Transcript Chapter 9
Chapter 9 Continental Tectonics and Mountain Chains 1 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 2 their margins? 3 Terranes • Geologically distinctive regions of Earth’s crust, each of which has behaved as a coherent crustal block 4 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) 5 Rifting • Triple junction – Three-armed grabens at plate boundaries – Associated with doming • Hot spot – May have multiple types of plate boundaries 6 Rifting • Formation of Atlantic Ocean – Red Sea provides modern analogue – Three-armed rift forms – One may die out • Failed rift – Mississippi River – Amazon 7 Rifting • Rift valleys – Extension breaks continental crust into fault blocks • Blocks subside rapidly – Accumulate sediments, lakes • Rift Valley, East Africa 8 Rifting • When rifting continues, continents separate along ridge axis • Margins cool, sink 9 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. 10 Bending and Flowing of Rocks • Rocks can bend and flow under stress – Metamorphosis at high pressures and temperatures – Compressive forces • Folding 11 Bending and Flowing of Rocks • Syncline – Rocks folded concave up – Vertices at bottom • Anticline – Rocks folded concave down – Vertices at top 12 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 of13fold 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 14 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 15 Bending and Flowing of Rocks • Plunging fold – When eroded, produces a curved outcrop pattern 16 Mountain Building • Orogenesis – Process of mountain building – Orogenies • Mountain building events 17 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 18 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 19 processes 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 20 surface Mountain Building • Folds and faulting – Increase folding – Develop overturned fold – Overturned fold can break 21 Mountain Building • Cross-section of Rocky Mountains • Thrust faults slice through previously folded rocks 22 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 23 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 24 Deformation Processes • Foreland basin sediments – Molasse – Flysch • Shales, turbidites – Floods rapidly – Turbidites accumulate • Nonmarine sediments – Mountain evolves – Fold and thrust moves inland – Chokes basin, folds flysch – Alluvial fans, floodplains, etc. • Clastic wedge 25 Andes Mountain Building • Igneous rocks added since Mesozoic • Continuing to build up – Bobs isostatically – Mountain chain is migrating inland 26 Andes Mountain Building • Magma shifting inland as subduction angle is reduced • Change in angle means change in plate movement 27 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 28 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 29 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 30 Continental Interiors • Black hills of South Dakota – Oblong dome 31 Continental Interiors • Domes and basins of North America 32 Continental Interiors • Michigan – Structural basin 33 34