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The Generation of Melting Anomalies by Plate Tectonic Processes

The Generation of Melting Anomalies by Plate Tectonic Processes. Gillian R. Foulger University of Durham. .... based on ideas developed over several years by a working group that includes ....

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The Generation of Melting Anomalies by Plate Tectonic Processes

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  1. The Generation of Melting Anomalies by Plate Tectonic Processes Gillian R. Foulger University of Durham

  2. .... based on ideas developed over several years by a working group that includes .... Don Anderson, Warren Hamilton, Jerry Winterer, Jim Natland, Dean Presnall, Peter Vogt, Anders Meibom, Hetu Sheth, Seth Stein, Mike O’Hara & Alan Smith .... etc for more see www.mantleplumes.org

  3. Two key elements: • Variations in lithosphere stress • Mantle inhomogeneity Simply put • Stress governs location of volcanism • Fusibility governs volume of magma

  4. MELT experiment EPR Mantle dehomogenising • ridges

  5. Mantle dehomogenising • ridges • subduction zones • eclogitisation of subducted crust

  6. A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite Pyrolite Eclogite Yaxley (2000) Eclogite is fusible

  7. Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup Cantal basalts model, Massif Central, France Pilet et al. (2005)

  8. Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup Schott et al. (2000)

  9. Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup

  10. Mantle dehomogenising adapted from Meibom & Anderson (2003)

  11. Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength

  12. Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength From Natland, 2004

  13. Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength adapted from Lundin & Doré (2005)

  14. Proposal “Hot spot” volcanism occurs where • stress is extensional • mantle is highly fusible

  15. Examples

  16. “Hot spots” on MORs • 1/3 of all “hot spots” are on or near MORs

  17. East African Rift • Afar • Other EAR “hotspots”?

  18. Basin & Range Province • Broad, intraplate extensional region • Associated with subduction of “Farallon slab” • Widespread volcanism

  19. NAVP & Iceland • Formed when continent rifted along Iapetus suture • Diverse data suggest not hot • Recycled Iapetus crust can explain geochemistry & melt volume Closure of the Iapetus

  20. Azores • Kinematic models • EQ focal mechanisms • bathymetry • suggest Azores branch is: • oblique, ultra-slow spreading (3-4 mm/yr) • diffuse plate boundary • dextral differential shear motion Lourenço et al. (1998)

  21. Time-progressive volcanism

  22. Predictions • Melt volumes can be explained by lithosphere extension + source fertility • Vertical motions related to shallow tectonic processes • Upper mantle is inhomogeneous • Migration of volcanism = migration of locus of extension

  23. Predictions • Seismic tomography anomalies indicate composition and melt, not only temperature • “Hot spot” lavas not required to be hot • Geochemistry can be explained by inhomogeneities in the shallow mantle

  24. That’s all folks!

  25. Example: mantle potential temperature, Iceland

  26. Temperature (Tp) from petrology • mid-ocean ridges: ~1280 - 1400˚C • Iceland: ~1280 - 1460˚C • Hawaii: ~1560˚C The only place on Earth hot enough for a weak upper-mantle plume is Hawaii

  27. Seismology does not reliably detect them in the lower mantle

  28. Iceland Vertical exaggeration x 10 DT ~ 200˚C DT ~ 60˚C Ritsema & Montagner (2003)

  29. Example:whole-mantle tomography: Iceland Ritsema et al. 1999

  30. Please read our book:Plates, Plumes & Paradigms

  31. Hudson Bay plume? Iceland: A plume from the core-mantle boundary? Bijwaard & Spakman (1999)

  32. Resolution of the “whole-mantle plume” The data used by Bijwaard & Spakman have no resolution in the lower mantle (courtesy of Karason & van der Hilst) From Foulger et al. (2001)

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