EVOLUTION/HISTORY OF THE CONTINENTS Chapter 10

Spreading center (divergent boundary) Subduction margin (convergent boundary) Transform fault Island arc Submarine deposits Ancient ocean floor Displaced continental fragments N O R T H A M E R I C A Aleutian Trench Cascadia Trench San Andreas Fault Wrangellia Much of western North America is made up of exotic terranes accreted over the past 200 million years. Wrangellia was transported from 5000 km away in the Southern Hemisphere.

ACCRETION OF A BUOYANT FRAGMENT TO A CONTINENT Continental crust Asthenosphere Lithosphere Fragment Accreted terrane A buoyant fragment is carried into a plate collision zone. The fragment is more buoyant than the lithosphere and is not subducted. The fragment becomes welded to the overriding plate.

ACCRETION OF AN ISLAND ARC TO A CONTINENT Continental crust Accreted terrane Island arc A plate carrying a continent subducts beneath an oceanic island arc. The continental crust is not subducted. The island arc crust becomes welded to the continent.

ACCRETION ALONG A TRANSFORM FAULT Terrane fragment Two plates slide past each other along a transform fault. A terrane fragment on plate B is carried along the margin of plate A. When the fault becomes inactive, the fragment becomes welded far from its original position. Transform fault Plate A Plate B Terrane fragment Accreted terrane

ACCRETION BY CONTINENTAL COLLISION AND RIFTING A plate carrying a continent subducts beneath another continental plate. The continents are welded together along a set of thrust faults. Later, rifting and seafloor spreading carry the plates apart, leaving a fragment of one welded to the other. Continental Plate A Continental Plate A Continental Plate B Continental Plate B Thrust faults Accreted terrane

Atlas Mountains AlpsCaucasus Zagros Mountains Tien Shan Tibetan Plateau Himalaya ASIA EUROPE AFRICA Saudi Arabia Saudi Arabia India China Earthquake depth 50 km deep (shallow focus) 50–300 km deep 300 km deep (deep focus) The African, Arabian, and Indian plates raised chains of mountains as they collided with the Eurasian Plate. Earthquake activity indicates that this orogeny is still in progress today.

Indian Plate Paleozoic sediments Mesozoic sediments Accretionary wedge Forearc basin Eurasian Plate (Tibet) Continental crust Oceanic crust Oceanic mantle lithosphere Oceanic mantle lithosphere 60 Ma As the Indian plate subducted under the Eurasian Plate, an accretionary wedge accumulated. Rising magma thickened the Eurasian Plate crust.

Main Central Thrust Main Central Thrust Ma India collided with Tibet, breaking along the Main Central Thrust fault.

Main Boundary Fault Ma As the collision continued, a slice of India crust was stacked onto the oncoming subcontinent. Main Central Thrust Eroded uplifted material

Main Boundary Fault 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 bulk of the Himalaya. Main Central Thrust HimalayaTibetan PlateauGanges Plain

Equator IAPETUS OCEAN IAPETUS OCEAN LAURENTIA BALTICA Middle Cambrian (510 Ma) After the breakup of Rodinia, the continent of Laurentia straddled the equator.

Equator IAPETUS OCEAN IAPETUS OCEAN LAURENTIA BALTICA Late Ordovician (450 Ma) An island arc collided with Laurentia, causing the Taconic orogeny. Taconic orogeny Taconic orogeny

LAURUSSIA GONDWANA Early Devonian (400 Ma) The collision of Laurentia with the continent of Baltica formed Laurussia. Caledonian orogeny Caledonian orogeny Acadian orogeny Acadian orogeny

LAURUSSIA GONDWANA Late Mississippian (340 Ma) The collision of Gondwana with Laurussia began with the Variscan orogeny… Variscan orogeny Variscan orogeny Shelf and submerged continent Shelf and submerged continent Equator

GONDWANA Upper Pennsylvanian (300 Ma) … and continued with the Appalachian orogeny. Siberia converged with Laurussia to form Laurasia. Variscan orogeny Variscan orogeny Equator LAURUSSIA Hercynian orogeny Hercynian orogeny Appalachian orogeny Appalachian orogeny Ural orogeny Ural orogeny

GONDWANA Early Permian (270 Ma) The end product was the supercontinent of Pangaea. Equator LAURASIA Ural orogeny Ural orogeny

Rifting splits the continent... Supercontinent Cycle

…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.