1. 4. Formation and Deformation of the Continental Crust
Geology of the Lithosphere
4. Formation and Deformation of the Continental Crust
Why are the Earth’s oldest crustal rocks found in continental areas?
What are the large-scale features of the continents?
How did the large-scale features of the continents form and how are they related to tectonic setting?
How did the continental areas form?
What forces are acting on the continental crust?
How do these stresses cause brittle & ductile deformation on all scales in crustal rocks?

2. Why are the Earth’s oldest rocks found in continental areas?
Average age of continents = 1,800 Ma
3800 Ma -Greenland
4000 million years -NW Canada
Average age of oceans = 65 Ma
170 Ma – Western Pacific Ocean
3800 Ma -Minnesota
170 Ma – Western Atlantic Ocean
3500 Ma – S Africa
3500 Ma – W Australia

3. Compare and contrast continental and oceanic lithosphere in terms of :
age,
structure, and
composition.
Differences :
Continental Lithosphere
Oceanic Lithosphere
Age
Structure
Composition
Similarities :
Base of lithosphere = 1300ºC isotherm
Lithosphere = zone above asthenosphere
Lithosphere = crust and upper mantle
Movement over the asthenosphere
Physical properties = brittle, solid, relatively cold

4. Continental
Oceanic
Crust - thicker 35 to 70km
Lithosphere - 40km (young) - 400km (cratons)
Crust - thinner 6 to 10km
Lithosphere – 10km (MOR) to 120km (subduction zones)
Granitic / andesitic
Basaltic
Less-dense – 2.7g/cm3 (does not subduct)
More dense – 3.0g/cm3(subducts)
Sedimentary, igneous and metamorphic rocks
Layered / 1, 2 and 3 (sediments, pillows, dykes and gabbros)
Older < 4.2Ga (billion years)
Tends to be older in "middle" with vertical stratigraphy
Average age – 1.8 billion years
Ridge / abyssal planes / trenches
Younger < 200Ma
Older away from ridge / horizontal stratigraphy
Average age – 65 Ma
(May be) folded and faulted ("all" types)
Transform faulting common
Spreading
Magnetic striping
Orogenic belts / cordillera
Island arcs

5. SEDIMENTARY BASINS CRATONS OROGENIC BELTS
What are the large-scale features of the continents?
SEDIMENTARY BASINS
CRATONS
OROGENIC BELTS
cratons
Cratons
Subduction Zone Orogenic Belts
Collision Zone Orogenic Belts
Sedimentary basins
The theory of plate tectonics explains the origins of all these large-scale features of the continents.

6. CANADIAN SHIELD Cratons stable for millions of years
no tectonic activity
old (0.5 – 3.5 billion years)
very little relief (highly eroded)
thick (300km?)
CANADIAN
SHIELD

7. Forebulge & oceanic trench
Features of Subduction Zone Orogenic Belts
Forebulge & oceanic trench
Accretionary prism
Fore-arc ridge & fore-arc basin
Volcanic arc
Back-arc basin
Paired metamorphic belts

8. Accretionary Prism

9. Back-arc Basin Formation

10. Back-arc Basin Formation

11. Back-arc Basin Formation

12. Features of Subduction Zone Orogenic Belts

13. Features of Collision Zone Orogenic Belts
6. Outer zone – fracture & faulting
1. Buoyancy prevents subduction
2. Energy dissipated by crustal deformation
7. Inner zone – folds & shear zones
3. Deformation starts at suture & spreads
4. Crustal thickening & shortening
5. Isostatic uplift

14. Outer Zone

15. Outer Zone

16. Outer Zone

23. Inner Zone

28. Describe, with the aid of labelled diagrams, the differing geometry of flexural (parallel) and flow (similar) folds.
10 marks)
PARALLEL FOLDS
SIMILAR FOLDS
same thickness on hinge and limbs (i.e. parallel around the fold)
radius of outer arc of fold greater than inner arc
beds thicker at hinge and thinner on limbs
radius of outer arc the same as radius of inner arc
radius of arcs decreases through the fold
can continue indefinitely due to changes in bed thickness
cannot continue indefinitely due to limited space
tight folds with low interlimb angle (0-30º)
open folds with high interlimb angle (70-120º)

29. Discuss the different conditions under which rocks of the same type can undergo either brittle or ductile deformation (15 marks)
1. Brittle & ductile deformation defined – stress & strain
2. Example structures of brittle deformation – jointing & faulting
3. Example structures of ductile deformation – folding & shear zones
4. Deformation variations in SAME rock type due to:
- temperature (geothermal gradient & depth of burial)
- pressure (depth of burial)
- rate (strain rate)
- pore fluids (chemical composition of the fluid)
the more chemically reactive the fluid the more ductile deformation

30. How did the North Sea Sedimentary Basin form?
Pmax
Pmin
Pmin
Tertiary
Cretaceous
Triassic
Jurassic
Pmax
Devonian
Metamorphic basement

31. Sedimentary Basins

32. a). Describe the major structural features of the continental lithosphere.
b). Explain the origin of these with reference to the theory of plate tectonics.

33. a). Describe how forces acting on continental lithosphere may cause brittle or ductile deformation.
b). Evaluate the importance of the depth in the lithosphere on the types of deformation produced.

35. How did continental crust form?

39. i). axial plane cleavage ii). parasitic folds iii). nappe structures
Geology of the Lithosphere
a). Describe, with the aid of labelled diagrams, the formation of two of the following:
i). axial plane cleavage
ii). parasitic folds
iii). nappe structures
b). Rocks of the same type can suffer brittle deformation or ductile deformation. Explain the conditions in which these different structures are formed (25)

40. Geology of the Lithosphere
With reference to plate boundaries, explain the formation of the following types of fault:
i). normal
ii). thrust
iii). transform (25)

41. Making Notes
1. Describe how the rate of seafloor spreading has been calculated at constructive plate boundaries and at hotspots.
2. Describe and explain how oceanic lithosphere may be absorbed back into the mantle.
3. Describe and explain the age distribution of rocks in continental and oceanic regions.