EVOLUTION/HISTORY OF THE CONTINENTS

Chapter 10

Spreading center(divergent boundary)

Subduction margin(convergent boundary)

Transformfault

Island arc

Submarine deposits

Ancient ocean floor

Displaced continental fragments

N O R T HA M E R I C A

AleutianTrench

CascadiaTrench

San AndreasFault

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 theSouthern Hemisphere.

ACCRETION OF A BUOYANT FRAGMENT TO A CONTINENT

Continentalcrust

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 ACONTINENT

Continentalcrust

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 ALONGA TRANSFORM FAULT

Terranefragment

Two plates slide past eachother 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.

Transformfault

Plate A

Plate B

Terranefragment

Accretedterrane

ACCRETION BYCONTINENTALCOLLISION ANDRIFTING

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.

ContinentalPlate A

ContinentalPlate B

Thrustfaults

Accretedterrane

AtlasMountains Alps Caucasus

ZagrosMountains Tien Shan

TibetanPlateau Himalaya

ASIAEUROPE

AFRICA SaudiArabia India

China

Earthquake depth50 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.

IndianPlate

Paleozoicsediments

Mesozoicsediments

Accretionarywedge

Forearcbasin

EurasianPlate(Tibet)

Continental crust

Oceanic crust Oceanic mantle

lithosphere

60 MaAs the Indian plate subducted under the Eurasian Plate, an accretionary wedge accumulated. Rising magma thickened the Eurasian Plate crust.

Main Central

Thrust

30-50 MaIndia collided with Tibet, breaking along the Main Central Thrust fault.

Main Boundary Fault

20-30 MaAs the collision continued, a slice of India crust was stacked onto the oncoming subcontinent.

Main Central Thrust

Eroded uplifted material

Main BoundaryFault

10-20 MaA 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

Himalaya Tibetan PlateauGanges Plain

Equator

IAPETUSOCEAN

LAURENTIA

BALTICA

Middle Cambrian (510 Ma)After the breakup of Rodinia, the continent ofLaurentia straddled the equator.

Equator

IAPETUSOCEAN

LAURENTIA BALTICA

Late Ordovician (450 Ma)An island arc collided with Laurentia, causing the Taconic orogeny.

Taco

nic

orogeny

LAURUSSIA

GONDWANA

Early Devonian (400 Ma)The collision of Laurentia with the continent ofBaltica formed Laurussia.

Caledonianorogeny

Acadianorogeny

LAURUSSIA

GONDWANA

Late Mississippian (340 Ma)The collision of Gondwana with Laurussia beganwith the Variscan orogeny…

Variscanorogeny

Shelf andsubmergedcontinent

Equator

GONDWANA

Upper Pennsylvanian (300 Ma)… and continued with the Appalachian orogeny.Siberia converged with Laurussia to form Laurasia.

Variscanorogeny

Equator

LAURUSSIA

Hercynian

orogeny

Appalachianorogeny

Uralorogeny

GONDWANA

Early Permian (270 Ma)The end product was the supercontinentof Pangaea.

Equator LAURASIA

Uralorogeny

Rifting splits the continent...

Supercontinent Cycle

…leading to the creation of new oceanic crust.

Passive margin cooling occurs and sedimentaccumulates.

Convergence begins: an oceanic plate subductsbeneath a continental plate, creating avolcanic chain.

Terrane accretion welds material to the continent.

Orogeny thickens the crust and buildsmountains, forming a new supercontinent.

The continent erodes, thinning the crust. Riftingmay begin the process again.