Presentation on theme: "4. Formation and Deformation of the Continental Crust"— Presentation transcript:
14. Formation and Deformation of the Continental Crust Geology of the Lithosphere4. Formation and Deformation of the Continental CrustWhy 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?
2Why are the Earth’s oldest rocks found in continental areas? Average age of continents = 1,800 Ma3800 Ma -Greenland4000 million years -NW CanadaAverage age of oceans = 65 Ma170 Ma – Western Pacific Ocean3800 Ma -Minnesota170 Ma – Western Atlantic Ocean3500 Ma – S Africa3500 Ma – W Australia
3Compare and contrast continental and oceanic lithosphere in terms of : age,structure, andcomposition.Differences :Continental LithosphereOceanic LithosphereAgeStructureCompositionSimilarities :Base of lithosphere = 1300ºC isothermLithosphere = zone above asthenosphereLithosphere = crust and upper mantleMovement over the asthenospherePhysical properties = brittle, solid, relatively cold
4ContinentalOceanicCrust - thicker 35 to 70kmLithosphere - 40km (young) - 400km (cratons)Crust - thinner 6 to 10kmLithosphere – 10km (MOR) to 120km (subduction zones)Granitic / andesiticBasalticLess-dense – 2.7g/cm3 (does not subduct)More dense – 3.0g/cm3(subducts)Sedimentary, igneous and metamorphic rocksLayered / 1, 2 and 3 (sediments, pillows, dykes and gabbros)Older < 4.2Ga (billion years)Tends to be older in "middle" with vertical stratigraphyAverage age – 1.8 billion yearsRidge / abyssal planes / trenchesYounger < 200MaOlder away from ridge / horizontal stratigraphyAverage age – 65 Ma(May be) folded and faulted ("all" types)Transform faulting commonSpreadingMagnetic stripingOrogenic belts / cordilleraIsland arcs
5SEDIMENTARY BASINS CRATONS OROGENIC BELTS What are the large-scale features of the continents?SEDIMENTARY BASINSCRATONSOROGENIC BELTScratonsCratonsSubduction Zone Orogenic BeltsCollision Zone Orogenic BeltsSedimentary basinsThe theory of plate tectonics explains the origins of all these large-scale features of the continents.
6CANADIAN SHIELD Cratons stable for millions of years no tectonic activityold (0.5 – 3.5 billion years)very little relief (highly eroded)thick (300km?)CANADIANSHIELD
7Forebulge & oceanic trench Features of Subduction Zone Orogenic BeltsForebulge & oceanic trenchAccretionary prismFore-arc ridge & fore-arc basinVolcanic arcBack-arc basinPaired metamorphic belts
28Describe, with the aid of labelled diagrams, the differing geometry of flexural (parallel) and flow (similar) folds.10 marks)PARALLEL FOLDSSIMILAR FOLDSsame thickness on hinge and limbs (i.e. parallel around the fold)radius of outer arc of fold greater than inner arcbeds thicker at hinge and thinner on limbsradius of outer arc the same as radius of inner arcradius of arcs decreases through the foldcan continue indefinitely due to changes in bed thicknesscannot continue indefinitely due to limited spacetight folds with low interlimb angle (0-30º)open folds with high interlimb angle (70-120º)
29Discuss 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 & strain2. Example structures of brittle deformation – jointing & faulting3. Example structures of ductile deformation – folding & shear zones4. 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
30How did the North Sea Sedimentary Basin form? PmaxPminPminTertiaryCretaceousTriassicJurassicPmaxDevonianMetamorphic basement
39i). axial plane cleavage ii). parasitic folds iii). nappe structures Geology of the Lithospherea). Describe, with the aid of labelled diagrams, the formation of two of the following:i). axial plane cleavageii). parasitic foldsiii). nappe structuresb). Rocks of the same type can suffer brittle deformation or ductile deformation. Explain the conditions in which these different structures are formed (25)
40Geology of the Lithosphere With reference to plate boundaries, explain the formation of the following types of fault:i). normalii). thrustiii). transform (25)
41Making Notes1. 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.