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General Geology: Convergent plate boundaries Mass wasting

General Geology: Convergent plate boundaries Mass wasting . Instructor: Prof. Dr. Boris Natalin. Topics . Orogeny and orogenic belts Subduction zones Continental collisions Transform boundaries Hot spots. Mountain belts. Planes and mountains (cratons and orogens).

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General Geology: Convergent plate boundaries Mass wasting

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  1. General Geology: Convergent plate boundariesMass wasting Instructor: Prof. Dr. Boris Natalin

  2. Topics • Orogeny and orogenic belts • Subduction zones • Continental collisions • Transform boundaries • Hot spots

  3. Mountain belts • Planes and mountains (cratons and orogens). • Orogenesis or orogeny – processes that create mountain belts (orogen or orogenic belt). • Composition: Volcanic rocks, intrusions, deep- and shallow water sedimentary rocks, ophiolites (ultramafic and mafic rocks of former oceanic crust) • Structures of orogens: folds, thrusts and strike-slip faults indicating compression

  4. Theories • Contraction of the Earth • Plate tectonics: Mountain building occurs at convergent plate boundaries • Subduction trigger partial melting • Magma intrudes (and makes!) the crust • Metamorphism • Compressional stress Jean-Baptiste-Armand-Louis-Léonce Élie de Beaumont (1798-1874)

  5. - Oceanic lithosphere is denser than asthenosphere • It pulls the slab (slab-pull force) • It caused mantle convection

  6. Major features of subduction zones • Deep-ocean trench • Volcanic (magmatic) arc (andesite, granodiorite) • Forearc region (fore-arc basin and fore-arc ridge • Back-arc region (back-arc basin)

  7. Types of subduction zones Volcanic island arc Andean-type plate margin Lower plate Upper plate • Magmatic arc is constructed onto oceanic crust (ophiolites) • Magmatic arc is constructed onto continental crust

  8. Peru-Chile trench – oceanic-continental. Advance of Southern America is faster than retreat of the trench. General compression. • Mariana trench – oceanic-oceanic. Extension in some places.

  9. Extension and backarc spreading • Slab-pull force acts vertically • Old oceanic lithosphere is heavier than young one (e.g. in Sothern America). • Trench retreat or trench roll-back • Slab-suction force pulls the upper plate • Extension in the upper plate Slab suction

  10. Compressional regime of subduction - Young lithosphere resist the subduction - Continental crust is weaker than oceanic crust and hence most deformation occur in it (thickening) - The Andes are 6000m high

  11. Distribution of earthquakes

  12. Benioff zone in NE Japan Deep active seismic area in a subduction zone The epicenters may be as deep as about 680 kilometers

  13. Plate boundary Earthquake focuses are shown by dots • Benioff zone in NE Japan • Double zone od earthquakes • Upper zone corresponds to the plate boundary • Lower zone indicate transformational faulting (serpentine → olivine)

  14. Origin of magmatic arc - At 100 km depth, fluids are released from the slab - Fluids trigger the melting of the overlying mantle wedge - Partial melting in presence of water - Basaltic magma ponds at crust/mantle boundary - Magmatic differentiation - Light product of the differentiation rise up

  15. Accretionary wedge and forearc basin

  16. Convergent boundaries • Oceanic-continental convergence (subduction zone) • Oceanic-oceanic convergence (subduction zone) • Continental-continental convergence (collision zone)

  17. Continental collisions Oceanic subduction → continent - continent collision → suture zone → crustal thickening → folding and thrusting

  18. Collision-related processes, structures, and rocks • Thickening of continental crust • Foreland basins filled with molasse deposits • Fold-and-thrust belts • Collision related granites • High-grade metamorphic rocks

  19. Foreland basin

  20. The collision of India and Asia 45 million years ago • The buoyant continental lithosphere cannot be subducted • Thickening of the crust • Formation of Tibetan Plateau

  21. Escape tectonics - Wedge-like shape of the India northern margin concentrate a larger stress near the Karakorum. - In Eastern Asia, blocks escape to the east from the zone of high compression. - There is no much space to move to the west of Karakorum.

  22. Rigid boundaries on both sides Free boundary on one side (Asia)

  23. Transform boundaries Oceanic transform fault Continental transform fault:The San Andreas fault

  24. Inconsistent displacements of magnetic anomalies (red) Terminations of fracture zones in the east?

  25. Transform faults ( Wilson,1965). Active Active Passive or inactive Transform and transcurrent displacement

  26. Strike-slip faults: The North Anatolian fault

  27. Hot spots

  28. Driving mechanisms

  29. Mass wasting

  30. Slopes are the common feature and they are not stable • Mass wasting refers to the downslope movement of rock, regolith, and soil under direct influence of gravity • Weathering disintegrate the rocks • Downslope transport by mass wasting • Ultimate transportation to the sea by running water

  31. Controls and triggers • Gravity is the controlling force (oversteepening of slopes; angle of response for sand is 30°) • Saturation of material with water reduces the internal cohesion • Vegetation • Ground vibration (e.g. earthquakes) • Liquefacation

  32. Example of the landslide triggered by an earthquake

  33. Classification of mass wasting processes

  34. Classification of mass wasting processes • Type of material: Soil/regolith or bedrocks) • Type of motion: ( free fall, slides, flow) • Rate of movement: rock avalanche (very fast) or creep (slow)

  35. Fall: Talus Bedrocks Talus (regolith)

  36. Fall: Rockfall Bedrocks

  37. Rock avalanche (200 km/hour)

  38. Slumps

  39. Slumps • Downward sliding of a mass of rock or unconsolidated material moving as a unit. • Rupture surface has a spoon shape • Motion is not fast and not far • Located on over steepened slopes • Occurred when strength of the material at the base of slope is decreased

  40. Slumps Wave erosion removes the support

  41. Slumps

  42. Rockslide (GrosVentre)

  43. Critical state just before the slide Rockslide Clay saturated with water (slippery material) River erosion removes the support of the sandstone layer

  44. Debris flow • Confined to river channels • Vegetation does not tie and support slopes • Loose material (soil, regolith, mud, water) slide down slope • Fast motion • Large volume of displaced rocks • Triggers: seasonal changes (snow melting), heavy rains or earthquakes

  45. Debris flow

  46. Earthflow • Slow (1 mm per day) flow of debris saturated with water • Liquefaction during earthquakes

  47. Lahars • Motion of volcanic ash and debris saturated with water • Mount St. Helen eruption – 50 million cubic meters

  48. Slow movement: creep Expansion caused by freezing Contraction during thaws

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