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.edu3University of Rochester, Rochester, NY, ebinger@earth.rochester.edu

4University of Leeds, Leeds, UK, D.Keir@leeds.ac.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 numbersPhysical 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/m3

Lower Crust 5e+12 2850

Upper Crust 4e+12 2590

Upper Crust Thinned 4e+12 2590

Mantlelithosphere 8e+12 3200

Sediment 3.5e+12 2560 Intrusion Weak (Magma) 1e+12 3000

Intrusion Strong (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

Northeast trending normal faults: ~230 Ma

Plate boundaries/sutures are one order of magnitude weaker than the surrounding lithosphere.

Pangaea: Model Results North America

ExtensionCompression

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

ExtensionCompression

North and Northeast trending dikes: ~200 Ma

Boundary between North and South America becomes significantly weakened.

Pangaea: Model Results North America

ExtensionCompression

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/m3

Lower Crust 5e+12 2850

Upper Crust 4e+12 2590

Upper Crust Thinned 4e+12 2590

Mantlelithosphere 8e+12 3200

Sediment 3.5e+12 2560 Intrusion Weak (Magma) 1e+12 3000

Intrusion Strong (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

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, 1997Beutel et al, submitted G3, 2009

60

40

20

0

60

40

20

0

km

300 km

ExtensionCompression

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

ExtensionCompression

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

200 km 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

ExtensionCompression

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

ExtensionCompression

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

ExtensionCompression

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

Predictions