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Mountains and Mountain Ranges • Orogeny – the process of mountain building – Crustal thickening • • • • Subduction zones Sub-plate magmatic activity Continental collision Compressive forces “squeeze” crust together – Thickening leads to isostatic adjustment • Rising of less-dense continental crust • Rising increases erosion, redistributes mass Island Arcs Andes(6,000 m) are the world’ s second highest mountain chain (Bolivia) Island arcs 1 The Andes • 190 mya (Jurassic) South America moves west • By 140 mya, Pacific plate had subducted to accommodate motion • By 130 mya, descending plate creates lots of basaltic magma The Andes: Subduction at a continental margin As the South American lithospheric plate moved westward in early cretaceous time, about 140 million years ago, a subduction zone and a trench formed at the west coast of the continent. The Andes • Rising magma – Some rises to form volcanoes – Some cools inside forming plutons – Thickening leads to isostatic rise – Once thick enough, soft rock beneath oozed outwards creating thrust faults • Sediments eroding from risen landscape makes sedimentary rocks on both sides of mountains The Andes: Subduction at a continental margin By 130 million years ago, igneous activity began and a subduction complex and forearc basin formed. 2 The Andes: Subduction at a continental margin The Himalayas rose when a plate carrying oceanic crust converged with a plate carrying continental crust, followed by a continent–continent collision. Pictured here is Lamoshe, in china’s Sichuan Province. By 90 million years ago, the trench and region of igneous activity had both migrated eastward. Old volcanoes became dormant and new ones formed to the east. About 200 million years ago, India was part of a large continent located near the South Pole. The Himalayas • 80 mya, India lithosphere fragment splits from South Polar landmass • India “gallops” (20 cm/yr) north, creates subduction zone beneath Asia – Creates “Andean” margin – granitic plutons, stratovolcanoes, etc. At that time India, southern Asia, and the intervening ocean basin were parts of the same lithospheric plate. 3 By 80 million years ago, India was migrating northward, approaching the equator. The Himalayas • 40 mya – oceanic lithosphere between landmasses consumed. Continents collide. – Velocity decreases to ~5 cm/yr – Both continental crust, neither can sink. – India thrusts beneath Asia, crustal thickness doubles When India began moving northward, the plate broke and subduction began at the southern margin of Asia. By 80 million years ago, an oceanic trench and subduction complex had formed. Volcanoes erupted and granite plutons formed in the region now called Tibet. By 40 million years ago, India had moved 4,000 to 5,000 kilometers northward and collided with Asia. When India collided with Tibet, the leading edge of India was underthrust beneath southern Tibet. 4 The Himalayas • Today: – – – – Contains all three rock types Many sedimentary rocks from old sea-floor Plateau is ~4000m, mountains rise from this Crustal mass caused faulting as it spread • Extensional (normal) faults in mountains • Compressional (reverse/thrust) faults in foothills continued plate movement has doubled the thickness of continental crust, creating the high Tibetan Plateau and the Himalayas. – India still moving north – Alps, Urals, Appalachians all happened under similar circumstances Question Question • ________ is a fracture with displacement. • The right graph is showing a _____. A. B. C. D. A. B. C. D. Fold; Fault; Joint; None of the above anticline; Syncline dome basin 5