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 , 2University of Houston, Houston, TX, 3University of Rochester, Rochester, NY, 4University of Leeds, Leeds, UK,

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

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

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

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

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 Upper Crust 4e U pper Crust Thinn e d 4 e Mantleli thosp h e r e 8e Sediment 3.5e Intrusion Weak (Magma) 1e Intrusion Str ong (Cooled) 8e Intrusion2 Weak (Magma) 1e Intrusion2 Strong (Cooled) 8e Finite Element Model Pangea Beutel et al, submitted G3, 2009

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

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

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

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

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

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 Upper Crust 4e U pper Crust Thinn e d 4 e Mantleli thosp h e r e 8e Sediment 3.5e Intrusion Weak (Magma) 1e Intrusion Str ong (Cooled) 8e Intrusion2 Weak (Magma) 1e Intrusion2 Strong (Cooled) 8e Main Ethiopian Rift Model Beutel et al, submitted G3, 2009

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

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

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

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

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

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

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

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

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.

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

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.

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.

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.

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