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EVOLUTION/HISTORY OF THE CONTINENTS Chapter 10 Wrangellia was transported from 5000 km away in the Southern Hemisphere. Aleutian Trench Wrangellia Spreading center (divergent boundary) Subduction margin (convergent boundary) Cascadia Trench NORTH AMERICA San Andreas Fault Much of western North America is made up of exotic terranes accreted over the past 200 million years. Transform fault Island arc Submarine deposits Ancient ocean floor Displaced continental fragments ACCRETION OF A BUOYANT FRAGMENT TO A CONTINENT Continental crust A buoyant fragment is carried into a plate collision zone. Fragment Lithosphere Asthenosphere The fragment is more buoyant than the lithosphere and is not subducted. Accreted terrane The fragment becomes welded to the overriding plate. ACCRETION OF AN ISLAND ARC TO A CONTINENT A plate carrying a continent subducts beneath an oceanic island arc. Island arc Continental crust The continental crust is not subducted. The island arc crust becomes welded to the continent. Accreted terrane ACCRETION ALONG A TRANSFORM FAULT Transform fault Two plates slide past each other along a transform fault. Terrane fragment Plate A Plate B A terrane fragment on plate B is carried along the margin of plate A. Terrane fragment Accreted terrane When the fault becomes inactive, the fragment becomes welded far from its original position. ACCRETION BY CONTINENTAL COLLISION AND RIFTING Continental A plate carrying a continent subducts beneath another continental plate. Continental Plate B Plate A The continents are welded together along a set of thrust faults. Thrust faults Accreted terrane Later, rifting and seafloor spreading carry the plates apart, leaving a fragment of one welded to the other. The African, Arabian, and Indian plates raised chains of mountains as they collided with the Eurasian Plate. Atlas Mountains Alps Caucasus Zagros Mountains EUROPE AFRICA Tien Shan Tibetan Plateau Himalaya ASIA Saudi Arabia China India Earthquake depth 50 km deep (shallow focus) 50–300 km deep 300 km deep (deep focus) Earthquake activity indicates that this orogeny is still in progress today. Paleozoic Mesozoic sediments sediments Indian Plate Oceanic crust Accretionary Forearc wedge basin Eurasian Plate (Tibet) Continental crust 60 Ma As the Indian plate subducted under the Eurasian Plate, an accretionary wedge accumulated. Rising magma thickened the Eurasian Plate crust. 30-50 Ma India collided with Tibet, breaking along the Main Central Thrust fault. Eroded uplifted material 20-30 Ma As the collision continued, a slice of India crust was stacked onto the oncoming subcontinent. Ganges Plain Himalaya Tibetan Plateau 10-20 Ma A second thrust fault developed, stacking a second slice of crust onto India and lifting the first slice. These overthrust slices make up the Middle Cambrian (510 Ma) After the breakup of Rodinia, the continent of Laurentia straddled the equator. LAURENTIA IAPETUS OCEAN BALTICA Late Ordovician (450 Ma) An island arc collided with Laurentia, causing the Taconic orogeny. LAURENTIA IAPETUS OCEAN BALTICA Early Devonian (400 Ma) The collision of Laurentia with the continent of Baltica formed Laurussia. Acadian orogeny Caledonian orogeny GONDWANA Late Mississippian (340 Ma) The collision of Gondwana with Laurussia began with the Variscan orogeny… Shelf and submerged continent Variscan orogeny GONDWANA Upper Pennsylvanian (300 Ma) … and continued with the Appalachian orogeny. Siberia converged with Laurussia to form Laurasia. Ural orogeny LAURUSSIA Appalachian orogeny GONDWANA Variscan orogeny Early Permian (270 Ma) The end product was the supercontinent of Pangaea. Ural orogeny LAURASIA GONDWANA Supercontinent Cycle Rifting splits the continent... …leading to the creation of new oceanic crust. Passive margin cooling occurs and sediment accumulates. Convergence begins: an oceanic plate subducts beneath a continental plate, creating a volcanic chain. Terrane accretion welds material to the continent. Orogeny thickens the crust and builds mountains, forming a new supercontinent. The continent erodes, thinning the crust. Rifting may begin the process again.
