Mountain Building and the Origin of the Continents

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Mountain Building and the Origin of the Continents

• Chapter 10

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Goals

• Understand the origin of the continents
• When were the continents formed?
• How were the continents formed?
• Why did it happen the way that it did?
• Understand the difference between continental
  and oceanic crust and lithosphere
• Understand why continents are high and oceans are
  low
• Understand the role of mountain building in
  construction of the continents
• Recognize interactions between tectonics and
  climate in mountain building
• Know three main types of mountain building

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How Continents are Built
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Cratons

• A craton is crust that hasnt been deformed in 1
  Ga.
• Low-geothermal gradient cool, strong and stable
  crust.

• Two cratonic provinces.
• Shields Outcrops of Pre-C igneous and
  metamorphic rocks.
• Platforms Shields covered by layers of
  relatively undeformed Phanerozoic sedimentary
  rocks.
• Usually surrounded by highly deformed Phanerozoic
  orogenic terranes

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Shields

• Intensively deformed old (gt 1 Ga) metamorphic and
  igneous rocks.
• Record of voluminous silicic magma production ca.
  3.5-2.5 Ga
• Form central core, or foundation, of modern
  continents

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Shield Formation

• Archean Tectonics
• Higher geothermal gradients (known from high-T
  magmatic rocks)
• Rapid plate motions
• Large volumes of silicic melt production (from
  fertile mantle)
• Granitic magmas form silicic microcontinents
  embedded in tectonic plates composed mainly of
  mafic ultramafic magmatic rocks (archean
  oceanic crust)
• Microcontinents collide and are welded together
  (3.3-1.0 Ga)
• Shields composed of
• Granite/Gneiss complexes (microcontinent
  fragments)
• Greenstone belts (archean oceanic crust trapped
  in suture)

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Cratonic Platforms

• Sedimentary rocks covering Pre-Cambrian basement.
• Deposited in shallow seas (shield continental
  margins)
• Some in lacustrine (lake) or swamp environments
• Exhibit large domes and basins.
• From vertical crustal adjustment.
• Created by stresses transmitted to interior from
  an active margin.

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Economic Importance

• Shields high temperatures of archean magma
  genesis yields unusually high concentrations of
  econimically important metals
• Nickel
• Magnesium
• Platinum, gold, silver
• Platforms Basins and domes provide depositional
  centers and structures suitable for concentrating
  energy resources
• Oil
• Gas
• Uranium

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How Continents Grow Mobile Belts, Accreted
Terranes, and Orogeny
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Mountains

• Occur in elongate linear belts, typically at the
  edges of the cratons.
• Mountains are constructed by tectonic plate
  interactions in a process called orogenesis.
• Geologists call mountain belts orogens, even if
  eroded flat

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Mountains

• Mountains provide vivid evidence of tectonic
  activity.
• They embody
• Uplift.
• Deformation.
• Metamorphism.
• Exhumation (by faulting erosion)

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Mountains

• Mountain building involves
• Structural deformation.
• Jointing.
• Faulting.
• Folding.
• Partial melting.
• Foliation.
• Metamorphism.
• Glaciation.
• Erosion.
• Sedimentation.
• Constructive processes build mountains up
  destructive processes tear them back down again.

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Orogenesis and Rock Genesis

• Orogenic events create many kinds of rocks.
• Igneous rocks Intrusive and extrusive.
• Subduction related volcanic arc.
• Rift related decompressional melting.
• Metamorphic rocks Regional and contact.
• Igneous intrusion.
• Deep burial.
• Horizontal compression.

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Orogenesis and Rock Genesis

• Orogenic events create many kinds of rocks.
• Sedimentary rocks Weathering and erosion.
• Erosional debris is shed to adjacent regions.
• Sediments accumulate in basins created by crustal
  flexure.
• Sediments can preserve evidence of mountains
  eroded away.

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Erosional Sculpting

• Mountainous terrain is often steep and jagged due
  to erosion by water and ice.
• Mountain heights reflect the balance between
• Uplift.
• Erosion.
• Rock structures can effect erosion.
• Resistant layers form cliffs.
• Easily eroded rocks form slopes.

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Orogenic Collapse

• The Himalayas are the maximum height possible.
• There is an upper limit to mountain heights.
  Why?
• Erosion accelerates with height.
• Weight of high mountains overwhelms rock
  strength.
• Deep, hot rocks eventually flow out from beneath
  mountains.
• The mountains then collapse downward like soft
  cheese.
• Uplift, erosion and collapse exhume deep crustal
  rocks.

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Causes of Orogenesis

• Mountain building is driven by plate tectonics.
• Convergent plate boundaries.
• Continental collisions.
• Rifting.
• Orogenic phases may last several hundred Ma.
• Ancient mountains are deeply dissected by erosion.

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Causes of Orogenesis

• Convergent tectonic boundaries create mountains.
• Subduction-related volcanic arcs grow on
  overriding plate.
• Accretionary prisms (off-scraped sediment) grow
  upward.

• Compression stacks thrust faults on the far side
  of mountain belt.

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Causes of Orogenesis

• Island fragments of continental lithosphere may
  be carried into trenches but they wont subduct.
• These blocks are added to the overriding plate.
• These exotic terranes have geologic histories
  unlike surrounding rocks.

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Causes of Orogenesis

• Continental collisions.
• Oceanic lithosphere can completely subduct.
• This closes the pre-existing ocean basin.
• Brings two blocks of continental crust together.
• Buoyant continental crust will not subduct.
• Instead, subduction is extinguished.

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Causes of Orogenesis

• Continent continent collision
• Creates a broad welt of crustal thickening.
• Thickening due to thrust faulting and flow
  folding.
• Center of belt consists of high-grade metamorphic
  rocks.
• Fold and thrust belts extend outward on either
  side.
• The resulting high mountains may eventually
  collapse.

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Causes of Orogenesis

• Continental rifting.
• Continental crust is uplifted in rift settings.
• Thinned crust is less heavy mantle responds
  isostatically.
• Decompressional melting adds asthenospheric
  magma.
• Increased heat flow from magma expands and
  uplifts rocks.
• Rifting creates linear fault block mountains and
  linear basins.

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Case Study - Appalachians

• A classic example of a complex orogenic belt.
• The Appalachians formed by 3 separate orogenic
  events.
• Preserves a complete Wilson cycle (the opening
  and closing of an ocean basin).

• The Appalachians today are the eroded remnants of
  the former mountains.

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Appalachians

• A giant orogenic belt existed before the
  Appalachians.
• The Grenville orogeny (1.1 Ga), formed a
  supercontinent.
• By 600 Ma, much of this orogenic belt had eroded
  away.

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Appalachians

• The Grenville orogenic belt rifted apart 600 Ma.
• This formed a new ocean (the proto-Atlantic).
• Eastern North America developed as a passive
  margin.
• A thick pile of sediments accumulated along this
  margin.
• An east-dipping subduction zone built up an
  island arc.

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Appalachians

• Subduction carried the margin into the island
  arc.
• The collision resulted in the Taconic orogeny 420
  Ma.
• A doubly-dipping subduction zone developed.
• Exotic blocks of continental crust were carried
  in.
• These blocks were added to the margin during the
  Acadian orogeny 370 Ma.

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Appalachians

• East-dipping subduction continued to close the
  ocean.
• Africa collided with North America 270 Ma during
  the Alleghenian orogeny.
• Created a huge fold and thrust belt and mountain
  range.
• Assembled the supercontinent of Pangea.

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Appalachians

• Pangea began to rift apart 180 Ma.
• Faulting and stretching thinned the lithosphere.
• Rifting led to development of a divergent margin.
• Sea-floor spreading created the Atlantic ocean.

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Modern Orogenesis

• Modern instrumentation can measure mountain
  growth.
• Global positioning systems (GPS) measure rates
  of

• Horizontal compression.
• Vertical uplift.

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Isostasy

• High mountains are supported by thickened
  lithosphere.
• Thickening is caused by collisional orogenesis.
• Average continental crust 35 to 40 km thick.
• Beneath orogenic belts 50 to 70 km thick.
• This thickened crust helps buoy the mountains
  upward.