curst is made here.
The volcaics here are more andersitic than basaltic due to the mixing with water.
Magmatics and volcanism usually occurs above the subducting slab intersects with the asthenosphere.
Describe continent-ocean convergen plate boundaries.
Continent - ocean convergent plate boundaries:
Oceanic crust always subducts under continental crust.
Trenches are formed.
Mountain building can occur = uplifted buoyant continental crust.
Volcanics, new felsic crust added to continent edge - continent grows. There are more plutons - magma gets stuck in the crust and doesn’t necissarily extrude. Causes igneous intrustions and crustal thickening.
Earthquakes occur.
E.g. the Andes on the West cost of South America.
Describe continent-continent convergent plate boundaries.
Nothing can subduct.
High mountins are formed.
Mixture of highly deformed continental crust, sediments and remaining oceanic crust.
E.g. The Himalayas in the junction between the Indian and Asian plates.
Draw and label a diagram of an ocean-continent collisional zone.
Explain continent building.
Uplift and accretion of low density rocks creartes continents from isalnd arcs.
As continental crust doesn’t subduct, bits accrete to form bigger continents.
The only way continents can get smaller is by erosion or rifting.
Explain rifting.
Separation of continents into smaller pieces.
Divergent boundary forms within a continent, thinner edges of continent become shelves.
Eventually full spreading redge breaks contenet apart, and sea floods in to form a new ocean.
E.g. East Africa - the Black Sea.
Show how the progressive development of a continental rift towards separation and formation of new oceanic crust occurs.
Show an idealised cross section of a spreading ridge.
Show fault geometry in extentional settings.
Show how when extension stops, and cooling of asthenosphere causes broad sagging.
Draw a transform fault.
A transform fault’s movement can be seen through thte displacement of geological features such as rivers.
In this diagram, the zones moving in the same direction are relatively inactive faults, and the areas moving in opposition are active faults.
What is a positive and negitive flower structre, and how are they formed?
Transpressional fault jog or ‘restraining bend’ produces local shortening and a ‘pop-up’ commonly in the form of a positive flower structure. (Lots of thrust faults).
Transtensional fault jog or ‘releasing bend’ produces loal extension commonly in the form of a negitive flower structure. (Losts of normal faults).
Exaplin the brittle ductile transition.
Outer layer = Cool = Many faults and slow folds.
Middle layer = Warm = Few faults, mostly folds.
Inner layer = Hot = Entirely folds.
How can you work out strike and dip, and what are they?
They help to work out what is happening underground by using information on the surface.
See lab manual for instruction on how to work this out.
What are the different kinds of faults?
Normal = extensional.
Reverse = compressional (thrust = low angle reverse).
Strike slip = transform = lateral motion.
Look at the in detail terminology in the prac manual!!
What are the different faults and strain axis orientations?
Note that:
The z axis is the maximum compressional axis.
The x axis is the maximum extensional axis.
The y axis is pretty irrelivant.
They are not cartesion axis’!!!
What is autochton and allochton?
Autochton stays in the same place when allochton is transported over the top of it.
What are foliations?
What is a decolloment fault?
A fault surface parallel to a mechanically weak horizon or layer, or parallel to bedding, that detaches or separates deformed rocks above from undeformed or differently deformed rocks below. Decollements, or decollement surfaces, are typical of regions of thrust faulting such as the Alps.
