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 1. 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? (Atwater, 2004) RTJ

4. Why Sonora?

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

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

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

8. 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 depleted in HFS NVZ Andes 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 All andesites & basaltic andesites from Sonora Less enriched in LILS Less depleted in HFS

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

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

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 Modified from Lawton & McMillan, 2000 Phase 1

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

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

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

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

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

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 Phase 2 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989 arc migrates west

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 Phase 4 Concept from Severinghaus & Atwater, 1990 & Atwater, 1989 Future Gulf rift

32. 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 Conclusions