Feedbacks between lithospheric stress and magmatism in incipient continental rift zones

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.edu2University of Houston, Houston, TX, jwvanwijk@uh.edu3University of Rochester, Rochester, NY, ebinger@earth.rochester.edu4University of Leeds, Leeds, UK, D.Keir@leeds.ac.uk

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

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

4. Methods 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

5. Pangaea: Observables North America 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 Beutel, 2009

6. Finite Element Model Pangea P h y s i c a l P r o p e r t i e s o f R i f t M o d e l s T A B L E 1 B a s e d o n T e s s e m a a n d A n t o i n e ( 2 0 0 4 ) K e y t o Y o u n g ﾕ s D e n s i t y 3 a n d b a s i c r o c k m e c h a n i c s C a r t o o n M o d u l u s P a k g / m M o d e l s L e e o w r C r u s t 5 + 1 2 2 8 5 0 U p p e r C r u s t 4 e + 1 2 2 5 9 0 U e e p p e r C r u s t T h i n n d 4 + 1 2 2 5 9 0 M a n t l e l i t h o s p r 8 e + 1 2 3 2 0 0 e h e S e d i m e n t 3 . 5 e + 1 2 2 5 6 0 I n t r u s i o n W e a k ( M a g m a ) 1 e + 1 2 3 0 0 0 o n g ( C o o l e d ) 8 e + 1 2 3 0 0 0 I n t r u s i o n S t r I n t r u s i o n 2 W e a k ( M a g m a ) 1 e + 1 2 2 8 0 0 I n t r u s i o n 2 S t r o n g ( C o o l e d ) 8 e + 1 2 2 8 0 0 Evolving applied tectonic force due to continental motion: Result show relative magnitudes not absolute numbers Beutel et al, submitted G3, 2009

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

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

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

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

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. Main Ethiopian Rift Model P h y s i c a l P r o p e r t i e s o f R i f t M o d e l s T A B L E 1 B a s e d o n T e s s e m a a n d A n t o i n e ( 2 0 0 4 ) K e y t o Y o u n g ﾕ s D e n s i t y 3 a n d b a s i c r o c k m e c h a n i c s C a r t o o n M o d u l u s P a k g / m M o d e l s L e e o w r C r u s t 5 + 1 2 2 8 5 0 U p p e r C r u s t 4 e + 1 2 2 5 9 0 U e e p p e r C r u s t T h i n n d 4 + 1 2 2 5 9 0 M a n t l e l i t h o s p r 8 e + 1 2 3 2 0 0 e h e S e d i m e n t 3 . 5 e + 1 2 2 5 6 0 I n t r u s i o n W e a k ( M a g m a ) 1 e + 1 2 3 0 0 0 o n g ( C o o l e d ) 8 e + 1 2 3 0 0 0 I n t r u s i o n S t r I n t r u s i o n 2 W e a k ( M a g m a ) 1 e + 1 2 2 8 0 0 I n t r u s i o n 2 S t r o n g ( C o o l e d ) 8 e + 1 2 2 8 0 0 Applied force due to continental motion: Results are shown as relative stress intensity. Beutel et al, submitted G3, 2009

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

14. Results: Strong magmatic bodies below thinned rift zones 0 0 20 20 km 40 40 60 60 Compression Extension 300 km 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

15. Results: Weak magmatic bodies below thinned rift zones 0 0 20 20 km 40 40 60 60 Compression Extension 300 km 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

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

17. Results: Strong magmatic bodies in a thinned rift zone at ~8 km depth Compression Extension 200 km 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. Results: Weak magmatic bodies in a thinned rift zone at ~8km depth Compression Extension 200 km 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

19. Results: Strong magmatic body with weak dike intrusions Compression Extension 50 km 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

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

28. Predictions