1. Feedbacks between lithospheric stress and magmatism in incipient continental rift zones Erin Beutel *(1), Jolante van Wijk (2), Cindy Ebinger (3), Derek Keir (4), 1College of Charleston, Department of Geology and Environmental Sciences, 66 George St., Charleston, SC 29424-0001, beutele@cofc.edu 2University of Houston, Houston, TX, jwvanwijk@uh.edu 3University of Rochester, Rochester, NY, ebinger@earth.rochester.edu 4University of Leeds, Leeds, UK, D.Keir@leeds.ac.ukebinger@earth.rochester.edu

2. Hypothesis: The location, shape, and extent of magmatic injection in continental rift zones may be controlled predominantly by tectonic forces and lithospheric strength. Hypothesis

3. Pangaea Study Areas East African Rift: Main Ethiopian Rift Beutel et al, submitted G3, 2009 Beutel, 2009

4. Testing: Use finite element program FElt to test stress evolution during evolving continental lithosphere extensional processes. Assumptions: 1) The lithosphere behaves elastically 2) Magma injection will be more likely to occur in areas of extension Methods

5. Early Rifting Evolution Sequence ~230 Ma Northeast trending normal faults ~200 Ma Northwest trending dikes ~200 Ma North trending dikes ~200 Ma Northeast trending dikes Pangaea: Observables North America Beutel, 2009

6. Evolving applied tectonic force due to continental motion: Result show relative magnitudes not absolute numbers Physical Properties of Rift Models TABLE 1 Based on Tessema and Antoine (2004) and basic rock mechanics Key to Cartoon Models Young ﾕ s Modulus Pa Density kg/m 3 L ow e r Crust 5 e +12 2850 Upper Crust 4e+12 2590 U pper Crust Thinn e d 4 e +12 2590 Mantleli thosp h e r e 8e+12 3200 Sediment 3.5e+12 2560 Intrusion Weak (Magma) 1e+12 3000 Intrusion Str ong (Cooled) 8e+12 3000 Intrusion2 Weak (Magma) 1e+12 2800 Intrusion2 Strong (Cooled) 8e+12 2800 Finite Element Model Pangea Beutel et al, submitted G3, 2009

7. Northeast trending normal faults: ~230 Ma Plate boundaries/sutures are one order of magnitude weaker than the surrounding lithosphere. Pangaea: Model Results North America Extension Compression

8. Northwest and North trending dikes: ~200 Ma Plate boundaries/sutures are one order of magnitude weaker than the surrounding lithosphere Pangaea: Model Results North America Extension Compression

9. North and Northeast trending dikes: ~200 Ma Boundary between North and South America becomes significantly weakened. Pangaea: Model Results North America Extension Compression

10. Large scale lithospheric strength and tectonic motion controls early rifting and magmatism. Pangaea: Model Results North America

11. Main Ethiopian Rift (MER) Observed In the East African rift strain and activity is focused in the 15 km x 60 km mafic magmatic intrusions in the mid- to shallow crust. Beutel et al, submitted G3, 2009

12. Applied force due to continental motion: Results are shown as relative stress intensity. Physical Properties of Rift Models TABLE 1 Based on Tessema and Antoine (2004) and basic rock mechanics Key to Cartoon Models Young ﾕ s Modulus Pa Density kg/m 3 L ow e r Crust 5 e +12 2850 Upper Crust 4e+12 2590 U pper Crust Thinn e d 4 e +12 2590 Mantleli thosp h e r e 8e+12 3200 Sediment 3.5e+12 2560 Intrusion Weak (Magma) 1e+12 3000 Intrusion Str ong (Cooled) 8e+12 3000 Intrusion2 Weak (Magma) 1e+12 2800 Intrusion2 Strong (Cooled) 8e+12 2800 Main Ethiopian Rift Model Beutel et al, submitted G3, 2009

13. Models: Cartoons of Elastic Finite Element Models 300 km Mantle Lithosphere Lower Crust Upper Crust Cartoon model of cross-section through Afar type rift zone including lithospheric properties after Tessema and Antoine (2004) 200 km Upper Crust Thinned Upper Crust Elastic Finite Element Program FElt by Gobat and Atkinson, 1997 Beutel et al, submitted G3, 2009

14. 60 40 20 0 60 40 20 0 km 300 km Extension Compression Key: Background colors indicate maximum stress magnitude and type, bars indicate maximum and minimum stress vectors (black is extensional, white is compressional) Results: Strong magmatic bodies below thinned rift zones Beutel et al, submitted G3, 2009

15. Extension Compression Key: Background colors indicate maximum stress magnitude and type, bars indicate maximum and minimum stress vectors (black is extensional, white is compressional) 40 20 0 60 40 20 0 km 60 300 km Results: Weak magmatic bodies below thinned rift zones Beutel et al, submitted G3, 2009

16. 200 km Mapview model of Northern MER crust with intrusions (about 8 km depth) Beutel et al, submitted G3, 2009

17. 200 km Results: Strong magmatic bodies in a thinned rift zone at ~8 km depth Extension Compression Key: Background colors indicate maximum stress magnitude and type, bars indicate maximum and minimum stress vectors (black is extensional, white is compressional) Beutel et al, submitted G3, 2009

18. 200 km Extension Compression Key: Background colors indicate maximum stress magnitude and type, bars indicate maximum and minimum stress vectors (black is extensional, white is compressional) Results: Weak magmatic bodies in a thinned rift zone at ~8km depth Beutel et al, submitted G3, 2009

19. 50 km Extension Compression Key: Background colors indicate maximum stress magnitude and type, bars indicate maximum and minimum stress vectors (black is extensional, white is compressional) Results: Strong magmatic body with weak dike intrusions Beutel et al, submitted G3, 2009

20. Results/Implications MER Stress at dike tips is higher in cooled magmatic bodies than in the surrounding lithosphere Dikes will often propagate within the magmatic body before they propagate through adjoining crust

21. Results/Implications Stress is concentrated in thinned lithosphere Once continental crust has thinned to a given point, that rift will continue to focus stress and thin.

22. Results/Implications MER Stress is concentrated in strong magmatic bodies Once strong magmatic bodies in weak crust are created they will become a stress (strain) foci. Resulting in magmatically segmented rift zones

23. Results/Implications MER Stress is concentrated around weak magmatic bodies If a magma body becomes highly magmatic and weak, stress will be concentrated at its tip and propagation is possible.

24. Results/Implications Overall The location and style of magmatism in rift zones is controlled by the distribution of stress in the lithosphere. Stress in the lithosphere is controlled by tectonically applied stresses and evolving lithospheric strength.

25. Results/Implications Overall Large scale tectonic stresses can remain largely constant and the evolving stress field due to evolving lithospheric properties will cause changes in the magmatic intrusion location, style and extent.

27. AG BK FD Do the model results follow with the observables? Seismicity in low magmatism segments Felsic Volcanoes near segment ends Predictions