Chapter 14: Mountain Belts and the Continental Crust

  • every main continent has a shield

orogenies = the processes that build mountain belts

  • collisional boundaries are a show of plates moving around

  • name of the orogeny that created the mountain belt is often different than the name of the mountain chain (Alleghenian orogeny = Appalachian Mts)

Intense deformation:

  • plate convergence causing folding, faulting, sedimentation, volcanism, metamorphism, foliation

isostasy: vertical movement of crust due to differences in density between the crust (ex. wood floating in water)

Weathering & erosion:

  • rate and nature of weathering and erosion

  • based on climate, type of rock, and height

new crust has been added to continents over time but how?

  1. magmatic addition: vertical transport of mafic materials (basalt), magmas created at subduction zones; flux melting

  2. continental accretion: horizontal transport of material (island arcs, old mt. ranges, seamounts, basaltic plateaus). crustal fragments are glued onto more stable cratons

    • accreted terrains

    • exotic terrains

paleo magnetic records help map from millions of years ago

  • fossils that once lived in different climates and at different times found next to each other bc they’ve been put together by moving of continents

  • large faults bound the accreted terrains

  • rocks of different age and lithology next to each other

  • paleomagnetic differences (can’t tell eastern or western hemisphere)

supercontinent events create and build crust which are convergent boundries that create mountains.

The Wilson Cycle: the IDEALIZED plate tectonic cycle of supercontinents and then no supercontinents (over time, there are cycles of building mountains)

cycles of building mountain - Appalachians have been activated/reactivated

  1. rifting within a continent splits the continent

    • Rodinia (1.2 billion years old), supercontinent

  2. leading to the opening of a new ocean basin and creation of new oceanic crust, starting the cycle

    • Tethys Ocean forms (equatorial sea way)

  3. as spreading continues and an ocean opens, passive margin cooling occurs and sediment accumulates during seafloor spreading

  4. convergence begins; an oceanic plate subducts beneath a continental plate, creating a volcanic chain at the active margin

    • North American-Europe (Laurentia) begins to converge with Africa-South America-Australia (Gondwana)

  5. Terrane accretion - from the sedimentary accretionary wedge or fragments carries by the subducting plate - welds material to the continent

  6. As two continents collide, orogeny thickens the crust and builds mountains, forming a new supercontinent

    • all continents collide to form Pangaea, supercontinent

  7. Continent erodes, thinning the crust = process may begin again

Tectonic History of North America

1.6 - 1.1 bya:

  • southern US was converging with an oceanic plate and formed the supercontinent Rodinia

400 - 200 mya:

  • eastern US was converging toward South America, Europe and Africa forming Appalachians and Pangea, then rifted apart forming Atlantic Ocean

80 - 55 mya:

  • Laramide Orogeny created the Rockies

Today:

  • SW US is extending forming the Rio Grande Rift, Basin and Rang and San Andreas Fault (passive margin to east)

Tectonic History of North America: Appalachian Mountains

Taconic Orogeny: Ordovician period

  • island arc formed and wash pushed into N.A.

Acadian Orogeny: Devonian period

  • continental crustal fragment pushed onto N.A.

Alleghanian Orogeny: Permian period

  • N.A. and Africa collide = Pangea

Today: history of these events in geologic record (rock from alabama all the way to florida, and basically all of the East Coast)

  • passive margin in Gulf of Mexico and Atlantic Ocean

West Coast

Cretaceous Period: subduction, formation of the rockies

EPR (left plate) Rockies (right plate)

Oligocene Epoch: subduction of EPR, formation of SAF

Today: SAF, Cascades, Basin and Range, Rio Grande Rift