Download Document

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?
Why are the Earth’s oldest rocks found in continental areas?
Average age of continents = 1,800 Ma
4000 million years
-NW Canada
3800 Ma Greenland
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
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
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
What are the large-scale features of the continents?
SEDIMENTARY
BASINS
CRATONS
cratons
OROGENIC
BELTS
•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.
Cratons
• stable for millions of years
• no tectonic activity
• old (0.5 – 3.5 billion years)
• very little relief (highly eroded)
CANADIAN
SHIELD
• thick (300km?)
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
Accretionary Prism
Back-arc Basin Formation
Back-arc Basin Formation
Back-arc Basin Formation
Features of Subduction Zone Orogenic Belts
Features of Collision Zone Orogenic Belts
1. Buoyancy prevents subduction
6. Outer zone – fracture & faulting
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
Outer Zone
Outer Zone
Outer Zone
Inner Zone
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
• radius of arcs decreases through
the fold
• cannot continue indefinitely due to
limited space
• open folds with high interlimb angle (70120º)
• beds thicker at hinge and thinner on
limbs
• radius of outer arc the same as radius
of inner arc
• can continue indefinitely due to changes
in bed thickness
• tight folds with low interlimb angle (030º)
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
How did the North Sea Sedimentary Basin form?
Pmax
Pmin
Pmin
Tertiary
Cretaceous
Jurassic
Triassic
Pmax
Devonian
Metamorphic
basement
Sedimentary Basins
a). Describe the major structural features of the continental
lithosphere.
b). Explain the origin of these with reference to the theory
of plate tectonics.
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.
How did continental crust form?
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)
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)
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.