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Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley © McGraw-Hill Education. Permission required for reproduction or display. Mountain Belts and the Continental Crust Physical Geology 15/e, Chapter 20 © McGraw-Hill Education. Permission required for reproduction or display. Mountains and Mountain Building Mountain belts – chains of mountain ranges that are 1000s of kilometers 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 © McGraw-Hill Education. Permission required for reproduction or display. Mountains and Mountain Building Mountain belts Major controlling factors: • Intense Deformation – mainly compressional folding and faulting • Isostasy- vertical movement of mountain belts during and after an orogeny • Weathering and Erosion – rates affected by many factors such as climate, rock type and heights. © McGraw-Hill Education. Permission required for reproduction or display. Characteristics of Major Mountain Belts Size and Alignment • Very long compared to width • In North America they tend to be parallel to coastlines Ages of Mountain Belts and Continents • 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 the continents • Precambrian Shield – Precambrian metamorphic and igneous rock exposed by erosion over large areas © McGraw-Hill Education. Permission required for reproduction or display. Characteristics of Major Mountain Belts Thickness and Characteristics of Rock Layers Mountain belts •Typically contain thick sequences of folded and faulted sedimentary rocks, 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 at convergent boundaries © McGraw-Hill Education. Permission required for reproduction or display. Characteristics of Major Mountain Belts Metamorphism and Plutonism • Regional metamorphic rock and plutonic rock is found in the most intensely deformed portions of major mountain belts • Migmatites – intermixed granite and metamorphic rock caused by partial melting of metamorphic rock Normal Faulting • Older portions of some major mountain belts have undergone normal faulting as a result of later uplift and horizontal extension Thickness and Density of Rocks • Gravity measurements indicate that continental crust beneath mountains is less dense and thicker than oceanic crust Features of Active Mountain Ranges • Frequent earthquakes and deep ocean trenches parallel to ranges © McGraw-Hill Education. Permission required for reproduction or display. Evolution of Mountain Belts Orogenies and Plate Convergence •Continental crust becomes thicker with tight folds and 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) © McGraw-Hill Education. Permission required for reproduction or display. 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 crustal root of a mountain range rises by isostatic adjustment • Normal Faulting • Tension in uplifting and spreading crust results in normal faulting and fault-block mountain ranges © McGraw-Hill Education. Permission required for reproduction or display. Evolution of Mountain Belts Post Orogenic Uplift and Block Faulting • Delamination • Thin crust and block faulting in the Basin and Range • Basin-and-Range province – may be the result of delamination • Overthickened mantle lithosphere beneath old mountain belt may detach and sink into asthenosphere • Resulting inflow of hot asthenosphere can stretch and thin overlying crust, producing normal faults © McGraw-Hill Education. Permission required for reproduction or display. The Growth of Continents Continents grow larger as mountain belts evolve along their margins • Accumulation of sediment and igneous activity add new continental crust Displaced Terranes Geologic continuous areas within mountain belts where the age and characteristics of the bedrock appear unrelated to that of the adjacent regions • Terrane boundaries are usually faults Concluding Comment - Plate Tectonics is a great advance but many new problems will need to be solved as science builds on the past. © McGraw-Hill Education. Permission required for reproduction or display. End of Chapter 20
