Lecture Outlines

Physical Geology, 15/e

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Plummer, Carlson & Hammersley

Mountain Belts and theContinental Crust

Physical Geology 15/e, Chapter 20

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Mountain belts – chains of mountain ranges that are 1000s ofkilometers long

• commonly located at or near the edges of continental landmasses• part of the geosphere• as they grow higher and steeper, erosion rates increase• air rising over mountain ranges results in precipitation and erosion

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Mountains and Mountain Building

Mountain beltsMajor controlling factors:• Intense Deformation – mainly

compressional folding and faulting• Isostasy- vertical movement of

mountain belts during and after anorogeny

• Weathering and Erosion – ratesaffected by many factors such asclimate, rock type and heights.

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Mountains and Mountain Building

Size and Alignment• Very long compared to width• In North America they tend to be parallel to

coastlines

Ages of Mountain Belts andContinents• Higher mountain ranges tend to be

geologically younger• Appalachians are much older than the North

American Cordillera• Ancient mountain belts have eroded nearly flat

to form the stable cores (cratons) of thecontinents

• Precambrian Shield – Precambrianmetamorphic and igneous rock exposed byerosion over large areas

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Characteristics of Major Mountain Belts

Thickness and Characteristics of Rock LayersMountain belts•Typically contain thick sequences of folded and faulted sedimentaryrocks, often of marine origin•Originally deposited on continental margins

Patterns of Folding and Faulting•Open folds common where deformation is not vey intense•Tighter folds and recumbent folds indicate greater deformation•Indicate crustal shortening produced by compression common atconvergent boundaries

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Characteristics of Major Mountain Belts

Metamorphism and Plutonism

• Regional metamorphic rock and plutonic rock is found in the mostintensely deformed portions of major mountain belts

• Migmatites – intermixed granite and metamorphic rock caused by partialmelting of metamorphic rock

Normal Faulting

• Older portions of some major mountain belts have undergone normalfaulting as a result of later uplift and horizontal extension

Thickness and Density of Rocks

• Gravity measurements indicate that continental crust beneath mountains isless dense and thicker than oceanic crust

Features of Active Mountain Ranges

• Frequent earthquakes and deep ocean trenches parallel to ranges

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Characteristics of Major Mountain Belts

Orogenies and Plate Convergence•Continental crust becomes thicker with tight foldsand reverse faults due to intense compression•Orogenies and Ocean-Continent Convergence

• Andean Type (ex. Andes Mountains)• Accretionary Wedge• Gravitational collapse and spreading

•Arc-Continent Convergence• The arc and continent are too buoyant to be

subducted (ex. New Guinea, Sierra Nevada)• Flipping subduction zone

•Orogenies and Continent-Continent Convergence• The two continents are too buoyant to be

subducted (ex. Urals, Alps, Himalayas)

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Evolution of Mountain Belts

Post Orogenic Uplift and Block Faulting

Block-faulting – a long period of erosion, uplift

occurs after convergence stops

• Isostasy

• As erosion removes overlying rock, the crustalroot of a mountain range rises by isostaticadjustment

• Normal Faulting

• Tension in uplifting and spreading crust resultsin normal faulting and fault-block mountainranges

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Evolution of Mountain Belts

Post Orogenic Uplift and BlockFaulting

• Delamination

• Thin crust and block faulting in theBasin and Range

• Basin-and-Range province – maybe the result of delamination

• Overthickened mantle lithospherebeneath old mountain belt maydetach and sink into asthenosphere

• Resulting inflow of hotasthenosphere can stretch and thinoverlying crust, producing normalfaults

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Evolution of Mountain Belts

Continents grow larger as mountain beltsevolve along their margins

• Accumulation of sediment and igneous activityadd new continental crust

Displaced Terranes

Geologic continuous areas within mountain beltswhere the age and characteristics of the bedrockappear unrelated to that of the adjacent regions

• Terrane boundaries are usually faults

Concluding Comment - Plate Tectonics is a greatadvance but many new problems will need to besolved as science builds on the past.

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The Growth of Continents

End of Chapter 20