Continental Rifting, Mountain Chains & Ophiolites

1. Chapter 9 Continental Tectonics and Mountain Chains

2. 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?

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

5. 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)

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

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

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

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

10. Rifting • Passive 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.

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

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

13. 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

14. 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

15. 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

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

17. Mountain Building • Orogenesis • Process of mountain building • Orogenies • Mountain building events

18. 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

19. 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

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

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

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

23. 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

24. 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

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

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

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

28. 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

29. The Pyrenees • Formed when Iberia collided with Eurasia • 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

30. 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

31. Continental Interiors • Black hills of South Dakota • Oblong dome

32. Continental Interiors • Domes and basins of North America

33. Continental Interiors • Michigan • Structural basin