feedbacks between lithospheric stress and magmatism in incipient continental rift zones
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Feedbacks between lithospheric stress and magmatism in incipient continental rift zones Erin Beutel *(1), Jolante van Wijk (2), Cindy Ebinger (3), Derek Keir (4), - PowerPoint PPT PresentationTRANSCRIPT
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, [email protected]
2University of Houston, Houston, TX, [email protected] of Rochester, Rochester, NY, [email protected]
4University of Leeds, Leeds, UK, [email protected]
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