Continental Tectonics and Mountain Chains Chapter 9 Continental Tectonics and Mountain Chains

Guiding Questions How does continental rifting begin and what environments of deposition does it produce? How do rocks become folded? How does a mountain chain form when a continental margin encounters a subduction zone? Why does a foreland basin accumulate large volumes of sediment on the continent? Why is continental crust not subducted? What is the significance of ophiolites? What is the zonation of a typical mountain chain? How have the Andes formed? How did the Pyrenees form? What is an exotic terrane? What broad features does rock deformation create in continents far from their margins?

Terranes Geologically distinctive regions of Earth’s crust, each of which has behaved as a coherent crustal block

San Andreas Fault 5.5 cm/year (~2 inches) Moving L.A. (on the Pacific plate) closer to San Francisco (on the North American plate)

Rifting Triple junction Three-armed grabens at plate boundaries Associated with doming Hot spot May have multiple types of plate boundaries

Rifting Formation of Atlantic Ocean Red Sea provides modern analogue Three-armed rift forms One may die out Failed rift Mississippi River Amazon

Rifting Rift valleys Extension breaks continental crust into fault blocks Blocks subside rapidly Accumulate sediments, lakes Rift Valley, East Africa

Rifting When rifting continues, continents separate along ridge axis Margins cool, sink

Rifting Passive margin Active margin Tectonically inactive areas of continental crust that accumulate sediment along shallow shelves Eastern U.S. Active margin Zones of tectonic deformation and igneous activity Western U.S.

Bending and Flowing of Rocks Rocks can bend and flow under stress Metamorphosis at high pressures and temperatures Compressive forces Folding

Bending and Flowing of Rocks Syncline Rocks folded concave up Vertices at bottom Anticline Rocks folded concave down Vertices at top

Bending and Flowing of Rocks Dip Angle that the bed forms with the horizontal plane Strike Compass direction that lies at right angles to the dip Always horizontal Regional strike Overall trend of fold axes

Bending and Flowing of Rocks Axial plane Imaginary plane that cuts through fold and divides it symmetrically Overturned fold If either limb is rotated more than 90° from its original position

Bending and Flowing of Rocks Axis of a fold Line of intersection between axial plane and beds of folded rock Plunging fold Axis lies at an angle to the horizontal

Bending and Flowing of Rocks Plunging fold When eroded, produces a curved outcrop pattern

Mountain Building Orogenesis Process of mountain building Orogenies Mountain building events

Mountain Building Continental Collision Continental crust cannot be subducted Suturing Unification of two continents along a subduction zone Ophiolite Remnant of seafloor pinched up along suture

Mountain Building Magma rises into overlying continental slab Volcanoes form, elevate crust Mountain peaks Plutons cool to form igneous core Metamorphic Belt Rocks on either side of core are deformed by core’s heat and other processes

Mountain Building Fold and Thrust belt Thrust sheets On continental edge Turned over away from core Brittle deformation Thrust sheets Large slices of crust formed by thrust faulting Slide along basal surface

Mountain Building Folds and faulting Increase folding Develop overturned fold Overturned fold can break

Mountain Building Cross-section of Rocky Mountains Thrust faults slice through previously folded rocks

Deformation Processes Deformation caused by Pressure applied by subducted plate Pushes mountain chain toward interior of continent Folding near igneous arc and inland Gravity spreading Rock deforms under its own weight, spreads out Deformation along folds and thrusts

Deformation Processes Foreland basin Downwarping of lithosphere beneath actively forming mountain chain beyond fold and thrust belt Axis is parallel to mountain chain Rapid formation Deep, often flooded

Deformation Processes Foreland basin sediments Flysch Shales, turbidites Floods rapidly Turbidites accumulate Molasse Nonmarine sediments Mountain evolves Fold and thrust moves inland Chokes basin, folds flysch Alluvial fans, floodplains, etc. Clastic wedge

Andes Mountain Building Igneous rocks added since Mesozoic Continuing to build up Bobs isostatically Mountain chain is migrating inland

Andes Mountain Building Magma shifting inland as subduction angle is reduced Change in angle means change in plate movement

Andes Mountain Building 10 M years ago Foreland basin connected to Atlantic along thin seaway Infilling of foreland basin led to formation of Amazon River from seaway Stranded marine animals that adapted to freshwater

The Pyrenees Formed when Iberia collided with Eurasia Exotic terrane Cretaceous and Paleogene Iberia originally part of Eurasia Subduction began, reattached toward north Ophiolites in northern Pyrenees mark suturing Foreland basin received flysch then molasse Exotic terrane

Continental Interiors Structural basin Circular or oval depression of stratified rock Structural dome Circular or oval uplift of stratified rock Erosion leads to circular pattern

Continental Interiors Black hills of South Dakota Oblong dome

Continental Interiors Domes and basins of North America

Continental Interiors Michigan Structural basin