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From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora, Mexico

From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora, Mexico. Christy B. Till Phillip B. Gans Frank J. Spera University of CA, Santa Barbara. Research Questions How does the character of volcanism change as the tectonic setting changes?

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From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora, Mexico

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  1. From Subduction to Extension/Transtension: A Case Study in Transitional Geochemistry from Sonora, Mexico Christy B. Till Phillip B. Gans Frank J. Spera University of CA, Santa Barbara

  2. Research Questions • How does the character of volcanism change as the tectonic setting changes? • Distinct changes in major element chemistry? Trace element? • Timescale for geochemical change? • Is change sharp or progressive? 2. Do extant petrotectonic models explain the changes we observe?

  3. Why Sonora? RTJ RTJ (Atwater, 2004)

  4. Why Sonora?

  5. Why Sonora? Increasing Distance from Gulf of California West East Volcanic Sections From Regions Sampled

  6. Subduction-related volcanism (modified from Francis, 1993) (modified from Wilson, 1989)

  7. Rift-related volcanism (Wilson, 1989) (modified from Francis, 1993) (Atwater, 2004) (modified from Best &Christiansen, 2001)

  8. Major Element Geochemistry

  9. Major Element Geochemistry

  10. The major element geochemistry reveals little or no change after subduction shuts off. • What does the trace element geochemistry tell us?

  11. Trace Element Geochemistry All andesites & basaltic andesites from Sonora

  12. Trace Element Geochemistry enriched in LILS NVZ Andes depleted in HFS All andesites & basaltic andesites from Sonora

  13. Trace Element Geochemistry All andesites & basaltic andesites from Sonora

  14. Trace Element Geochemistry All andesites & basaltic andesites from Sonora

  15. Trace Element Geochemistry Less enriched in LILS Less depleted in HFS All andesites & basaltic andesites from Sonora

  16. Trace Element Geochemistry greater arc signature La/Nb through time

  17. Trace Element Geochemistry SSU greater arc signature coast SL eastern SSU SL coast La/Nb through time

  18. Trace Element Geochemistry

  19. Geochemical Chronology • Until 15 Ma: subduction & arc volcanism migrates west • 15 - 12.5 Ma: subduction of very young hot slab, no volcanism • 12.5 - 8 Ma: progressive change from subduction-related toward rift-related volcanism • How does this compare to models?

  20. Passive Rifting/Slab Roll Back Model Phase 1 Modified from Lawton & McMillan, 2000

  21. Passive Rifting/Slab Roll Back Model Phase 2 Modified from Lawton & McMillan, 2000

  22. Passive Rifting/Slab Roll Back Model Thick, cold, old slab required Phase 2 Modified from Lawton & McMillan, 2000

  23. Passive Rifting/Slab Roll Back Model Phase 3 Modified from Lawton & McMillan, 2000

  24. Passive Rifting/Slab Roll Back Model never get Phase 3 volcanism Phase 3 Modified from Lawton & McMillan, 2000

  25. Passive Rifting/Slab Roll Back Model Future Gulf rift Phase 3 Modified from Lawton & McMillan, 2000

  26. Active Rifting/Slab Assimilation Model Phase 1 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  27. Active Rifting/Slab Assimilation Model Phase 2 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  28. Active Rifting/Slab Assimilation Model arc migrates west Phase 2 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  29. Active Rifting/Slab Assimilation Model Phase 3 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  30. Active Rifting/Slab Assimilation Model Phase 4 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  31. Active Rifting/Slab Assimilation Model Future Gulf rift Phase 4 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989

  32. Conclusions • A geochemical change in Sonora is only evident on the trace element scale • Geochemical change in Sonora is progressive and spans several million years (time scale to flush mantle wedge) • Sonora does not resemble petrotectonic models: • Age/temp wrong for slab roll back • Arc migrates wrong direction • Need to be cautious when interpreting historical geochemistry

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