Presentation on theme: "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."— Presentation transcript:
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? 4000 million years -NW Canada 3800 Ma - Greenland 3800 Ma - Minnesota 3500 Ma – W Australia 3500 Ma – S Africa 170 Ma – Western Atlantic Ocean 170 Ma – Western Pacific Ocean Average age of continents = 1,800 Ma Average age of oceans = 65 Ma
Compare and contrast continental and oceanic lithosphere in terms of : age, structure, and composition. Similarities : Differences : Continental LithosphereOceanic Lithosphere Age Structure Composition Base of lithosphere= 1300ºC isotherm Lithosphere= zone above asthenosphere Lithosphere= crust and upper mantle Movement over the asthenosphere Physical properties = brittle, solid, relatively cold
ContinentalOceanic Crust - thicker 35 to 70km Lithosphere - 40km (young) - 400km (cratons) Crust - thinner 6 to 10km Lithosphere – 10km (MOR) to 120km (subduction zones) Granitic / andesiticBasaltic Less-dense – 2.7g/cm 3 (does not subduct)More dense – 3.0g/cm 3 (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 / cordilleraIsland arcs
What are the large-scale features of the continents? Cratons Subduction Zone Orogenic Belts Collision Zone Orogenic Belts Sedimentary basins cratons The theory of plate tectonics explains the origins of all these large-scale features of the continents. CRATONS OROGENIC BELTS SEDIMENTARY BASINS
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
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
Back-arc Basin Formation
Features of Subduction Zone Orogenic Belts
Features of Collision Zone Orogenic Belts 1. Buoyancy prevents subduction 2. Energy dissipated by crustal deformation 3. Deformation starts at suture & spreads 4. Crustal thickening & shortening 5. Isostatic uplift 6. Outer zone – fracture & faulting 7. Inner zone – folds & shear zones
Describe, with the aid of labelled diagrams, the differing geometry of flexural (parallel) and flow (similar) folds. 10 marks) 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 PARALLEL FOLDSSIMILAR FOLDS 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 (0- 30º) open folds with high interlimb angle ( º)
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 Tertiary Cretaceous Jurassic Triassic Devonian Metamorphic basement
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?
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) Geology of the Lithosphere
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.