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Lecture 7: Convergent margins II

Lecture 7: Convergent margins II. Original: Molnar et al. (1979). K&V 2 nd Edition. KK&V 3 rd Edition. KK&V Fig 9.3. Why are back-arc basins preferentially found in the Western Pacific?. Green = 0 – 50 km Yellow = 50 – 250 km Red = > 250 km. Peru-Chile trench.

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Lecture 7: Convergent margins II

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  1. Lecture 7: Convergent margins II

  2. Original: Molnar et al. (1979) K&V 2nd Edition KK&V 3rd Edition

  3. KK&V Fig 9.3

  4. Why are back-arc basins preferentially found in the Western Pacific?

  5. Green = 0 – 50 km Yellow = 50 – 250 km Red = > 250 km Peru-Chile trench

  6. Note: Altiplano (center) Fold and Thrust Belts (on the east)

  7. Note shallow dip beneath Peru and Central Chile Green = 0 – 50 km Yellow = 50 – 250 km Red = > 250 km Nazca Ridge Juan Fernandez Islands

  8. Processes occurring on the “landward trench slope” Accretionary versus non-accretionary environments Atacama Desert Filled barbs = accretionary Open barbs = non-accretionary

  9. Accretionary prisms grow over time Outer arc high

  10. Anak Krakatoa Nias Sumatra Krakatoa Java Islands south of Sumatra and Java are “outer-arc highs” The deeps between the islands and the mainland are “fore-arc basins” Indonesian Arc: Sumatra-Java trench Movie title: Krakatoa, East of Java

  11. The decollement is the boundary between undeformed sediments that are being subducted and the scraped off, accreted material The style of faulting in the accretionary prism is called imbricate thrusting

  12. Imbricate thrusting Coulbourn, 1981

  13. Chikyu (Earth)

  14. Nankei Trough: classic accretionary prism KK&V Fig 9.20

  15. Drilling shows that 20 Ma ago lower slope was very shallow Some process is removing lower slope material from underneath

  16. Subduction erosion by interaction with horst and graben topography Chile Trench Von Huene and Ranero 2003

  17. Horsts and grabens develop on the flexural bulge seaward of Chile trench

  18. Gravity Horsts and grabens develop on the flexural bulge seaward of the Chile trench

  19. Depth to the slab beneath volcanic arcs ≅125 km Profiles are aligned on volcanic axis

  20. The answer is #3

  21. orange = zones of phase changes Shallowest zone supplies H2O for melting arc magmas Deeper zones are source of deep earthquakes

  22. KK&V Fig 9.25

  23. Nankei Trough Izu-Bonin Trench Shikoku Basin Philippine Sea Mariana Basin Mariana Trench Parece-Vela Basin Challenger Deep (10,920 m) Back-Arc Basins

  24. Active Arc Seafloor ages Fossil Spreading Center 25 – 15 Ma Active arc Fossil Spreading Center 60 – 35 Ma Active Back-arc basin Remnant Arc Remnant Arc

  25. KK&V Fig 9.3

  26. Development of a series of progressively younger back-arc basins Driven by trench roll-back

  27. Pacific Plate Fossil Trench Active arc Fiji Lau Bain Tonga Trench Remnant Arc Active Arc Australian Plate

  28. Lau Basin: 6 Ma old fast spreading ~ 100 mm/yr KK&V Figs 9.31 and 9.32

  29. Lau Basin Good magnetic anomalies Note: Central Anomaly, J, 2, 2A Zellmer & Taylor 2001

  30. Solomon Islands New Guinea Woodlark Basin

  31. Ontong-Java plateau

  32. Woodlark Basin Small basin actively spreading in front of the arc; great seafloor spreading magnetic anomalies Spreading axis is being subducted at the east end Spreading axis is propagating into New Guinea at the west end, rifting this piece of continent See KK&V 7.8.2 in Continental rifting chapter

  33. Ontong Java Plateau Solomon Arc Vitiaz trench Arc-reversal Until ~20 Ma the Pacific plate was underthrusting Australia all along the arc from the Solomons to the Vitiaz trench By 10 Ma, the arc had reversed and the Australian plate was underthrusting the Pacific plate Attributed to the collision of the Ontong-Javav plateau with the trench

  34. Abandoned Vitiaz Trench Fiji Fiji Basin New Hebrides Trench

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